The Oscilloscope [How To Understand and Use TV Test Instruments (1953)]

INTRODUCTION

It does not take a serviceman long to discover that one of his most valuable tools in the servicing and alignment of television receivers is the oscilloscope. For servicing, the oscilloscope is used to reveal the waveshape of the signal in each of the circuits through which it passes. Of particular interest are the video amplifiers, the sync separator circuits, and the horizontal and vertical sweep systems. In each of these circuits comparison of the signal as it is with the waveform diagrams furnished by the manufacturer provides an excellent method of determining whether or not a circuit is operating properly.

For alignment, the oscilloscope is a natural companion to the sweep generator, depicting graphically the response curve of the circuit into which the sweep signal is fed. This superior method of aligning wideband circuits not only provides an instantaneous picture of the circuit conditions as they exist, but any changes that are wrought by adjusting coil cores and/or trimmer capacitors in the circuit become immediately apparent. The technician is thus kept fully informed at all times of the condition of the circuit being worked on.

Much of the mystery which once surrounded the oscilloscope, its mode of operation and the circuits it employs, has now been replaced by everyday familiarity. T his is because an oscilloscope and the deflection circuits in a television receiver operate by the same basic principles. In a television receiver, saw-tooth deflection currents (or voltages, depending upon whether there are deflection coils or deflection plates) sweep the electron beam from side to side or from top to bottom. This, too, is the action in an oscilloscope with the exception that in the oscilloscope the saw-toothed deflection voltage is applied only to the horizontal deflection plate s. The vertical deflection plates receive the incoming signal. Focus and centering controls ( of one sort or another) are similar in purpose in TV sets and oscilloscopes. An intensity control on an oscilloscope becomes the brightness control on a television receiver. These are some of the more obvious similarities -- others will become apparent as we describe and examine many of the oscilloscopes which are currently available to the serviceman.

BASIC OSCILLOSCOPE CIRCUITS

The heart of an oscilloscope is the cathode-ray tube, for it is on the fluorescent screen of this tube that the various waveforms applied to the unit are depicted. A beam of electrons is developed in a gun structure located at the narrow or neck end of the tube. Electrons emitted by a hot cathode are accelerated forward and as they travel through a series of metallic cylinders, they are formed into a narrow beam. This beam then travels down the length of the tube to the fluorescent screen. See Figure 1. Wherever the beam strikes the screen, visible light is produced.

As in the case of the conventional vacuum tube, the control grid regulates the number of electrons which travel past it. Since the extent of this electron flow directly affects the intensity of light which is emitted by the fluorescent screen, the illumination level is controlled by varying the grid' voltage on the tube. This control is placed on the front panel of the instrument and is called the "Intensity" control. See Figure 2. Turning this knob counterclockwise will cause the trace produced by the beam to become dimmer until it finally disappears; turning the control knob to the right or clockwise will gradually raise or increase the beam intensity.

There is no one correct position for this control. Just how intense the beam should be will depend upon the amount of surrounding light. If the oscilloscope is used in an area where the light level is high, then, in all probability, the "Intensity" control will be turned well to the right. Where the surrounding light is not too bright, a less intense trace will prove sufficient. It all depends on the serviceman and where he works.

Figure 1. The Internal Structure of an Electron Gun and the Path Travelled by the Beam in Reaching the Fluorescent Screen.

The electrons emitted by the cathode have a tendency to spread out, and it is necessary to control and focus them into a narrow beam. This is accomplished by the focusing and accelerating electrodes, which act in the same manner as the optical lens system of a camera, except that in this case, it is an electron beam which is focused rather than a light beam. By adjusting the voltage applied to the focusing electrode1 the beam diameter is controlled. The potentiometer which performs this function is also mounted on the front panel and is called the "Focus" control. See Figure 2.

Figure 2. The Front Panel Controls of an Oscilloscope. Courtesy Simpson Electric Company.

After passing through the focusing and accelerating electrodes, the electron beam reaches two sets or pairs of deflecting plates which a r e mounted at right angles to each other. Remembering that electrons are inherently negative and that opposite charges attract and like charges repel, we can appreciate what these plates do. Looking in at the face of the tube ( Figure 3) we would see the fluorescent screen, the plates, and the white (or light) dot caused by the electron beam striking the screen.

If now we apply a positive voltage to Plate H1 and a negative voltage to plate H2, H1 will attract the beam, H2 will repel it, and the beam will move closer to H1. Therefore, the dot of light is displaced to the left. By the same token, if the reverse voltages are applied to the plates, the beam would move to the right.

Figure 3. The Horizontal Deflection Plates (P1 and P2), the Vertical Deflection Plates (V1 and V2), and the Fluorescent Screen of a CRT as Seen Head-On.

With no Deflection Voltages Applied to any of the Plates, the Electron Beam Will Strike the Screen at the Center.

Figure 2. The Front Panel Controls of an Oscilloscope. (Model 476, Courtesy of Simpson Electrical Co.)

Figure 4. An Alternating Voltage Applied to the Horizontal Deflection Plates H1 and H2 Will Produce a Line on the Screen.

Figure 6. To Reproduce this Curve on a Scope Screen, then as the Beam Travels from Left to Right, it Must also be Made to Move Vertically.

Figure 5. The most Common Type of Voltage which is Applied to the Horizontal Deflection Plates in a CRT iIs the Saw-Tooth Wave.

If an alternating voltage is applied to both plates, the beam will continuously deflect from one side to the other. If the voltage is made to change quickly enough, the result will be a horizontal line.

See Figure 4. The most common type of voItage which is applied to the horizontal deflection plates is the saw-tooth wave shown in Figure 5. From A to C the voltage rises steadily and linearly, moving the beam across the face of the oscilloscope screen at an even rate. At point C, it drops sharply, returning to the same level as point A. This drop causes the electron beam to retrace rapidly.

Half of the applied saw-tooth wave is negative (points A to B) and while this portion of the wave is active, the beam is at some point to the left of center.

At A the beam is farthest to the left, but as the saw tooth voltage gradually rises, the beam is drawn in toward the center, reaching this point when the volt age reaches point B. As the voltage continues to rise, the forward motion of the beam brings it to the far right-hand section of the screen when the saw tooth voltage reaches point C. From point C to point D, the saw-tooth voltage drops sharply, causing the electron beam to retrace quickly back to the left-hand side of the screen again.

We see from this sequence that the application of a saw-tooth voltage to the horizontal deflection plates moves the beam first one way across the screen, and then the other way. If this back and forth motion is repeated often enough per second, the various traces blend into each other, producing a steady horizontal line (also known as a base or axis) of uniform intensity.

To reproduce a certain waveform, say the response curve shown in Figure 6, then as the beam travels on its way from left to right, we also want it to move vertically ( or up and down). This can be accomplished by applying the wave to be reproduced to the vertical deflection plates. When the voltage applied to the vertical plates increases, the beam moves up; when it decreases, the beam moves down.

In this way the beam moves up and down as it travels across the face of the cathode-ray tube and the wave shape of any voltage applied to the vertical deflection plates is traced out.

It is perfectly feasible to apply waveforms to be observed directly to the deflection plates them selves and this is sometimes done (as we shall see later). However, in order to obtain any sizeable deflection of the beam, either straight across or up and down, a considerable amount of voltage is required. A much more flexible arrangement is achieved by inserting amplifiers between the deflection plates and the applied voltages. Now, with rather small input voltages, a sizeable deflect ion of the electron beam can be obtained and the usefulness of the oscilloscope as a test instrument is increased.

The block diagram of the oscilloscope, thus far, appears as shown in Figure 7. There is one set of amplifiers leading to the vertical deflection plates and a similar set feeding the horizontal plates. Just how many amplifier stages are contained in each system depends upon how elaborately the oscilloscope is designed. In the more expensive units there may be as many as four stages; in the lower priced instruments only two stages will be found. In practically all instances, the final or output stage is push-pull, providing balanced voltages for each set of deflection plates. The use of such balanced voltages produces a trace which is uniformly wide at both ends of the trace line. When single-ended amplifiers are used, this is not true.

Figure 7. A Simplified Block Diagram of an Oscilloscope.

Aside from a power supply, only one more circuit is needed in Figure 7 to complete the basic diagram of an oscilloscope. This is the sweep or saw-tooth generator, the stage which develops the saw-tooth wave which is applied to the horizontal deflection plates through the horizontal deflection amplifiers. Two types of saw-tooth generators are in general use today, the multivibrator and the thyatron tube. The multivibrator is a resistance-capacitance coupled oscillator in which the frequency of oscillation is determined by the values of the capacitances and resistances used in the circuit. The saw-tooth wave is developed across a capacitor which is allowed to charge (producing the trace or forward motion of the electron beam in the oscilloscope) and then rapidly discharged (producing the faster retrace portion). This multivibrator is identical in form to the multivibrators used in the sweep systems of the television receivers.

The thyratron tube is a gaseous tube of special construction. In the thyratron circuit, Figure 8, when the power is first applied, the capacitor C, connected across the tube, will charge up at an approximately uniform rate until it reaches a certain potential known as the ionizing potential of the tube. (Until this level is reached, the tube is nonconductive.) When the tube ionizes, it conducts heavily, effectively placing a short-circuit across capacitor C and causing it to discharge rapidly. When the potential across the capacitor, and therefore the tube, drops below a level known as the deionization level, the tube ceases to conduct and the charge again builds up across the capacitor. This deionization level is close to zero and so the voltage build-up across the capacitor C ranges between the ionizing potential at which the tube fires and the de-ionizing potential at which it ceases to conduct. See Figure 8. The frequency of the saw-tooth wave generated depends upon how rapidly C charges and this, in turn, is governed entirely by the relative values of C and R. The foregoing circuits represent the basic components of every oscilloscope. Just how many front panel controls will be found on the instrument depends upon how elaborate its circuits are. In the discussion to follow, we will examine the operating controls of a number of modern oscilloscopes in order to see not only what these controls do, but how they are employed in television service and alignment work.

Figure 8. Thyratron Tube Method of Developing Saw Tooth Waves.

OSCILLOSCOPE OPERATING CONTROLS

The oscilloscope in Figure 2 contains what might be termed a typical number of controls. The "Focus" control is used to adjust the sharpness of the trace or point of light on the screen. This knob is rotated until the trace is as sharp and as clearly defined as it can be. The "Intensity" control enables the operator to adjust the brilliance of the spot or trace. The proper method of adjusting this control was previously discussed. To move the beam vertically or horizontally, "Vert. Centering" and "Horiz. Centering" controls are provided. Rotating the vertical centering knob moves the trace or pattern up or down while rotating the horizontal centering knob moves the pattern left or right. With the aid of these two controls, the pattern can be positioned anywhere on the face of the tube.

The next three controls, "Sync," "Range Frequency," and "Sweep Range," are all associated with the saw-tooth generator contained in this instrument.

The simplest of the three controls, and actually the one to be set first, is the " Sweep Range" control.

In the "OFF" position, this control turns the sweep generator off and the beam appears as a pin point of light at the center of the screen. In other words, by turning the sweep generator off, we have removed all deflection voltages from the horizontal deflection plates and the electron beam impinges on the screen at a single point, usually the center.

If the beam is permitted to remain stationary at one point on the screen with even normal intensity, it will soon burn the screen, with the result that in future use this area may become insensitive and not produce any light at all as the beam passes over it.

If, in the process of working with an oscilloscope, it becomes necessary to shut off the sweep oscillator, then reduce the beam intensity until the spot is only faintly visible.

The question now arises, "When is it necessary to set the ' Sweep Range' control to the 'OFF' position?" The answer: When a special 60-cycle sine wave deflection voltage is fed to the o s c ill o scope from a sweep generator during an alignment. It will be remembered that this particular point was discussed in the previous section on sweep generators.

The 60-cycle sine wave voltage from the generator is fed into the oscilloscope at the terminals marked "Horiz. Input" and it is used in place of the saw-tooth voltage to deflect the beam horizontally. We will return to this point again when we discuss TV receiver alignment.

Beyond the "OFF" position, the " Sweep Range" switch selects the operating frequency range for the horizontal saw-tooth oscillator. The first position beyond "OFF" is labeled 15 to 75 cycles, which means that the frequency of the saw-tooth deflection voltage produced will lie within this range.* The exact frequency generated between 15 and 75 cycles will be established by the position of the "Range Frequency" control. This potentiometer is a fine tuning or vernier adjustment on the "Sweep Range" control. At the low end of its rotation the saw-tooth frequency generated is at its lowest value for any position of the " Sweep Range" switch; at the other end (extreme clockwise position), the saw-tooth frequency is at the highest value for the range chosen by the " Sweep Range" control. In the case of the 15 to 75 cycle position, this would be 75 cycles (approximately). Sweeping ranges of this particular instrument (of Figure 2) extend from 15 cycles to 60,000 cycles.

* The frequency of the saw-tooth wave tells you how many times a second the beam sweeps across the screen from left to right and how many times it re traces. For example, a 60-cycle saw-tooth wave will move the beam 60 times per second from left to right and 60 times from right to left on retrace.

This permits us to see one cycle of any wave having a frequency between 15 cycles and 60,000 cycles. Any wave with a frequency above 60,000 cycles will develop more than one cycle on the oscilloscope screen; any wave having a frequency less than 15 cycles will develop less than one cycle during one forward trace of the electron beam.

To use the two foregoing controls to observe one or more cycles of any wave applied to the vertical input terminal (and ground, of course), set the "Sweep Range" switch to the range within which the signal frequency falls. Then rotate the "Range Frequency" control until one cycle ( or two or as many as desired) appears on the oscilloscope screen.

If you don't happen to know the approximate frequency of the applied signal, it takes but a minute to try each of the five range positions of the " Sweep Range" switch until you find the best range to use.

After a little practice it takes longer to explain how to do it than it does to do it.

In order to work with any pattern on the screen, the pattern should be held stationary. With the "Range Frequency" control, it is possible with some patience, to adjust the frequency of the saw-tooth generator until it exactly equals ( or is an exact multiple of) the frequency of the applied vertical signal.

But, unless this control is constantly adjusted, the frequency of the saw-tooth generator will change ( even if only a few cycles) and the pattern will drift.

To keep the trace or pattern steady without frequent recourse to the "Range Frequency" control, a portion of the incoming signal is fed to the saw tooth generator and serves as a synchronizing pulse which locks the generator in step with its own frequency. The "Sync" control enables the operator to vary the amount of synchronizing pulse or signal fed to the sweep oscillator. The optimum position for this control is at that point where the smallest amount of sync signal causes the pattern to become stationary. Thus, you start with the "Sync" control at zero and slowly turn it to the right ( clockwise) until the pattern locks in. The three controls just described and especially the "Sync" control, are to be used in conjunction with the "Function" switch situated just below them.

It is important before using the "Sync" control to adjust the "Range Frequency" knob until the pattern is close to being stationary.

This "Function" switch is a 5-position switch which controls the power input and selects the desired horizontal deflection signal. The power is off in the OFF position of the switch and is on in the remaining four positions. In addition, the switch makes the following connections in its 5 positions:

1. OFF. Opens the circuit for the power input.

2. INT. SYNC. A linear (or saw-tooth) sweep voltage is applied to the horizontal amplifier. At the same time, a portion of the "Vert. Input" signal is fed into the sweep oscillator through the "Sync" control. If the frequency of the applied (or "Vert. Input") signal is near frequency, the pattern can be locked steady on the screen of the cathode tube.

3. LINE SYNC. The saw-tooth sweep voltage is still applied to the deflection plates through the horizontal amplifier. However, now, the synchronizing voltage is not taken from the incoming signal, but from the 60-cycle power line. The 60-cycle pulse is injected into the sweep oscillator through the "Sync" control and therefore locks the saw-tooth oscillator in sync with the line frequency. This is usable for applied signals having frequencies of 20, 30, 60, 120, and 180 cycles, and for other sub-harmonics of 60 cycles.

4. EXT. SYNC. The saw-tooth sweep voltage is still active. However, now, a synchronizing pulse can be obtained only from an external signal injected into the sweep circuit through the "Ext. Sync" terminall. The setting of the "Sync" control still deter mines how much of this sync pulse reaches the saw-tooth oscillator.

5. HORIZ. AMP. In this position the saw-tooth voltage of the oscilloscope's sweep generator is disconnected from the horizontal amplifier and the beam is stationary at the center of the screen in a small, round spot. To obtain any horizontal deflection of the beam, an external signal must be applied to the "Horiz. Input" terminal at the front of the oscilloscope. This signal will be amplified by the horizontal amplifier and applied to the horizontal deflection plates of the cathode-ray tube.

When the "Function" switch is in the "Horiz. Amp." position, no use is made of the internal sweep oscillator of the oscilloscope. Consequently, to prevent stray voltages from the oscillator reaching the horizontal system, it is always best to place the " Sweep Range" switch in the OFF position when the "Function" switch is set at "Horiz. Amp." The most frequent use that the TV serviceman will make of the "Horiz. Amp." position will be in TV or FM receiver alignment. As discussed previously, nearly all sine wave sweep generators supply their own 60-cycle deflection voltage for the oscilloscope and this should be used in preference to a 60-cycle saw-tooth or sine wave voltage which the oscilloscope may be capable of supplying.* * An exception to this occurs when the oscilloscope contains specific provision ( in the form of a phasing control) enabling it to develop a properly phased pattern. Such units will be examined presently.

On either side of the "Function" switch are the gain controls for the vertical and horizontal amplifiers. Thus, the "Vert. Gain" control adjusts the amplitude of the signal fed into the vertical pre-amplifier and hence it controls the height of the pattern on the viewing screen. The "Horiz. Gain" potentiometer adjusts the input to the horizontal amplifier to produce the desired pattern width on the cathode-ray screen. This control is effective when ever any voltages, external or internal, are .applied to the horizontal amplifier.

Just beneath the "Vert. Gain" control is the "Vert. Attenuator" switch. This is a voltage divider network which limits the amount of the vertical input signal reaching the first vertical amplifier. There are four switch positions labeled: .5, 5, 50, and 500 volts. These figures indicate the maximum value of input signal that should be applied with the switch in each position. Thus, with the attenuator switch in the .5 position, no signal having an amplitude greater than 5 volts rms should be applied to the "Vert. Input" terminal (and ground). Excess voltage on any range may produce a distorted pattern or may possibly cause damage. On the other hand, setting the control pointer too high -- say at 500 volts for an input voltage of 30 volts will produce a pattern which is too small. In other words, do not set the control too high or too low.

The "Vert. Attenuator" switch receives the applied signal before the "Vert. Gain" control. Hence, the attenuator switch may be considered as a rough adjustment with the gain potentiometer as its vernier.

The last remaining control on the front panel of this instrument ( Figure 2) is the "Horiz. Sens." switch. This two-position switch is in series with the line from the "Horiz. Input" terminal to the horizontal amplifier and it is marked "High" for the closed position of the switch. The circuit (Figure 9) indicates that the full signal is fed to the horizontal amplifier when the switch is in the "High" position.

The system is thus most sensitive at this setting and affords any applied signal maximum amplification (provided, of course, that the "Horiz. Gain" control is turned up, too.)

Figure 9. The Horizontal Input Circuit of the Instrument Shown in Figure 2.

The "Horiz. Sens." is marked "Low" for the open position of the switch and a 1 megohm resistor is placed in series with the input to the "Horiz. Gain" control.

These markings of "High" and "Low" can be confusing since they refer to the sensitivity of the horizontal amplifier system and not to the applied voltages. Actually, strong voltages are applied with the switch in the "Low" position be c au s e with a strong voltage, less amplification is required. On the other hand, with a weak signal, the "Horiz. Sens." switch would be set to "High." At the bottom of the oscilloscope there are six terminal posts through which connections are made to the instrument circuits. · The posts are employed as follows: "GND." Two ground terminal posts are pro vided, one on each side of the front panel for grounding input circuits.

"VERT. INPUT". Any signal connected between this terminal and "GND." will be coupled through an isolating amplifier to the vertical amplifier to be used for vertical deflection.

"60-CYCLE TEST SIGNAL".

A 6.3 volt , 60 cycle signal is available at this connection for any outside use including calibration of the sweep frequency of the instrument itself.

"EXT. SYNC". When the "Function" control is in the "Ext. Sync" position, any signal connected be tween the "Ext. Sync" terminal and the "GND." terminal will be coupled to the "Sync" control.

"HORIZ. INPUT". Any signal connected between this terminal and "GND." can be fed into the horizontal amplifier if the "Function" switch is in "Horiz. Amp." position. (Turn " Sweep Range" to OFF.)

Figure 10. The Terminal Board Located Behind the Front Panel Cover Plate of the Oscilloscope Shown in Figure 2.

We have now covered all the front panel controls and terminals of the oscilloscope. Also located on the front panel, at the bottom, is a cover plate marked "Remove for Internal Oscilloscope Connections." When the four mounting screws are removed and the plate taken off, a terminal board with 10 numbered terminals appears, as shown in Figure 10.

Note that jumper wires connect the following terminal pairs: 1 and 6, 2 and 7, 3 and 8, 4 and 9, and 5 and 10. The jumper wires between 4 and 9, and between 5 and 10 connect the output of the horizontal amplifiers to the horizontal deflection plates; the wires between 1 and 6, and 2 and 7 connect the output of the vertical amplifiers to the vertical deflection plates; and between terminal 3 and terminal 8 they feed a blanking pulse (from terminal 3) to the cathode (terminal 8) of the cathode-ray tube.

To use the plates directly, we would first remove the jumpers, thereby breaking the circuit between the vertical and horizontal amplifiers and their respective deflection plates. Next, to apply a voltage to the horizontal plates, we would connect it between terminals 9 and 10. To bring a voltage to the vertical plates, we would apply it across terminals 6 and 7.

The television serviceman will seldom have occasion to use the plates directly. This becomes necessary when the signals to be observed either have very large amplitudes or their frequencies are too high to be successfully passed through the vertical deflection amplifiers. It is also possible to use the plates directly for the measurement of DC voltages (which will not be passed by the R-C coupled vertical amplifiers). Either the vertical or the horizontal deflection plates may be used. The oscilloscope is calibrated first by the applying of a known voltage to the selected pair of plates and noting how much the spot on the screen is moved or shifted. The known voltage is then removed and the unknown DC voltage applied in its place. The deflection of the spot that this latter voltage produces is now com pared with that caused by the known or calibrating voltage. From the equation below, the value of the unknown DC voltage is determined.

D Known

E Known

D Unknown

E Unknown

Where: D Known = Distance in inches spot is moved by known voltage.

D Unknown = Distance in inches spot is moved by unknown voltage.

E Known = Value of known voltage.

E Unknown= Value of unknown voltage (to be found).

When direct use is made of the deflection plates in the oscilloscope of Figure 2, not only are the vertical and horizontal amplifiers disconnected from the plates, but the normal centering voltages are also removed. In this condition, rotation of the front panel centering controls will have no effect on the beam. In fact, the only front panel controls which will have any effect are the power on and off, the "Intensity" and the "Focus". (In some oscilloscopes, the positioning controls are effective under all conditions.)

"INTENSITY MODULATION". The cathode of the C. R. tube is connected to terminal No. 8 on the front panel board through a capacitor. Ordinarily, terminal No. 8 is connected to terminal No. 3, there by receiving (from terminal 3) a short, sharp, positive pulse of voltage every time the beam retraces rapidly from right to left. The application of a positive pulse to the cathode of a tube produces the same result as the application of a similar negative voltage to the control grid. If the pulse is sufficiently powerful, it will cut the beam off during the retrace interval and that is precisely what happens here. This action is desirable because there is seldom any reason for having the beam produce a visible marking during the retrace interval. In fact, by blanking out the beam, the desired pattern stands out clearer.

It is not necessary to employ the cathode solely for the purpose of blanking out the retrace. If it is desired to intensify the beam during certain portions of its sweep, then negative pulses can be fed to the cathode via terminal 8. Simply remove the jumper between 3 and 8 and apply the intensifying v o It age between 8 and one of the "GND." terminals. By feed i n g in different types of voltages, we can either brighten the trace or cut it off during certain selected portions of its cycle. This process of varying the beam intensity is known as intensity modulation and many oscilloscopes contain some provision for achieving it. While not much use is made of this facility in television service work, it is employed in frequency measurement. For those interested in this application, additional information will be found in the instruction manual which comes with their oscilloscope.

In working with oscilloscopes or in reading the instruction manuals that accompany them, it will be found that the horizontal input terminals are sometimes called the "X" axis. This terminology was borrowed from mathematics where it is customary to refer to all horizontal axes as "X" axes. By the same token, the vertical input terminals are referred to as "Y" axis. And, to complete the analogy, the intensity modulation terminal is known as the "Z" axis. Despite the names, all three systems operate as outlined above.

HOW TO SET UP SCOPES

The foregoing discussion has dealt with the function of each control found on the oscilloscope of Figure 2. To someone who is unfamiliar with this instrument, even the relatively s mall number of controls can prove quite confusing unless he knows how to set the instrument up and prepare it for use.

Toward that end, the following outline will prove helpful.

A. Before the power is turned on, the following controls should be set to approximately mid-position.

1. "FOCUS"

2. "VERT. CENTERING"

3. "SYNC"

4. "RANGE FREQUENCY"

5. "HORIZ. CENTERING"

6. "INTENSITY" B. Set the "Vert. Gain" to zero and "Horiz. Gain" to 5.

C. Set "Vert. Attenuator" to 500 V position.

D. Set "Horiz. Sens" to "Low".

E. Set " Sweep Range" to 75-350-cycle position.

F. Now turn the "Function" switch from OFF to "Int. Sync" and the "Operate" pilot light should come on: Let the instrument warm up for approximately five minutes.

On the screen a single straight line should appear. Rotate the two centering controls until the line is centered on the screen. Adjust the "Intensity" control until the line is as bright as desired and then bring the line into sharpest focus by rot at in g the "Focus" control. If the horizontal line extends be yond the edges of the screen, it may be desirable to reduce the "Horiz. Gain" setting until the ends of the line are visible.

Thus far we have proceeded on the assumption that with the various controls set as indicated, a line will appear on the screen when the power is turned on.

But suppose it doesn't. What then? The first thing to check is the "Operate" pilot light. Is it on? If it is, then turn next to the "Intensity" control and set it to at least three-fourths position, or possibly to the maximum clock w i s e position. Note whether any trace (or dot) appears on the screen.

If the trace is still missing, move the "Vert. Centering" and "Horiz. Centering" controls as the line may be off screen.

If the instrument is operating normally, these adjustments will produce a trace on the screen. Absence of the trace indicates that the unit is somehow defective.

Once the oscilloscope has been set up as indicated, it is in position to receive voltages for its vertical amplifiers. The voltage should be applied between the "Vert. Input" terminal and "GND." and the "Vert. Gain" control is rotated until the pattern occupies as much height as desired. If you find that the pattern is too small at any setting of the "Vert.

Gain" control, switch the "Vert. Attenuator" to a lower position.

To produce a stationary pattern of the wave, adjust the " Sweep Range" first, then the "Range Frequency", and finally the "Sync" controls as outlined previously.

It is possible to check the vertical system of the oscilloscope before any signals are applied. To do this, run a short jumper from the "60-cycle Test Signal" terminal to the "Vert. Input" terminal. This will place a 60-cycle voltage across the vertical input terminals. Then adjust the "Vert. Gain" and "Attenuator" controls until the sine wave pattern appears on the screen.

Figure 11. The Genescope -- a Multi-Purpose Instrument Containing an AM Generator, a Crystal Calibrator, an Oscilloscope, and a Sweep Generator. Courtesy Simpson Electric Company.

The oscilloscope shown in Figure 2 possesses the particular shape that it does because it is also available in combination with an AM generator and an FM and TV sweep generator. see Figure 11. At the time of this writing this is the only instrument of its kind.

Figure 12. An Oscilloscope which Is Available in Kit Form. Courtesy Eico.

Figure 12. An Oscilloscope Which Is Available in Kit Form. (Model 425, Courtesy Eico.)

Fig. 13

OTHER OSCILLOSCOPES

The front panel of another oscilloscope is shown in Figure 12. This unit, which can be procured in kit form as well as assembled, contains essentially the same controls as the previous oscilloscope. How ever, the physical placement of the controls differs and, in some instances, their names have been alter ed. Thus, in place of the previous " Sweep Range", we now have "Coarse Frequency". The markings on this latter control start at 30 cycles and extend up to 30,000 cycles. Actually, you will discover, after working with different oscilloscopes, that the numbers inscribed at each position of this control are only approximate and in many instruments there is considerable overlap between adjacent ranges. Thus, although the lowest range of the unit in Figure 12 is marked as 30, it is possible to go somewhat below this. Also, it will be found that sweeping frequencies above 30 M, or 30,000 are attainable.

The "Fine Frequency" control in Figure 12 serves the same purpose as the "Range Frequency" knob in the previous unit. And the "Sync Amp." is the circuit which is employed to lock the horizontal sweep to the frequency of the signal being observed.

Power to the instrument is controlled by a separate switch instead of being part of a multi-position switch. The "Horiz. Input" switch connects the horizontal amplifiers to either the internal saw-tooth generator or to the horizontal input binding post. The "Sync" switch transfers the horizontal synchronization from internal sync to the "Sync Input" jack on the front panel. There is no line synchronization switch in this oscilloscope and to lock the sweep generator in with the power line frequency, a short wire would have to be connected from the 60-cycle test binding post to the "Sync Input" post. Then the "Sync" switch would be placed in the "Ext." position.

In this oscilloscope, an intensity modulation jack is located on the rear panel and is used for the same purpose as it was in the previous oscilloscope.

Also on the rear panel is a terminal strip from which connections can be made direct to the vertical deflection plates. No such facility is available for the horizontal deflection plates. Connecting term in a 1 jacks on the front panel include "Horiz. Input", "GND.", "Vert. Input", "Sync Input", and "60-Cycle Test". Another oscilloscope, shown in Figure 13, differs in certain respects from the previous two instruments. Of minor interest is the fact that the power on-off switch is now ganged to the "Intensity" control. When this control is in the extreme counter clockwise position, the power is off. To turn the power on, rotate this knob in a clockwise direction.

The pilot light will come on, indicating that the instrument is in operation. Further rot at ion of the knob will increase the intensity of the beam on the cathode-ray tube screen.

While this is the first oscilloscope, thus far, which employs this particular arrangement, it is a fairly common one.

The instrument contains a set of terminals on the front panel labeled "Z-Input". This is for modulating the beam. In addition, there is "Z-Axis" amplifier which will amplify any signal applied to the "Z-Input" terminals before coupling this signal to the control grid of the cathode-ray tube. The "Z Axis Gain " control regulates the amount of voltage applied to this amplifier. When the control is in the extreme counterclockwise position, none of the "Z-axis" voltage reaches the cathode-ray tube grid.

Rotating the knob in a clockwise direction permits more and more voltage to reach the "Z-Axis" amplifier, producing a greater effect on the cathode-ray tube beam. In use, connect the desired "blanking" or "brightening" signal to the "Z-Input" terminals, and, starting in the extreme counterclockwise position, rotate the control clockwise until the desired blanking or brightening is observed on the trace. Never use more "Z-Axis" gain than is necessary, since too much blanking or brightening distorts the picture.

Figure 14. A resistive Voltage Divider Network Suggested by Supreme, Inc. for use with their Oscilloscope of Figure 13.

Figure 13. An Oscilloscope Containing a Special" Z Axis" Amplifier. Courtesy Supreme, Inc.

The "Sweep Frequency" control on this instrument is a six-position switch with the various frequency ranges identified by letters, A, B, C, D, etc., instead of the more conventional method of using the frequencies themselves. However, by referring to the instruction manual, it will readily be found that the various letters stand for the following sweep frequencies.

A. 6 hz to 60 hz B. 20 hz to 200 hz C. 90 hz to 700 hz D. 550 hz to 4 khz E. 3.5 khz to 25 khz F. 24 khz to 150 khz

In practice, the time it takes to find the proper settings of the "Sweep Frequency" and "Fine Frequency" controls (to obtain one cycle of the input wave on the screen) is so short that reference to the table in the manual is seldom necessary.

Incidentally, while we are on the subject of instruction manuals, it is strongly recommended that these manuals be carefully and thoroughly studied.

No one knows better than the manufacturer what the capabilities of his instrument are and this information is, for the most part, found only in the manual.

To illustrate the point, the oscilloscope in Figure 13 contains vertical input terminals and a vertical gain control. There is, however, no special attenuation control for handling large signals and looking in the manual we find that signals larger than 25 volts peak-to-peak should not be applied directly to the input terminals since they will produce a distorted trace. The manufacturer suggests that if it is de sired to view the waveform of higher voltages, they first be reduced to a suitable value by the use of a resistive network such as shown in Figure 14. This network is so proportioned that the scope receives 1/10 of the total applied voltage.

At the bottom of the front panel (Figure 13) there is a socket receptacle marked "Probe". Into this receptacle is plugged a male socket which, in turn, is connected to a special high-frequency probe.

The oscilloscope, through the socket, feeds filament and plate voltages to a small 6C4 tube contained within the probe housing. The probe, because of its high input impedance ( 5 megohms) and low shunting capacitance {9 mmf.), can be used to pick up RF and IF signals directly at various points within a radio receiver. The 6C4 acts as an RF cathode follower, feeding whatever signals it receives to the vertical system. (The vertical amplifiers have a response that extends out to 7 mhz.) The probe cannot be used with FM or television receivers because of the very high RF and IF frequencies in these circuits.

PHASING CONTROLS

It was noted earlier in this section that when an oscilloscope is used in conjunction with a sweep generator, a 60-cycle sine wave beam s wee ping voltage must ordinarily be obtained from the generator. However, if the oscilloscope contains the proper auxiliary circuits, this extra connection can be dispensed with and the sine wave beam sweeping voltage obtained from the oscilloscope itself.

If the front panel of the oscilloscope shown in Figure 15 is examined carefully it will be found to contain a control labeled "60-Cycle Phasing". In addition, if you glance at the "Coarse Frequency" control, you will find a position, at the extreme left hand side of this scale, marked "60-Cycle Sweep". With the knob in this position, the saw-tooth oscillator is turned off and a 60-cycle sine wave voltage is applied to the horizontal amplifier. This, then, duplicates what the connecting lead from the sweep generator would do.

The next step is to be able to adjust the phase of this 60-cycle voltage in order that it may be brought in step with the sine wave driving voltage in the sweep generator. When the in-phase condition is attained, essentially one trace ( or response pattern) will be observed on the screen. This is the function of the "60-Cycle Phasing" control on the oscilloscope of Figure 15.

PHASING CONTROL

Figure 15. An Oscilloscope which can provide its own 60 Cycle Sweep and Phasing Control. Courtesy Sylvania Electric Products Company.

At this point an interesting question could be raised: "When an oscilloscope s u c h as shown in Figure 15 is used with a sweep generator for a circuit alignment, and the oscilloscope is made to supply its own sine wave deflection voltage, can a single response pattern be obtained by varying the sweep generator's phase control instead of the oscilloscope's phase control?" The answer is "No", be cause the phase control in the sweep generator varies the phase of the 60-cycle voltage that it has available for connection to the oscilloscope. This control does not vary the phase of the 60-cycle voltage which it itself uses to sweep the oscillator frequencies back and forth. And, if we do not use the 60-cycle voltage that the sweep generator has available for the oscilloscope, then, of course, the control will have no effect.

In the oscilloscope of Figure 15, the "60-Cycle Phasing" control does not become effective until the "Coarse Frequency" s witch has be en set to the "60-Cycle Sweep" position.

One further point concerning this oscilloscope is the "Vert. Attenuation" switch. The switch reduces the height of the vertical trace by attenuating the signal fed from the ''Vert. Input" terminals to the cathode follower. The 1:1 position applies full signal; the 100:1 applies 1/100th of the signal, etc.

Figure 15. An Oscilloscope Which Can Provide its Own 60 Cycle Sweep and Phasing Control. (Model 400, Courtesy Sylvania Electric Products Inc.)

Figure 16. The Precision Model ES-500A Oscilloscope. (Courtesy Precision Apparatus Company.)

Figure 16. The Precision Model ES-500AOscillo scope. Courtesy Precision Apparatus Company.

Figure 17. Non-Symmetrical Waveforms which Could Make it Difficult to Obtain Suitable Synchronization in an Oscilloscope. See text.

PRECISION OSCILLOSCOPE

To illustrate the wide variety of names that different manufacturers assign to controls which per-form exactly the same functions, the oscilloscope shown in Figure 2 used the name "Range Frequency" for the potentiometer which varied the saw - tooth sweep frequency within any given range. In the following instruments this was changed to "Fine Freq." (Figure 12), and "Freq. Vernier" (Figure 15). Now, in Figure 16, the name has been shortened to "Vernier". In general, the names most frequently employed are either "Fine Frequency" or "Vernier". Under "Synchronization" in Figure 16 we find first the usual "Sync" control (or, as it is labeled here, the "Sync Lock") with which we can regulate the amplitude of the sync voltage applied to the internal saw-tooth generator. Next to it is a switch which, in previous instruments, was labeled as the sweep or sync selector. However, instead of having three positions, EXT., INT., and LINE, there are four positions here, with two positions for INT.,i. e., INT. NEG., and INT. POS. In either position, a portion of the signal or voltage applied to the vertical input terminals is fed to the saw-tooth generator to provide synchronization or lock-in of the pattern observed on the screen.

The reason for incorporating this additional position can be seen if we examine Figures 17 A and 17 B. Non-sinusoidal waveforms may occasionally have a large negative voltage as compared to the positive voltage (or vice versa). If the polarity of the synchronization circuits in the scope is not selectable from the oscilloscope panel, a waveform of the type illustrated in Figure 17B could be synchronized only by its relatively small peak. In such a case, insufficient voltage might be available for synchronization and an unstable pattern could result. This instability can be overcome by designing the sync selector switch so that sync voltage can be obtained from either the positive or negative peak of the wave to be observed.

With a sine wave or any other symmetrical waveform, the same lock-in action will be obtained with the switch in either position.

There are two knobs labeled "Horizontal Phase" and "Blanking Phase" at the bottom of the front panel. The first of these controls, "Horizontal Phase," has already been encountered on the previous oscilloscope, Figure 15, under the name of "60-Cycle Phasing". All the remarks made there are applicable here, too.

Figure 18. FM Detector Response Obtained on an Oscilloscope with Blanking Control in Sweep Generator. (B) Usual Dual Pattern with no Blanking.

Figure 19. The Vertical Polarity Reversing Switch in the Precision Oscilloscope with Permit Curves to be Displayed in Either Direction.

Figure 20. Simplified Diagram of the Vertical Reversing Switch Circuit in the Precision Oscilloscope.

The second control, "Blanking Phase", when turned from its OFF position, applies a sine wave voltage to the control grid of the cathode-ray tube.

This has the effect of intensifying the trace during a portion of the sine wave cycle, and of blanking it out during its most negative portion. The most useful application of this function is the elimination of one of the dual patterns obtained in the sweep alignment of TV and FM receivers. The 60-cycle sine wave applied to the control grid of the CRT is so phased, by means of the "Blanking Phase" control, that when the second pattern should be traced out, the beam is blanked out. This results in one pattern which is much easier to work with.

Blanking controls, it will be remembered, were previously encountered on sweep generators. When this control was turned on, the sweep oscillator was prevented from functioning during its backward sweep and, as a consequence, a single response pattern trace was produced on the screen. In essence, then, the blanking switch on the sweep generator and the "Blanking Phase" control on the oscilloscope of Figure 16 perform the same function.

There is, however, this difference. When the blanking control is in the sweep generator, the pattern seen on the scope screen will appear as shown in Figure 18A. During the backward trace, when the sweep generator is inoperative, the sine wave deflection voltage applied to the scope will bring the beam straight back across the screen, forming a base line.

This base line is helpful as a reference or zero axis against which the serviceman can compare the response curve. In aligning discriminators, for example, both halves of the "S" curve should be symmetrical. By having a reference line to work against, the job of judging when the balanced condition is reached is considerably eased. Figure 18A shows the "S" curve with a base line, while Figure 18B illustrates the usual dual pattern when no blanking is employed. The advantage of the base line arrangement is evident.

Now, when the blanking control is contained in the oscilloscope instead of the sweep generator , turning it on and adjusting it will remove one of the dual patterns. However, unlike the previous arrangement, no base line will appear. The reason, of course, stems from the fact that the blanking control in the oscilloscope removes the second pattern by cutting off the C.R. T. beam. During this interval no electrons reach the screen. On the other hand, when the blanking control is in the sweep gene rat or , the sweep oscillator stops working for one-half cycle and the generator output drops to zero. Since the oscilloscope beam is not affected, it still continues to be seen. However, with no signal voltage coming in during this interval, it moves unaffected across the screen, producing the base line. During the next for ward trace, the sweep generator resumes normal output and the response pattern is produced.

To operate either type of blanking control, first produce the response pattern with the dual traces.

Next, bring the two patterns as close together as possible. Then rotate the blanking control until only one trace is visible.

VERTICAL POLARITY SWITCH

Another useful feature of the oscilloscope of Figure 16 is the "Vertical Polarity" reversing switch.

This switch has two positions marked NORMAL and REVERSE. When the switch is moved from one position to the other, the pattern on the screen is flipped over or reversed in the vertical direction. Thus, if the pattern appears as shown in Figure 19A when the switch is in one position, it will appear as shown in Figure 19B when the switch is turned to the other position. This is a useful device for interpreting TV video IF and sync or sweep waveforms, e spec i all y when the manufacturer's manual shows the pattern in one position and your scope shows it in the opposite position.

Reverse patterns on the screen is a source of confusion to many servicemen. Why, for example, should the manufacturer's bulletin show the pattern pointing up when your scope has it pointing down? Does it mean that your equipment is hooked up wrong or does it indicate a defective circuit? Actually, the answer is neither one. The polarity of the signal at various points in a television receiver is important and in general the manufacturer attempts to show the signal as it should be at each specific point. Now, this same signal, fed to your scope may come out (at the screen) right side up or upside down, depending upon whether your scope has an even or an odd number of stages in its vertical amplifier system. Any voltage, passing through an amplifier, is reversed in phase by 180°. Therefore, if the signal shown in Figure 19A is fed into the grid of an amplifier, it will appear at the plate as shown in Figure 19B. Pass this second signal through one more stage and lo and behold, it is right-side up again.

As a general rule, two or any even number of stages will produce a signal at the output which has the same phase (approximately) as the input signal.

By the same token, pass a signal through one or any odd number of amplifier s and its phase will be reversed.

Figure 22. Hickok Model 195-B Oscilloscope. (Courtesy Hickok Electrical Instrument Company.)

Figure 21. (A) Video IF Response Curve with the Low Frequencies at the Left and the High Frequencies at the Right. (B) Same Curve in Reversed Position.

Figure 22. Hickok Model 195 B Oscilloscope. Courtesy Hickok Electrical Instrument Company.

Thus, the polarity of the wave seen on the screen of an oscilloscope will depend upon the number of amplifiers in its vertical system.

In the instrument of Figure 16, this limitation is circumvented by the polarity reversing switch. To obtain a reversal, the designers of this instrument merely reversed the connections made to the final push-pull vertical amplifier stage. See Figure 20.

In the NORMAL position of this switch, the grid of V2 receives its signal from the plate of V1 while V3 receives its signals from the cathode of V2. In the REVERSE position, the input signals to V2 and V3 are interchanged. Note that the same phase reversal can be incorporated in the horizontal system. Thus, one oscilloscope may show a video IF response curve with the low frequency and at the left (Figure 21A) while another may have it on the right (Figure 21B). Textbooks and service manuals usually prefer to show curves with their low frequencies at the left and high frequencies at the right and that is the way most ser vice men come to recognize them. The curve in Figure 21B may be transformed into the one in Figure 21A by reversing the voltages applied to the horizontal deflection plates. Sometimes an oscilloscope contains such a horizontal phase reversal switch, although it is not often found.

*ADDITIONAL OSCILLOSCOPE CONTROL NAMES

Additional changes in the names of some of the controls is encountered in the oscilloscope shown in Figure 22. Thus, what was previously known as the "Coarse Frequency" control is here labeled as the "Steps" control. Associated with this control is the "Vernier" knob. The "Locking" control is similar in operation to the previous "Sync" lock-in controls.

In the horizontal system there is a " Horizontal Gain" potentiometer and just beneath it is a 4-position selector switch (unnamed on this panel) to determine what i s fed into this system. The designations at each position are as follows:

A. AMP. OUT. Any voltage connected to the horizontal input binding posts is fed directly to the horizontal deflecting plates through a DC blocking capacitor.

B. AMP. IN. The voltage at the horizontal binding posts is fed through the horizontal amplifier to the deflecting plates. This would be the position to use, for example, when a 60-cycle sine wave driving voltage is obtained from the sweep generator.

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*Horizontal reversal of a pattern can also be accomplished by reversing the phase of the 60 cycle driving voltage fed by the sweep generator to the oscilloscope.

This was previously noted in section 3.

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C. 60'v. The horizontal deflection voltage is a 60-cycle sine wave obtained from the power supply.

The position could be used, during a sweep-alignment, in place of the 60-cycle driving voltage from the genera tor. To superimpose the two response patterns produced on the screen, a "Phasing" control is available.

D. S.S. OSC. Now the horizontal sweep voltage is derived from the internal saw-tooth sweep oscillator and may be of any frequency from 10 cycles to 25 khz.

Just below this 4-position switch is a 3-position switch (up, center, and down) governing the synchronization or lock-in of the saw-tooth sweep oscillator.

When switched to the INT. position, the sweep oscillator may be synchronized at the frequency, or sub multiple of the frequency, of any voltage being applied to the vertical deflecting circuits.

In the center, it is in the EXT. position and the oscillator may be synchronized from any external source of frequency. An arrow points to the binding post where this external synchronizing voltage is applied. (Note that on the left-hand side of the pilot light there is another terminal marked EXT., but this one is associated with the "Intensity Mod." switch.

The third or final position of the "Sync" control (down) is labeled 120 cycles FM and in this position, 120-cycle pulses are obtained from the power supply and used to synchronize the saw-tooth sweep oscillator. (In a full-wave power supply, the frequency of the rectified pulses is 120 cycles. In a half-wave power supply, it is 60 cycles.) This particular position of the "Sync" switch is designed to meet the recommendations that some manufacturers of FM receivers make concerning the alignment of their sets. More information on this point will be given in a later section.

The "Intensity Modulation" switch (at the left) contains two positions - INTERNAL and EXTERNAL. The EXTERNAL position, of course, is the familiar one wherein any external voltage may be applied to the EXT. binding post and employed to intensity or blank out a portion of the beam trace. The new position, labeled INT. is used only when 60 cycles is being used for horizontal deflection and, in this position, the return trace is blanked out.

Note, therefore, that this instrument ( Figure 22) contains a "Phasing" control with which to super impose the forward and return traces (when 60-cycle sine wave deflection is being used) and an internal blanking circuit to remove the return trace. In the previous oscilloscope ( Figure 16), it was further possible to adjust the phase of the blanking voltage by rotating the "Blanking Phase" control. This provided some latitude in choosing the position of the forward or backward trace to be blanked out which is not available here.

PROBES

Before we leave the oscilloscope of Figure 22, it might be of interest to note that in the vertical selector switch (which is located just beneath the "Vertical Gain" control) there is one position labeled "De mod". When the vertical selector switch is in this position, any modulated RF voltage applied to the vertical input terminals is detected and then applied to the vertical amplifier system for presentation on the screen. In essence, we have here a built-in RF probe. Thus, in a radio receiver, any of the RF or IF signals could be applied directly to this scope.

It is important to remember, however, that be cause leads must be used between the vertical input terminal of the scope and the receiver, that only relatively low-frequency signals can be detected success fully. If we probed high-frequency FM and TV circuits with these same leads, circuit operation would be disturbed to such an extent that the usefulness of this arrangement would be questionable. However, for low-frequency signals , this added convenience is desirable.

There are occasions, m signal tracing and in sweep alignment, when it would be helpful if the signal at various points in the RF or IF stages could be viewed directly on the scope screen. To permit this to be done, many oscilloscope manufacturers have available RF probes which can be used in circuits operating at frequencies as high as 200 mhz or more.

These RF probes are similar to VTVM RF probes, containing a germanium crystal ( or a miniature diode) which detects the received signal and then applies the low-frequency modulation of the signal to the vertical input terminals of the scope for screen presentation.

The principal difficulty which is encountered in using these probes is the small amount of signal which is present in the RF and IF stages of a receiver. A good oscilloscope will require about .015 volts (rms) of input signal to produce a pattern 1-inch high. Low priced units will generally need considerably more voltage, sometimes as much as .5 volts, to produce the same size pattern. In the final IF stages of a receiver it may be possible to obtain this large a signal; seldom in the RF section, or first IF stages.

Figure 23A. Low-Capacity Probe Circuit.

The word "probe" has a variety of meanings in radio and television parlance. Thus, in VTVM's there is an RF probe and a high voltage probe. Also, the word probe is applied frequently to the test lead or prod which connects to the AC input jack and to the lead which goes to the DC input terminal. With oscilloscopes, there is the RF probe we have just discussed. In addition, there is frequently found another type of probe known as a low-capacity probe. Its purpose is to permit the proper observation of wave forms in low-capacity circuits.

The vertical input circuit of an oscilloscope contains a certain amount of capacitance - on the order of 30 to 50 mmf. To this we can add (on the average) another 25 to 50 mmf of capacitance arising from the test lead.* Thus, when you place your test prod or probe at some point in a circuit to observe the waveforms present there, you are automatically shunting this point with 80 to 100 mmf of additional capacity. In some circuits, this additional capacity will have virtually no effect on the circuit operation; in other circuits, especially where the wave for m s contain relatively high frequencies (such as square sync pulses), the additional capacitance will actually alter the shape of any wave present here.

• A pair of plain leads will shunt less capacitance across the circuit than a section of coaxial cable . However, the cable is shielded, reducing or eliminating spurious signal pickup and, because of this, is more desirable.

Figure 23B. How the Low-Capacity Probe Might Appear When Constructed. Courtesy Hickok Electrical Instrument Company.

To minimize the disturbing effect of the oscilloscope test lead, a special low capacity probe can be designed. A small, sem1variable capacitor and resistor connected as shown in Figure 23A, are encased in a special housing to form a low capacity probe.

The value of the capacitor will usually be about 10 to 15 mmf. while its parallel resistor will be about 2.2 meg. The reduction in shunting capacity occurs because the padder capacitor is actually being placed in series with the 80 to 100 mmf of combined capacity present in the connecting test lead and at the vertical amplifier input. And since capacitances in series produce a total value which is less than the lowest capacitor, the addition of 10-15 mmf of series capacitance reduces the effective overall capacitance to a decided improvement over the 80 mmf or so present before the addition.

One disadvantage of this arrangement is that the voltage actually reaching the vertical amplifiers of the scope is reduced in the same proportion as the input capacitance. Thus, if the total capacitance is decreased by 1/100, so is the voltage reaching the scope. In television service work, the observation of waveforms using the low-capacity probe usually is required in the video amplifier and sweep systems and in these stages sufficient voltage is available, even with the probe reduction.

Figure 25. (A) Padder Capacitor Value too High.

(B) Padder Capacitor Value too Low. (C) Padder Properly Adjusted.

Figure 24. Wave Shown at (A) Was Obtained Without use of Low-Capacity Probe; Wave at (B) Did have Probe. Note the Difference Between the Two.

Figure 25. (D) Padder Capacitor Properly Adjusted for Normally Operating Video Detector Source.

The two illustrations in Figure 24 show clearly how a much truer picture of the circuit waveform is obtained when a low-capacity probe is employed. The steep sides of the waveform, containing the higher frequency components, are much more in evidence, in Figure 24B than they are in Figure 24A. It is in the reproduction of steep sided pulses and waveforms that the low-capacity probe is most useful.

Anyone not possessing a low-capacity probe may build one from the circuit shown in Figure 23.

Then connect it to the oscilloscope it is to be used with and apply a square wave to the probe. Use a frequency between 1,000 and 10,000 cycles. Adjust the padder condenser until the wave on the screen is also square.

Figure 25A illustrates a condition where the value of the padder capacitor is too high and Figure 25B where the padder capacitor value is too low.

Figure 25C is the square wave as it appears when the padder is properly adjusted.

In the absence of a square wave generator, Philco recommends that the probe be connected to the video detector of a TV set known to be in good working order. Set oscilloscope for composite video signal, and adjust padder so that amplitude of vertical and horizontal sync pulses are equal. See Figure 25D. (Oscilloscope sweep frequency is set at 30 cycles to observe composite waveforms.)

Once a low-capacity probe is adjusted for a certain oscilloscope, it should be used only with that instrument. Use with any other oscilloscope should not be attempted unless the padder is returned. It is also well to keep in mind that the probe cannot compensate for limitations in the frequency response of the vertical amplifier system of the oscilloscope.

In recognition of its usefulness, a low-capacity probe is built into the oscilloscope shown in Figure 20, Whatever voltage the probe picks up will reach the vertical amplifiers when the selector switch located just above the probe is turned to one of the three positions on the side marked PROBE. In the PROBE-1 position, the probe signal receives its greatest amplification. In the PROBE - 1/10 position, only 1/10 of the voltage of the PROBE-1 position reaches the vertical amplifier. And in the PROBE 1/100 position, even less of the signal reaches the vertical amplifier . Thus, there is a certain amount of attenuation in the probe circuit itself, plus an additional amount in the 1/10 and 1/100 positions, if desired.

When the low-capacity probe of this oscilloscope is not in use, signals are applied to the terminal marked AC. This terminal is similar to the vertical input terminals on the previous oscilloscopes. A separate attenuating system (AC-1, AC-1/10 and AC 1/100) is available for these signals.

Figure 26. Oscilloscope Containing a Built-In Low Capacity Probe. Provision is Also Available for Measuring Peak-To-Peak Values of Any Waves Depicted on Screen. Courtesy General Electric Company.

Figure 27. The Wave Shown Above Covers a Height of Ten Squares on the Ruled Mask.

PEAK-TO-PEAK VOLTAGE MEASUREMENTS

In this instrument ( Figure 26) we meet for the first time a feature which is finding increasing usage in television servicing. This is the "Calibrate Volts PK-PK (Peak-Peak)" control. The values shown in the various positions represent the peak-to-peak volt ages which will be obtained from the " Cal Volts Out" terminal. For, example, when the "Calibrate" switch is in the 0.3 position, the sine wave voltage obtained from "Cal Volts Out" terminal is 0.3 volts, peak-to peak. Additional peak-to-peak voltages of 1.5, 3.0, 15, 150, and 300 are also available.

Having voltages available whose peak-to-peak value is known is extremely helpful when servicing the vertical or horizontal sweep systems of television receivers. In checking through any sweep system, two facts concerning the waveforms found there are important--wave shape and peak-to-peak amplitude.

The wave shape can be determined by inspection on a scope screen; the wave amplitude, however, must be measured and having known calibration voltage on hand is one way to accomplish this. The method of measurement is as follows: Apply the wave whose amplitude is to be deter mined to the oscilloscope input terminals and, with the vertical gain control, adjust the amplitude of this wave until it occupies a reasonable height --- say one-half the size of the screen. To fa c i lit ate the measurement, it is generally best to have the wave extend over a specific number of vertical squares on the screen mask. A simple figure to work with, in this respect, is 10 or 15 squares. (The wave in Figure 27 covers 10 squares.) Whatever the figure, make a specific note of it.

Now remove the signal and, without touching the vertical gain control, apply the calibration voltage to the same (vertical) input terminals. A sine wave will appear on the screen. See Figure 28A. Compare its height with that of the previous wave and if the two cover as many vertical squares, then both possess the same peak-to-peak value. Assume that this happens when the calibration switch is in the 30 volts peak-to-peak position. Then both waves have a peak to-peak value of 30 volts. (Some servicemen like to turn down the horizontal gain control until the calibration sine wave is only a vertical line. See Figure 28B. They claim that such a line is easier to work with. Either method will give similar results since we are not interested in the horizontal spread of the wave, but only its height.

Figure 26. Oscilloscope Containing a Built-In Low Capacity Probe. Provision Is Also Available for Measuring Peak-to-Peak Value of any Waves Depicted on the Screen. (Model ST-2C, Courtesy General Electric Company.)

Figure 29. This Oscilloscope Contains a Built-In Peak-to-Peak Voltmeter Together with Several Calibrating Controls. (Model 34-31, Courtesy of Triplett Electrical Instrument Co.)

The fortunate occurrence of both waves having the same peak-to-peak value is not likely to be the most common experience of the serviceman. More often than not, the amplitudes of both waves will differ. Let us· say that when the unknown wave is adjusted to cover a height of 10 squares, the calibration sine wave, with the "Calibrate" switch in the 30 volts position, covers only 8 squares. Then the peak-to peak amplitude of the unknown signal is computed as follows:

CALIBRATION Voltage Value Figure 28.

Unknown Voltage No. of vertical squares it covers.

No. of vertical squares it covers.

The CALIBRATION voltage value here is 30 volts, and the number of vertical squares it covers is 8.

The unknown voltage covers 10 vertical squares .

Hence, then, or and, 30 volts X 8 = 10 8 X = 300 X = 300 8 X = 37.4 volts peak-to-peak. (Approx.) (The same formula would be employed had the amplitude of the calibration wave been greater than that of the unknown wave.) Always measure unknown peak-to-peak amplitudes using calibrating voltages which approximate the unknown value as closely as possible. Not only does this lead to more accurate results, but it is also easier to work with.

The convenience of having a calibrating voltage handy to measure the peak-to-peak values of any voltage applied to the vertical input terminal is re cognized in another oscilloscope (Figure 29) by incorporating a peak reading voltmeter and several controls for varying the amount of the calibrating voltage Perhaps the best way of illustrating the operation of these controls is by means of an example.

Assume there is a pattern on the screen and we wish to determine its peak-to-peak value. As a first step, we would adjust the "Vertical Gain" control until the pattern height extended over a convenient number of screen mask lines - say 10 or 20. The number makes no difference. Note the height of the pattern so that the calibration wave can be made to approach this value. Also, observe the position of "Vertical Atten. Cal." selector switch. Let us assume that it is in the second position marked 1.

With this data in hand, turn the "Vertical Atten. Cal." switch to the 3V CAL. position. This removes the original pattern from the screen and substitutes in its place a 60-cycle sine wave. The peak-to-peak value of this sine wave can be read from the 0-3V scale of the voltmeter on the front panel.

Rotate the "Calibrating Voltage" control until the sine wave covers as much height as the unknown wave and when this condition is reached, the peak-to peak value of the sine wave is simply read from the meter scale.

Figure 29. This Oscilloscope Contains a Built-In Peak-To-Peak Voltmeter Together with Several Calibrating Controls. Courtesy Triplett Electrical Instrument Company.

Available, too, is a 0-10 volt position of the "Calibration" switch, if more voltage is desired.

Note that it is the meter which gives you the peak-to-peak value of the calibrating sine wave. The "Vertical Atten. Cal." switch merely indicates which meter scale to read - 0-3 or 0-10.

To continue, suppose in the above example, that the sine wave occupied as much vertical distance as the unknown wave when the peak-to-peak reading on the meter was 2.5 volts. Then this value multiplied by the 1 to which the "Vertical Atten. Cal." switch was set when the unknown wave was applied tells us that the peak-to-peak value of this wave is 2.5 volts.

Had the "Vertical Atten. Cal." switch been set at 10 when the original wave was applied, the meter reading (2.5 volts) would have had to be multiplied by 10 (to give 25 volts). Or, had the switch been set to the 1st position, or .1, the meter reading would have had to be divided by 10. Which brings up an interesting question. Why, for example, do we take the attenuator control setting into account here when this was not done before? The answer is to be found in the manner in which the calibration voltage is applied to the vertical system. If this calibration voltage is applied to the vertical input terminals in the same manner as the unknown wave, then obviously whatever value we determine for the calibration voltage will be the same for the unknown wave.

In this oscilloscope (Figure 29), however, the calibration voltage is applied internally and, as it happens, it is fed in at a point which is beyond the attenuation network. Therefore, when the incoming signal is attenuated, the amount of attenuation it receives must be taken into account because we wish to know the true peak-to-peak value of the voltage as it is at the vertical input terminals and not what it be comes after it has passed through the attenuator net work. It is to arrive at this true value that we follow the multiplication procedure outlined above.

In this respect, remember that when a low-capacity probe is used to display a wave on an oscilloscope screen, the attenuation caused by the probe must be considered, too. If this information is not available, dispense with the probe and use an ordinary pair of leads while the peak-to-peak measurement is being made. This may result in some distortion of the waveform, but, in general, it will not alter the peak amplitude appreciably.

To the reader who wonders how to determine when to use which procedure there can only be one answer--study the instruction manual of your particular instrument. This cannot be emphasized too strongly. If you think enough of an instrument to buy it, then spend the little extra time required to learn what it will or will not do.

OTHER PEAK-TO-PEAK MEASURING METHODS

When the oscilloscope is incapable of supplying its own calibrated voltages, recourse may be made to a variac and an ordinary VTVM, to a special voltage calibrating instrument, or to a VTVM capable of measuring peak-to-peak values. Each method will be described in turn.

"'---------r RMS VALUE

=.707 PEAK PEAK-TO - PEAK

=2,828 RMS

__ L _________ _ PEAK : 1.414 RMS

Figure 30. Relationship Between RMS, Peak, and Peak-To-Peak Values of a Sine Wave.

A. VARIAC AND VTVM. To start, feed the TV signal into the vertical in put terminal of the oscilloscope. Then, with the wave present on the screen, adjust the vertical gain control of the oscilloscope until the display occupies a height between one-half to three-quarters of the .full screen height. Note the exact number of lines that the pattern covers vertically. (In the absence of a mask, use a crayon to mark off the top and bottom of the wave.) This done, remove the signal from the oscilloscope and apply in its place an AC voltage obtained from the output of a variac. Rotate the Variac control until its sine wave equals the peak-to-peak height of the previous TV signal.

None of the controls of the oscilloscope are touched during this second operation.

Now, with a VTVM or an AC voltmeter, measure the AC voltage being fed to the oscilloscope by the Variac. The figure thus obtained is the RMS value of the wave. The peak value may be computed by multi plying this RMS figure by 1.414. See Figure 30. Since the peak-to-peak value is twice the peak value, the RMS meter reading is multiplied by 2 x 1.414 or 2.828.

To illustrate, suppose the input AC voltage applied by the Variac to the oscilloscope is 10 volts.

The peak-to-peak voltage of this wave is 10 x 2.828 or 28.28 volts. If the unknown wave covers the same height on the screen, its peak-to-peak amplitude is also 28.28 volts.

When you cannot obtain a calibrating voltage whose amplitude equals that of the TV signal, the next best approach is to come as close as possible with the calibrating voltage and then estimate the amplitude of the unknown sign al by comparing its height with that of the calibrating wave. The formula to use is:

Peak-to-Peak Value

Peak-to-Peak value … of unknown signal Screen height of Screen height of AC wave unknown wave

The screen height of both waves may be either in inches or in the number of lines or squares covered.

In other words, they may be in any units, just as long as both measurements are expressed in the same units.

Figure 31. A Voltage Calibrator. Courtesy Hickok Electrical Instrument Company.

Figure 32. Block Diagram of the Hickok Model 630 Television Voltage Calibrator.

B. OSCILLOSCOPE CALIBRATORS.

There are, at the present time, about a half dozen v o l t age calibrator instruments by means of which it is possible to determine the peak-to-peak voltages of any wave in the television receiver sweep or video circuits. All are simply designed, lending themselves to quick and easy application.

The block and schematic diagrams of the calibrator shown in Figure 31 are given in Figures 32 and 33. A multivibrator using a 6SN7 tube develops a symmetrical square wave output. The multivibrator frequency is approximately 440 cycles, but this has no bearing on the application of the instrument. A low frequency was purposely chosen to enable the square wave to be depicted on the scope screen and also to permit the use of a conventional copper oxide metering circuit. The peak-to-peak output of the multivibrator is measured by the meter while, at the same time, the wave itself appears on the scope screen. See Figure 34. When the square wave occupies the same height as the unknown wave, the peak-to-peak value is read from the meter.

Specifically, the calibrator is connected to equipment under test as shown in Figure 35. The voltage to be measured is connected to the "Voltage Input" terminals while the "Voltage Output" terminals connect to the oscilloscope. Throw the " Direct Cal." switch to the DIRECT position. This feeds the TV signal directly to the oscilloscope where its pattern may be observed. Then adjust the vertical gain controls of the oscilloscope until the pattern occupies a suitable height.

Figure 33. The Schematic Diagram of the Hickok Model 630 Television Voltage Calibrator.

The next step is to use the square wave in the calibrator to determine the peak-to-peak value of the TV signal. Throw the "Direct-Cal." switch to the CAL. position. This removes the TV signal from the oscilloscope and substitutes in its place the square wave of the calibrator. Actually, all you see of the square wave are the top and bottom horizontal lines.

Adjust the separation of these two lines, using the "Voltage Range Selector" and "Vernier" controls , until their distance is equal to the peak-to-peak separation of the TV signal. Then simply read the peak to-peak voltage from the meter scale. The meter range to read is determined by the "Voltage Range Selector" switch.

Figure 34. The Screen Presentation Produced by the Hickok Calibrator.

Figure 35. How the Hickok Calibrator is Connected for Use.

Peak-to-peak voltages up to 100 volts may be read directly from the meter. When the TV signal has a greater amplitude than 100 volts, the height it occupies is compared with that of the 100 volt peak to-peak square wave using the following formula: where

E cal calibrating voltage (i.e., 100 volts)

Deal number of squares or lines on scope screen covered by calibrating voltage

Ex unknown voltage

Dx Divisions of deflection for unknown voltage

This formula will be recognized as being similar to the estimating formulas previously given.

Another oscilloscope calibrating instrument is the Sylvania Electric unit shown in Figure 36. This, too, uses a symmetrical, square-topped wave which is controllable in amplitude from zero to 100 volts.

In place of a meter, the "Volts" control is calibrated directly and the value as indicated on this dial scale, multiplied by the setting of the "Multiplier" control, represents the actual peak-to-peak amplitude of the square wave observed on the oscilloscope s c re en . Estimation, using the formula given previously, is required when the peak-to-peak value of the TV signal being measured exceeds 100 volts.

A modified oscilloscope calibrator using sine waves in place of square waves is shown in Figure 37.

The signal to be measured is fed in at the "Input" terminals while the oscilloscope connects to the "Scope" terminals. The selector switch at the right is capable of providing six different peak-to-peak voltages; 1, 2.5, 25, 100, and 250. These values re present the maximum peak-to-peak voltages obtain able from their respective positions when the switch makes contact with them. The "Calibrating Voltage Adjustment" potentiometer at the left enables the operator to vary the voltage at each position from zero to its maximum value.

RMS and simple peak voltage values are also given on the selector switch and on the meter face.

In operation, the calibrator is plugged into an AC outlet to develop its sine wave voltages. Then, with the TV input signal and oscilloscope connected as indicated above, the instrument selector switch is set to any one of the circle positions located between each set of figures. There are six such circles and when the switch is in any one of these positions, the TV signal at the "Input" terminals passes through the calibrator and appears on the scope screen. Here its height is adjusted to cover the desired number of mask lines.

Figure 36. The Sylvania Oscilloscope Calibrating Standard. Courtesy Sylvania Electric Company.

The selector switch is then turned to one of the six calibrating positions containing the peak-to-peak figures. This removes the TV signal from the oscilloscope and substitutes in its place a sine wave. The peak-to-peak value closest to that of the TV signal amplitude is chose n and the "Voltage Adjustment" knob rotated until the sine wave occupies as much height as the TV signal. The exact peak-to-peak value is then read from the appropriate scale of the calibrator meter.

Note that the selector switch of the calibrator contains twelve positions with six producing sine waves of different maximum amplitudes and six alternate positions feeding the TV signal (at the "Input" terminals) directly to the oscilloscope. This arrangement makes it very simple to alternate between the calibrating voltages and the signal under test.

C. PEAK-READING VTVM. A VTVM which is specially designed to indicate peak-to-peak values of a wave will give this value directly when connected into the TV circuit. Precautions to observe in accepting the peak-to-peak readings of many VTVM's were given in Section 1 and it is suggested that the reader refer back to these before using a VTVM for this purpose.

Figure 37. The Oscilloscope Calibrator Manufactured by Simpson Electric Company.

BANDPASS VS. SENSITIVITY

The function of the vertical amplifiers in an oscilloscope is to amplify applied signals in order that they may be sufficiently powerful by the time they reach the deflection plates to produce a sizable pattern on the screen. The greater the amplification available in this system, the smaller the input signal needed to produce a given vertical deflection. In other words, the oscilloscope becomes more sensitive as a measuring device.

Of importance, too, in the vertical system is its bandpass or its ability to amplify a range of frequencies. The signals whose wave shapes are portrayed on the oscilloscope screen are those found in television receivers, i.e., square waves, saw-tooth waves, and video signals, to mention the more common ones. Each of these waves contain a number of frequencies and to accurately depict their wave shapes, every frequency contained in the wave should be passed by the vertical amplifier system. For accurate television signal observation, this can mean a 4-mhz (or more) bandpass.

Figure 38. An Oscilloscope Capable of Providing Either Wide-Bandpass (4 mhz) or High Gain.

(Model CRO-2, Courtesy of Jackson Electrical Instrument Co.)

The objectives, then, in designing an amplifier for use in the vertical system of an oscilloscope are wide bandpass and high amplification. Unfortunately, for any given circuit, bandwidth x gain is a constant which means that if we increase the bandpass by a certain amount, we decrease its gain by the same factor. Thus, one works against the other and if we desire a wide bandpass, we must make up for the resultant gain reduction by adding more amplifier stages.

This, in turn, raises the cost of the instrument.

Most manufacturers of moderately priced equipment resolve this conflict by designing the vertical system amplifiers to possess a nominal bandpass (between 300,000 cycles and 1,000,000 cycles). Sever-al manufacturers permit the instrument user to choose between high sensitivity and wide bandpass by providing a suitable switching arrangement. This, for example, is true of the oscilloscope shown in Figure 38. The "Vertical Input" control has three positions where the vertical system possesses a wide bandpass (response uniform within 10% from 20 cycles to 4.5 mhz) with a fair amount of gain and three positions where the bandpass is reduced (uniform within 10% to 100 khz), but the gain is up. For each of these positions, attenuation ratios of 100:1, 10:1, and 1:1 are available.

In the oscilloscope of Figure 29, there is a switch at the rear by means of which the bandwidth of the vertical system can be widened from its normal value of 2 mhz to a special value of 4 mhz. However, in the 4 mhz position, the system gain is reduced in half.

This matter of bandpass is a source of confusion to many servicemen. Just how wide should the vertical system bandpass be for suitable application to TV receiver servicing? The answer to this can be found by analyzing where ( and how) the oscilloscope is employed. Broadly speaking the scope is used either in conjunction with a sweep generator to trace out the response curve of a circuit or it is used in 'servicing to show whether or not a signal is present at a certain point in a circuit and, if so, what its shape is.

Oscilloscope use with a sweep generator re quires that it be capable of depicting a waveform having a repetition frequency of 60 cycles -- this being the rate at which the response pattern is swept out. Hence, the vertical system bandpass should ex tend down below 60-cycles, preferable to 30 cycles or less. The lower this limit, the more linear the response will be at 60 cycles.

Figure 39. An Oscilloscope Containing a Modified Sweep Generator. (Mode 1 505, Courtesy of Hickok Electrical Instrument Co.)

For servicing, the oscilloscope finds its greatest application in checking voltage waveforms in stages located beyond the video second detector.

These include the video amplifiers, sync separator stages, and the vertical and horizontal sweep systems.

In all but the video amplifier stages, the fundamental frequencies are low ( either 60 cycles for the vertical sweep system or 15,750 cycles for the horizontal sweep system). The rectangular pulse or saw-tooth waveforms in both systems have components that extend to the twentieth harmonic (or so) and taking this into consideration only requires that the bandpass be uniform to a maximum of 315,000 cycles (15,750 x 20 = 315,000). In the video amplifier stages, the full 4.0 mhz video signal is present, but in everyday servicing you are interested primarily in determining whether the video signal is present rather than accurately depicting its wave shape. Hence, any oscilloscope which possesses a uniform vertical frequency response in excess of 300,000 cycles will be suitable for television receiver servicing.

Understandably, the greater the vertical band pass, the more desirable the instrument, but the 300,000 cycle figure may be looked upon as representing a value below which it is not desirable to go.

SPECIAL FUNCTION OSCILLOSCOPES

The instruments we have discussed thus far in this section have been concerned solely with those functions that an oscilloscope would norm all y be called upon to perform. (The Genescope shown in Figure 2, of course, is three instruments in one and is not designed to be used principally for any one purpose, such as the other units are.) However, there are available certain oscilloscopes which will perform other duties that are not ordinarily considered as be in g within the province of an oscilloscope. One such unit is shown in Figure 39. In addition to being an oscilloscope, there is also available here a modified sweep generator.

The controls for this sweep generator are positioned in the lower left-hand corner of the front panel under the general heading of RADIO FREQUENCY. Contained within this instrument is and RF oscillator which can operate at a fixed frequency of 1,000 khz (1 mhz) or at a frequency of 50 mhz, depending upon the position of the "FM Selector" control. A reactance tube is connected to this oscillator and by means of this circuit the frequency of the oscillator can be shifted back and forth 60 times a second. At 1,000 khz, the frequency sweep range is 0-30 khz (depending upon the position of the "Sweep" control) and at 50 mhz, a sweep as wide as 450 khz is possible. Thus, what we have here is an FM oscillator capable of operating at one of two frequencies, 1,000 khz and 50 mhz.

Three controls on the front panel are associated with the FM oscillator: "FM Selector", "Sweep", and "Output". The "FM Selector" is a four position switch controlling the frequency modulated RF output:

a. 1,000 khz: 0-30 khz SWEEP - the 1,000 khz RF signal is frequency modulated with a sweep variable from 0-30 khz.

b. 1,000 khz: EXT - the 1,000 khz RF signal may be frequency modulated from an external source.

c. 50 mhz: 0-450 khz SWEEP - the 50 mhz RF signal is frequency modulated with the sweep variable from 0 to 450 khz.

d. 50 mhz: EXT - the 50 mhz RF signal may be frequency modulated from an external source.

The "Sweep" control determines the sweep range of the FM oscillator. When the FM oscillator is operating at 1,000 khz, this control varies the sweeping range continuously from 0 to 30 khz. When the out put frequency is 50 mhz, the sweep is continuously adjustable from 0 to 450 khz. This control is also in the circuit when external modulation is employed.

The "Output" control in the OFF position serves as an off-on switch for the RF oscillator. In the 0-100 position, it serves as a control of the RF output level.

At first glance, the usefulness of an FM oscillator which is capable of generating only two frequencies may hardly seem to justify the additional cost and effort necessary to include it in the oscilloscope. And this would be true if that is all this particular circuit did. However. a triode mixer is also included with the oscillator and external RF signals may be fed to this triode for mixing with the output of the FM oscillator. Thus, suppose the FM oscillator is operating at 50 mhz and another generator is connected between the "EXT.OSC. Input" and "GND" terminals (both located at extreme lower left-hand corner of the front panel). If the external generator is set to 60 mhz, its signal will mix with the frequency modulated 50 mhz signal in the mixer tube developing and making available at the "RF Output" terminal a 10 mhz FM signal. 10 mhz is the difference frequency although, as a matter of fact, the sum frequency ( 50 mhz + 60 mhz) of 110 mhz will also be present and may be used, if desired.

Thus, we have a flexible arrangement whereby a wide variety of output frequencies may be generated, limited only by the frequency range of the external oscillator. The frequency modulated signals so produced may be fed to a receiver and its response curve determined by feeding the receiver output back to the vertical input terminals of this same oscilloscope.

The remaining controls on this instrument, including the "Phasing" control, deal exclusively with the oscilloscope. They are, in fact, quite similar to the controls on the instrument shown in Figure 22, both units having been manufactured by the same firm.

The oscilloscopes covered in this section are those which are designed primarily for use in radio and television servicing, and therefore are the instruments of greatest interest to the TV technician.

Available but not mentioned are a host of other oscilloscopes, many highly specialized for specific industrial or engineering applications. There is probably no other single test instrument which is as versatile as the oscilloscope. Its range of application is truly science wide.