This Section shows how to adjust a radio to get all the performance that was designed into it. The tune-up procedure for a receiver is known as alignment. Not all restorers of antique radios will wish to try this procedure, especially when they are first getting into restoration, because it can be tricky and does require additional test equipment beyond the basic VOM or multimeter. Furthermore, it is at the very bottom of the list of things to do in restoring a receiver. For these reasons it is presented last in this guide. However, alignment is a very useful procedure to know about. Alignment is often necessary to get top performance out of a radio and is occasionally necessary to get even acceptable performance.
The tuned circuits of a radio receiver must be accurately adjusted to work together if the set is to achieve its maximum degree of operational efficiency. When the circuits are thus correctly related to one another, they are said to be in alignment.
The fixed frequency difference between the RF signal carrier and the heterodyne oscillator must be maintained with a high degree of accuracy over the entire tuning range of the receiver. Simultaneous tuning of the RF and oscillator circuits is achieved by ganging the tuning capacitors ( FIG. 1) and/or inductors ( FIG. 2) of the separate circuits and making them responsive to a single control.
FIG. 1 Ganged tuning capacitor, showing trimmers that are adjusted
during alignment.
In some difficult troubleshooting problems, it may be necessary to attempt alignment to locate the trouble; for example, a shorted trimmer capacitor across a low-resistance coil. Resistance readings are difficult to interpret in a circuit such as this, but failure of the circuit to respond to peaking during alignment will show that there is something definitely wrong with the tuned circuit.
Another reason for alignment is the replacement of one or more tubes in critical circuits, such as oscillators. This illustrates the point that when slightly weak tubes are replaced with new ones, the circuit characteristics can change because of the different interelectrode capacities of the original and new tubes. This can detune the grid circuits and cause low sensitivity.
The several capacitors or inductors are said to track if they retain their proper frequency relationships throughout the tuning range.
The aging of parts, the changing of the characteristics of tubes, climatic conditions and vibration are some of the reasons for mis alignment. Also, haphazard attempts at alignment and tinkering often do more harm than good, and may increase the time spent on relatively minor repairs.
Every receiver that is operating poorly requires maintenance, but it does not follow that every receiver that needs maintenance needs alignment. Repairs which require replacement of components or the redressing of wiring especially in high-frequency circuits, often make subsequent alignment necessary.
The usual indication of the need for alignment is low sensitivity and volume even though everything else is definitely good. Alignment is also needed if the RF circuits do not track properly; that is, if the dial reading does not agree with the frequency of the incoming signal.
ALIGNMENT PRECAUTIONS
Before alignment is attempted, you should read carefully and follow all available service literature on the radio. There have been many instances where a receiver has been thrown out of alignment by tampering.
FIG. 2. The ganged tuning inductors of a Sears Silvertone battery- operated
model.
No adjustments of any kind should be made before it has been definitely established that component part troubles are not causing the abnormal operation. Attempting alignment when other troubles are present can lead to complete realignment after finding the other fault. Alignment is at the very bottom of the list of operations performed after troubleshooting.
The alignment procedures and adjustments recommended in this Section are not meant to be used for all receivers; they are general. Use them only as a guide. The specific information on any receiver is contained in the service instructions written for that receiver. If the particular manual is not available, the general procedure in this Section may be used.
Signal Generator. An accurately calibrated RF signal generator is a prime necessity both for checking the alignment of a set and for aligning the circuits.
It is possible to do a rough job of alignment without a signal generator. Thus, trimmers and padders can be tuned for maximum output from the receiver, but the results obtained from this method are likely to be accurate.
Output Indicators. For best results, an output indicator should be used. This can be the AC scale of a multimeter or VTVM.
The loudspeaker of a receiving set can be used as an output indicator as a last resort. With such a device, however, the results obtained will depend on the accuracy of the restorer’s ear, and the human ear is not very sensitive to small changes in the level of sound. If a loudspeaker must be used, its output should be lowered as far as possible, and the weakest possible input signal should also be used. The signal should be weak in order to minimize or eliminate automatic gain control action, and the output should be lowered because the ear is best able to detect changes of sound level in the low-level range.
AM RECEIVER ALIGNMENT
In general, circuit alignment is best begun in the circuits that are farthest from the antenna. Adjustment then proceeds toward the antenna, with the antenna circuit proper usually being the last one adjusted.
In some receivers, it is necessary to disable the high-frequency oscillator so that unwanted beat frequencies cannot cause misleading signals. The oscillator tube can be removed from its socket, or the tuning capacitor can be shorted out to stop oscillations. This applies only to IF alignment.
The AGC circuit may be used or may be cut out of service during the alignment of the set. If the AGC circuit uses a separate tube, and it cannot be removed from its socket, the circuit can be disconnected at the common point to the stages that are AGC controlled, or the AGC bus can be grounded.
Output Measurements
The signal output level at the detector is an effective measure of circuit alignment. This output can be measured by connecting an electronic multimeter across the detector load resistor ( FIG. 3). An electronic multimeter is specified because a meter with a high sensitivity is required.
If the level at the detector is not strong enough to give a good reading on DC scale of the multimeter, an output meter, or the multimeter connected as an output meter by using the AC scale, may be connected to the audio output circuit. The meter test leads are connected to the voice coil ( FIG. 4).
The signal output may be at a low level at this point also, depending on the sensitivity of the receiver. In this case, the connection shown in FIG. 5 can be used at the highest point of signal voltage in the receiver. A DC blocking capacitor should be connected in the hot lead to protect the meter. If a regular output meter is available, the external capacitor need not be included because there is one connected internally.
LOCATION OF TRIMMERS
Most IF stages have double-tuned transformers; that is, the primaries and secondaries are separately tuned. Either adjustable capacitors or tuning slugs may be used as trimmers. The IF trimmers of old radios are usually small, adjustable mica capacitors that are located in a metal can along with the IF transformer (see FIG. 6). Generally, both adjustment screws (primary and secondary) are located at the top or bottom of the IF can. In some cases, one may be at the top and the other at the bottom of the can, both maybe at the side of the can, or both may be on the chassis. In some cases, it is necessary to use a special nonmetallic screwdriver or hex wrench to adjust the screws since the inductive effect of a metallic screw driver or hex wrench might affect circuit tuning. Special alignment tools are available from any electronics distributor.
FIG. 3. Signal voltage measurement at the detector load.
Fig. 4. Signal voltage measurement at the voice coil.
I-F ALIGNMENT
Before discussing alignment procedures, we assume that all alignment adjustments are not in their normal positions.
Alignment is begun at the final IF stage. Set the signal generator to the desired frequency and turn on the modulation switch. Connect a blocking capacitor in the hot lead between the signal generator and the grid of the last IF amplifier tube and ground. Connect the output meter or multimeter.
Turn the receiver gain controls on full, and set the signal generator attenuator to produce a midscale reading on the output meter. Adjust the primary and secondary trimmers in the output IF amplifier for maximum output.
Move the signal generator connection to the grid of the next IF tube toward the antenna, and adjust the trimmers of the stage for maximum output. The output of the signal generator must now be decreased because the signal strength has been increased by the additional amplification of this stage.
Other IF stages, if any, are aligned in the same manner. It is very important not to change the frequency of the signal generator during the alignment.
Mixer Output Alignment
Because the frequency of the mixer output signal is the same as the IF frequency, the same signal frequency that was fed into the IF stages can be fed into the mixer. Feed the signal into the grid of the mixer. Adjust the trimmers in the IF transformer between the mixer plate and the first IF grid for maximum indication on the output meter.
In some receivers, it may be very difficult to get at the under side of the mixer tube socket, especially in vhf circuits. Connect the signal generator hot lead to a metal tube shield. Push the shield down over the tube, but not so far that it touches the chassis. The signal will reach the grid by capacitive coupling. If a suitable shield is not available, wrap the signal generator lead around the mixer tube.
All of the circuits that are tuned to the IF frequency have now been aligned. At this point, the stages that were aligned previously can be touched up, with the signal generator connected to the mixer circuit. The purpose of this touching up or retuning procedure is to compensate for the slight change in frequency that often takes place because of interaction between stages during alignment.
FIG. 5. Signal voltage measurement in the plate circuit.
OSCILLATOR, MIXER INPUT AND RF ALIGNMENT
Aligning the RF amplifier stages, local oscillator and the mixer grid circuit is similar to aligning the IF circuits to track with the tuning dial. The RF trimmers are usually built into the main tuning capacitor ( FIG. 1). The other trimmers and padders are often small, adjust able mica capacitors, as shown in FIG. 7. (A padder is a trimmer capacitor in the oscillator circuit that is used for calibration at the low-frequency end of the tuning dial).
Set the receiver tuning dial to the highest frequency on the dial (on multiband receivers, the highest frequency of the band being aligned). Connect the signal generator to the antenna input and tune it to the same frequency as the receiver. Connect an output indicator and adjust the trimmer of each circuit for a maximum output indication as above.
Check the accuracy of the tracking. Set the signal generator to a frequency near the middle of the tuning range of the band under alignment. Tune the receiver to this frequency. If the signal from the generator produces a maximum output indication when the receiver is tuned to exactly the same frequency as the generator, then the receiver dial is tracking properly. If these results are not obtained, however, you will need to repeat the whole procedure. (Sometimes the adjustment of the padders will affect the alignment of the high-frequency end of the dial.)
Some receivers have adjustable inductors or capacitors only on the oscillator circuit for the low end of the dial. In these cases, it is still necessary to check the tracking of the dial at one or more intermediate places.
ALIGNING RECEIVERS WITH MORE THAN ONE TUNING RANGE
Receivers with more than one tuning range are aligned in the same way as one-band receivers. Each band is aligned beginning with the highest frequency.
The signal generator and receiver are set to the frequencies that are designated in the instructions. After one band is aligned, the next one is aligned in the same way, but the frequencies involved are different.
If there is not information on the location of the adjusting screws, they can be found by experimentation. Tune the receiver to a high frequency and set the signal generator to the same frequency. Note the band in use at the time. Turn the trimmers or other adjustment screws—one at a time—until the one that affects the output is found. Return all adjusting screws as closely as possible to their original positions. The fact that one affects the output means that the adjustment is in the circuit of the band in use. In some higher frequency units, the mere touching of a trimmer with an alignment tool can cause a change in the output. Because a given change in capacitance will be noticed more readily at higher frequencies, it is best to make the test at the high-frequency end of each band.
FIG. 7. Some trimmers take the form of small variable mica capacitors such
as this one.
FIG. 8. A view of the chassis of the Silvertone Model 7108, showing the unusual
ganged tuning inductor and foil-lined cardboard tube shield.
Trimmers in a multiband receiver are usually located in groups near the coils they tune. The trimmer associated with the band in use can be identified by the relative number of turns on the coil. Because the coil for the lowest frequency band will have the greatest number of turns, the coil in use can be identified by the number of its turns as compared with the turns on the other coils.
One of the wonderful things about antique radios is the diversity of them. It’s surprising the things you find in them. The Silvertone (Sears, Roebuck and Company) Model 7108, for example, had a three-stage ganged tuning inductor ( FIG. 8) instead of the usual tuning capacitor. Unusual circuit features such as this can complicate alignment procedures. Servicing information provided by the manufacturer or a publisher such as Gernsback, Rider, or Supreme is very helpful in such cases. This radio, by the way, had a No. 625 camera battery (small button type) in the circuit, though not in the schematic. Apparently it was added by a serviceman for some reason that now escapes me. This set also had cardboard tube shields ( FIG. 8). The inner surface of the shield was metal foil, which contacted the tube base when the shield was pushed down over the tube.
The Zenith Model 6G601M was an early portable. Like portables made by Zenith for quite a number of years, it had a detachable loop antenna called a Wavemagnet. Apparently the manufacturer didn’t want to leave too much to the radio buyer’s imagination at this point, because the antenna actually had a giant horseshoe magnet printed on it ( FIG. 9). This antenna didn’t have any special capability for pulling in stations—it was really just a loop antenna—but the printed magnet did suggest the way to point the antenna for best reception. Loop antennas in later Zenith models were without the printed magnet, but they were still called Wavemagnets.