Each report about an item of electronic test equipment is based on examination and operation of the device in the ELECTRONIC SERVICING laboratory. Personal observations about the performance, and details of new and useful features are spotlighted along with tips about using the equipment for best results.
By Carl Babcoke Model
DVM-56 is the first microprocessor-controlled digital multimeter manufactured by Sencore, and it has several very unusual features, including the following:
A choice of peak-to-peak, aver age, or true-RMS ac voltage;
direct-reading true-RMS decibels for the 1mW audio standard;
true-RMS decibels that are programmed for any selected level;
dc voltage to 10kV with a special probe and range;
high-power and low-power resistance tests to 99.99M-ohm;
basic accuracy of 0.075% ±5 digits for de voltage;
ac and dc current measurements;
a choice of display resolution;
automatic zeroing, auto decimal, and auto polarity;
display-area LEDs that indicate the prefix for all measurements (for example, mV, V or kV); and
the peak & null mode with LEDs that indicate an increasing reading, a decreasing reading, or a stable reading.
In addition to helping with the analog-to-digital conversion and other chores, the microprocessor provides these functions:
auto ranging of all functions, but with a range-hold button that can lock to any range;
a button to subtract test-lead resistance during ohms tests, and a way to substitute a different standard level during dBp tests that re quire another standard. These are activated by the ohms & dBp button; and
blanking the display until the reading has stabilized. This "thinking" is indicated by dashes that travel to the right across the read out.
Readout display DVM-56 has five 0.5-inch 7-segment red LED full digits (Figure 1) that are microprocessor controlled to produce a smooth transition be tween ranges during autoranging.
A choice of resolution is provided by three interlocked display push buttons (below the readout). For example, when the 4 1/2, 4, and 3 buttons are pressed in sequence during voltage tests of a 1.5V battery, the readout might display:
1.5539 1.554 1.55
Notice that the first three digits and the decimal did not move. In stead, the undesired digits were blanked. According to the button pressed, this readout could be called 4 1/2-digit, 3 1/2-digit or 2 1/2-digit.
Ranging versus decimal--The decimal is moved one position to the right each time the readout advances one digit beyond 9999. Also, autoranging occurs at one digit higher than 19999.
(Most autoranging DMMs move the decimal and change the range at the same point. The decimal moves between a 1.9999V reading and the next 2.000V reading.) The DVM-56 changes the decimal above 9.999V and changes the range above 19.999V. Then the sequence repeats at 99.99V, 199.99V and so on, as shown in these photographs of actual displays.
Figure 1--Microranger model DVM-56 by Sencore is microprocessor controlled,
has 11 functions including peak-to-peak ac voltage, range hold for the autoranging,
peak and null indications, automatic subtraction of test lead resistance and
a choice of visible resolution. The 4V2-digit LED readout has LEDs to indicate
the function and prefix in use.
Notice that the decimal did not move between 19.955 and 20.00.
When the 41/2-digit button is in operation and the 4-digit button is pressed, the fifth digit is blanked out only when the first digit is one.
For all others, no change occurs. If the 20.00V example is on the readout and the 4-digit button is pressed, there is no change. If the 3-digit button is pressed, the fourth digit is blanked out, as in the 20.0V example.
Autoranging to a lower range occurs at every 18000 downward count, regardless of the range or decimal position. Between 19999 upward and 18000 downward read outs is hysteresis that prevents false ranging.
One of the nine LEDs near the abbreviations (below the readout) lights to indicate the range and function that's in use. These are lighted by the microprocessor as part of the autoranging.
Function selector One rotary switch (Figure 2) selects any of the 11 functions.
Underneath the switch are four more pushbuttons.
A range-hold button locks the autoranging in the range that is in use when the button is pressed. The operating manual gives methods for finding the desired range. Readings are obtained faster without the slight delay of autoranging.
The next button is the peak & null. It activates an analog circuit (for fastest speed) that lights LEDs located next to the + and-arrows of the peak & null part of the display. A reading that is increasing lights the top or positive LED, and a decreasing reading lights the bottom or minus LED.
Unchanging readings either alternate between the two, or both are lighted.
Another button is marked ohms & dBp zero. These two functions are microprocessor controlled and will be explained in the ohms and dB sections. At the right is the power on-off button.
DC voltage The dcV-2kV position of the selector switch provides conventional measurement of dc voltages up to 1999.9V. Input impedance is 15M for all four ranges. The display with its mV, V and kV indications is direct reading in either hold or autoranging. The most-sensitive range reads from 000.1mV to 999.9mV.
Figure 2--One rotary switch and seven pushbuttons select all functions.
Measurements up to 9999V (nominal 10kV) can be made with the TP222 probe that slips over the regular probe, when the selector is turned to dcV-10kV position. The decimal is moved one digit to the right, so these measurements also are direct reading, while the input impedance becomes 150M-ohm. This slip-over probe is shown in Figure 3, along with a sheet of condensed operating instructions that can be slid out from below the meter.
Sencore advises that the TP222 probe should be used for all dcV measurements. It is true that the accuracy remains adequate for most uses, and the input resistance is higher with the probe. However, one voltage tested 019.2mV without the probe, or 0.038V with it. A battery tested 1.5706V without, or 1.581V with it. A 9V battery measured 10.077V without, or 10.075V with the probe. Therefore, the Test Lab recommendation is that the TP222 probe should be used for all dcV above about 10V, and that it be removed for lower voltages.
The TP222 probe is strongly recommended for constant use around horizontal-sweep and high-voltage circuits. No damage should occur to the DMV-56 if the TP222 probe is touched accidentally to the plate of a horizontal-output tube or to the damper-tube cathode.
Focus voltage measurements were made in a new RCA color TV. The voltage range was between 6.429kV and 8.893kV, with best focusing obtained at 8.370kV. This is excellent accuracy for such high volt ages. In fact, it is better than required.
Figure 3 A TP222 probe that slips over the regular test probe allows direct-reading
dc voltage measurements up to 9.999kV when the DCV-10KV PROBE function is
selected. A plastic card with condensed instructions for operating model DVM-56
can be slid out from beneath the DMM when needed.
AC voltage
Three types of acV measurements are provided. All have input impedances of 1M-O. Accuracy of the average mode (calibrated in RMS for sinewaves only) is 0.5% digits. For all waveforms, true-RMS accuracy is 0.5% 4 digits, and peak-to-peak accuracy is rated at 1% ±4 digits.
Frequency response is important for audio-gain tests, decibels, and peak-to-peak tests. Therefore, all three acV modes were tested for response.
When tested with sine waves against another meter having flat response, the peak-to-peak measurements were flat to above 100kHz with a minor peak at 18kHz. In actual tests, the PPV calibration was very accurate for all waveforms except horizontal-sweep waveshapes narrower than horizontal-sync pulses. Narrower fast-repetition pulses produce slightly low readings, which still are helpful for comparison purposes. This limitation was included deliberately to provide better DMM protection.
Response of the average-acV function was flat, except for a moderate high-frequency increase that reaches +1dB at 20kHz and +3dB at 100kHz.
True-RMS acV function measured less than 0.5dB variation over the 20Hz to 200kHz frequency span.
All three functions were tested on the 19.999VPP or 9.999V-RMS ranges. It is possible that higher ranges might show some slight loss of high frequencies. These performances are considered very good.
Decibels
Two types of decibel measurements are provided. Both use the true-RMS converter, so the response should be the same as previously mentioned for true-RMS ac voltage. Three automatically selected ranges cover from-43dB to +62.2dB, the input impedance is 1M-o, and the accuracy is rated at 1% ±.1dB for the 1 mW-in-600 ohm standard. The readout is in tenths of a decibel (+42.1dB, for example) for signals larger than 0.13V RMS.
Amplitudes below that point have only two digits.
Programmable decibels (dBp) is another feature made possible by the microprocessor. When a specific level is to be selected as reference, a signal of that voltage is used for input. Then the operator presses the ohms & dBp button until the readout shows 00.0. Any signals now supplied to the probes will be displayed as the difference between it and the standard. This can be helpful for many kinds of audio testing.
Ac and dc current
Current flow up to 1.9999A can be measured by the front-panel jacks. The optional CS223 shunt allows measurements up to 19.999A when it is added between the DMM and the test leads.
Accuracy for dc is 0.3% ±.4 digits or 1% digits for ac. Ac current frequency response is the same as true-RMS acV response.
Resistance measurements
A choice is offered of high-power or low-power measurements of resistances. Low-power measurements can be made between 00.01 n and 1.9999M-o, while high-power measurements cover the span between
000.1 ohm and 99.99M-ohm . Both have 3% ±4 digit accuracy. All ranges are selected automatically, unless locked by the range-hold button.
Most DMMs have the same ranges for high and low power. The DVM-56 high-power ohms function tests higher resistances than does the low-power ohms mode, which can measure lower resistances. Otherwise, they are equally recommended for testing pure resistances. However, both are needed for resistance tests in circuits containing transistors, diodes and ICs.
Low-power ohms should be selected when conduction through these solid-state junctions is not de sired (in-circuit, for example). High-power ohms should be the choice when continuity or comparative resistance of semi-conductor junctions is needed, either in-circuit or out-of-circuit. However, every ohmmeter range shows a different resistance for a solid-state junction. This is true of all brands and types. A forward-biased junction will show a high reading when a higher-resistance range is used. Unfortunately, these low-current measurements produce readings that exaggerate minor differences between individual diodes; therefore, such readings are of minor value. Consistent and dependable readings require junction currents of 1 mA or more.
Therefore, when measuring for ward-conduction resistances with a DVM-56, the lowest high-power ohms range should be used. Select high-power ohms, short together the test probes to obtain the lowest range, and press the range-hold button. Now measure the junction resistance. Silicon junctions usually read about 550 o (about 0.550V across the junction) and germanium junctions show between 200-ohm and 300-ohm. This test identifies silicon or germanium types and shows the approximate voltage drop for comparisons.
Overrange for any function produces a flashing 8888 display, but the automatic ranging usually eliminates over-ranging for all measurements except resistance tests.
One more feature is important for low-resistance tests: automatic subtraction of test-lead resistance.
Select either high-power or low-power ohms, short together the ends of the test leads, and then press the ohms & dBp button until the reading goes to 0000.0. Before subtraction with the sample DVM 56, the minimum resistance reading was about 10-ohm. This proves the need for correction; otherwise, the accuracy for low readings would be unacceptable.
After the minimum resistance has been subtracted from all readings, the action continues until ac power to the DMM is turned off.
Therefore, the subtraction routine should be performed each time before low-resistances are measured. The same precaution applies also to reprogramming the dBp function. It must be reset each time the standard is changed or the power turned off.
Comments
The Sencore DVM-56 digital multimeter performed flawlessly during these examinations. It is a large and highly accurate DMM with unusual features that make it appropriate for all lab and bench operations.
Price is $695, and many options are available.
Also see:
Preventing damage from transients
Locating AC leakage in industrial plants