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
----- Overrange indication is a non-flashing "OL" as
shown.
----- Diodes or translator junctions are tested by supplying-a current of 5mA and reading the dc voltage drop across the external diode.
Model TECH-310 from Beckman Instruments is a portable digital multimeter with several unusual features including 22M o dc input resistance, a special semiconductor test, a unique continuity indicator and effective overload protection.
Automatic dc polarity, automatic zero, automatically positioned decimal and an overrange that reads "OL" are additional features. The seven measurement functions have 29 ranges.
Digits of the Liquid Crystal Display (LCD) readout are almost 1/2-inch high. One large rotary switch selects all functions and ranges. It turns easily and indexes positively. Both the selector knob and the function/range markings are recessed. The DCA and ACA areas are darker tan to help separate the various functions.
Low-drain CMOS circuitry and the LCD display allow the 9V battery to supply about 2000 continuous hours with an alkaline battery or slightly less with a zinc-carbon type. When about 20 hours of use remain in the battery, the decimal blinks as a warning.
In a 3 1/2-digit display, the first digit can be blanked out, or it can show a number 1. No other numbers are possible. Therefore, this readout is limited to a maximum count of 1999 (1.999V or 199.9V, for example). If a higher count is called for, the circuit replaces the usual reading with an "OL" overrange symbol.
Accuracy specifications of the various functions are for 1-year operation. Undoubtedly the accuracies are better when the instrument is new.
Dc volts
Five decaded dc-voltage ranges start at 200mV (0.2V) full scale and end with a 2000V range that is marked 1500V because 1500V is the maximum recommended voltage. The decimal is moved automatically by the range switch, and the polarity indication changes automatically.
Accuracy of all dc-voltage ranges is ±0.25% of reading plus 1 digit.
Input impedance is rated at 22M-ohm, and an internal active filter is said to remove almost all extraneous ac signals above 49Hz. Model 310 gave accurate readings when measuring half-wave rectified dc, and this indicates proper integration of all waveforms.
Ac volts
The 200mV, 2V, 20V and 200V ac ranges are similar to those for do voltage. However, the highest range (actually 2000V full scale) should not be used to measure ac sine waves of more than 1000V RMS or 1400V peak. The limitation is necessary because peak voltages above 1500V exceed the safety ratings of the instrument.
Internally, the ac circuit is average-responding for sine waves, but is calibrated for RMS. Input impedance is rated at 2.2M-o . Frequency response was exceptionally good. (Many digital meters are not flat over the audio range.) TECH-310 measured within f1/3dB between 20Hz and 40kHz, with a small peak at 50kHz and rolled off rapidly above that frequency. This is better than the manufacturer's specifications.
Basic ac voltage accuracy is rated at ±0.75% of reading +3 least-significant digits.
Ac and dc current
Ranges for both dc and ac current are 20011A, 2mA, 20mA, 200mA, 2A and 10A. One test-lead jack is used for all except the l0A range, which has its own jack.
Rated accuracy for dc current except the l0A range is ±0:75% of reading +1 digit (l0A is ±1.5% of reading +1 digit). For ac current, the rating is X1.5% of reading +3 digits (l0A is ±2% of reading +3 digits). Polarity indication operates automatically for dc current. Frequency response of ac current is the same as for ac voltage.
Resistance Six resistance ranges measure from 200o full scale (resolution of 0.1-o ) to 20M full scale (10K 12 resolution) at an accuracy of ±0.5% of reading +l. digit (±1.5% of reading +1 digit for the 20M range). Maximum open-circuit voltage across the test leads was about 0.42V and the V s2 probe was positive. When measuring a resistance barely above the overrange point, the dc voltage was about 0.2V. This is not enough to cause silicon diodes or transistor junctions to conduct. Therefore, resistors usually can be checked in-circuit without large errors from solid-state conduction. A separate test is provided for diodes.
----------- Each safety-type test lead has a sleeve at the plug
to Insulate the metal contacts, plus a ridge on the test probe to prevent
fingers from touching the metal tip.
Insta-Ohms
One of the minor deficiencies of most digital resistance measurements is that rapid intermittent open or short circuits are not revealed properly because of the time necessary for the meter to count up or down. Beckman in the TECH-310 has solved the problem by adding an ohms symbol to the upper left corner of the display.
Any resistance reading between a dead short and about twice the value of the range that is in use causes the ohms symbol to appear long before the digital readout settles down to a stable reading.
When the probes are removed from a resistance (or when an intermittent open occurs), the ohms symbol disappears rapidly before the readout returns to zero.
On or off operation of this ohms symbol is rated at 100ms (0.1s), but it seems nearly instantaneous to the eye. For example, starting with the test probes not touching each other and the overrange signal showing on the readout, a series of rapid shorting together and separating the test probes causes the ohms symbol to flash on and off in step with the shorts and opens but without allowing the "OL" signal to disappear from the regular digital readout. Such fast responses can be helpful.
Diode test
Another important feature that resembles resistance measurements is the diode test. The switch position is next to the 200 s2 range, and it is identified by a diode symbol. A test current of 5mA is supplied to the diode under evaluation and the meter reads the voltage drop that's across the diode, up to a maximum of 1.999Vdc. This test can be very informative.
Germanium diodes and transistor junctions gave readings around 0.3V, ordinary silicon diode rectifiers showed about 0.6V and two LEDs each measured 1.59V. A damper diode out of an old tube-type color TV evidently had several diodes in series since over range was obtained. However, a silicon damper diode from a solid-state color receiver measured about 0.6V. Positive identification can be made of cathodes versus anodes or silicon versus germanium types.
LEDs not only show their characteristic voltages (not all require 1.6V) but dim light can be seen.
For reverse (or leakage) tests, use the regular resistance ranges.
Accuracy of the diode test is rated at ±0.25% of reading +2 digits.
Many diodes and transistor junctions can be tested sufficiently while in-circuit, providing the paralleling resistances are higher than 200-ohm . With one polarity of test leads, the reading should be the correct voltage drop for that kind of diode.
After the leads are reversed, the readout should show "OL". If both readings are near zero or overrange, the junction is shorted or open.
Comments
All voltage ranges are protected to 1500V peak by the circuit design, and spark gaps are provided for voltages between 1500V peak and 6kV peak. Resistance ranges are protected to 300Vdc or RMS ac, and the current inputs have a 2A fuse. But these added protections should be used only for accidents. Don't knowingly apply voltage in excess of ratings to any solid-state meter.
The Beckman one-year limited warranty is unusual. It agrees to repair or replace the ailing multi meter with a new or reconditioned one for $3 (postage and handling) and send it back by air, usually within one working day.
Performance of the Beckman TECH-310 was excellent in all ways. Features of Insta-Ohms indication and the diode test were found to be dependable and very helpful.
Another advantage not mentioned in the manufacturer's literature is that the digital multimeter has an internal metallic sheath that shields the circuits. Although the meter was not tested in an intense RF field, it should be stable under that condition. The performance was not affected in any way by bringing it almost in contact with the high-voltage section of a color TV receiver. This is a stringent test, and the lack of any problem indicates good shielding.
A similar meter, TECH-300, does not have the Insta-Ohms continuity indicator or the l0A range. Accuracy is slightly lower.
Model TECH-310 sells for $130 and TECH-300 is priced at $100.
In summary, the Beckman TECH-310 is a professional-quality digital multimeter that should be a valuable addition to any service operation.
Also see: Reports from the test lab (Oct, 1979)
Testing Sencore VA-48 performance