98 What about VTVM's?...
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
98 What about VTVM's?...
...I notice that you can get a "service grade" RCA
VoltOhmyst or a "laboratory grade" HP or Ballantine unit. Are the HP
and Ballantine meters really superior to the VoltOhmyst.
I have WV97A and WV98C VoltOhmysts, and HP400D AC and 412A DC
VTVM's. The meters that get the most use in my work are the WV98C and
the HP412A. I like the WV98C because it has both AC and DC capability,
and a reasonably good ohmmeter. It is also much more responsive to
signal changes, and is much easier to use for a lot of service tasks.
The 412A is extremely accurate across the scale, much better than the
WV98C, and has a maximum full-scale sensitivity of 100 microvolts,
compared to the WV98C's 500 millivolts. It also has a much wider ohms
range, and is more accurate there, too. It won't measure AC voltages,
and the design cuts off AC response at only a few Hz, so it is slow to
respond to changes. In short, the tradeoffs are not only construction
quality, but in flexibility. The WV98C required some component
replacement when I got it, but it is a much better setup for general
service work. The HP meter needed a light bulb replaced in the chopper,
but was essentially "plug 'n play" and didn't need recalibration.
The HP400D is an "AC meter," and the WV98C has "AC" voltmeter functions.
However, the 400D is a true RMS meter, while the WV98C is peak-reading.
That is a major difference. If you are looking at the calibrator output
of a Tek scope, which is a square wave, the WV98C reads the peak value,
and you have to read that value on the "P-P" scale. The 400D reads the
equivalent DC energy value of the square wave, information that is not
particularly useful if you want to see if the calibrator is anywhere
near accurate. RMS measurements have plenty of value when looking at
something like a class C amplifier. The 400D was also something of a
bear to get working properly, because it had been "fixed" by someone who
did not know the principles of operation, and who "repaired" all sorts
of things except what was actually wrong with it. In short, all three
meters have their place on my bench, and in several applications, one
will not substitute for another very well.
One point that should not be overlooked is that the sophisticated
circuitry of a laboratory grade instrument can be a nightmare to trouble
shoot and repair. The WV98C does not have a chopper with lightbulbs, a
clock motor, and optical switches, which had problems in the HP412A, and
has only a rudimentary power supply, so does not have the regulator
problems that were the actual fault found in the HP400D.
You can get a great deal of good use out of a service grade VTVM over
and above what you can get from a passive VOM. Input impedance is much
higher, so you can accurately measure things that the VOM can't see.
You can invert the DC sensing and measure negative voltages directly,
and the ohmmeter function has a lot more capability than is available
with a VOM. While one quickly gets used to the backward-reading
ohmmeter scale on a VOM, I still like the VTVM's forward-reading
ohmmeter.
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