Testing a VTVM?

[JMFahey]

Originally Posted by PRR

Being an AC VTVM, the trick of calibrating against a fresh flashlight cell is not useful (no AC in a battery).

Well, yes and no.
A classic VTVM is *always* a DC measuring instrument, just check classic schematics.
The trick they use to measure AC is thanks to a probe, which does the rectification and conversion.

They generally use 2 kinds:

* the "RF probe" type.
It uses a series cap which must also stand any DC voltage present so typically 600V rated, diode (usually Germanium) which generates a positive voltage and a high value series resistor (usually 4M7 or so) which in series with 10M input impedance scales doen the peak rectified voltage to RMS equivalent.


*the "Peak to Peak" probe
It uses a full wave rectifier-doubler.
Sometimes a vacuum tube double diode is used, think 6H6, 6AL5 and similar.
Meter panel has a Peak to Peak scale, and old TV schematics included waveforms and their PP values.

So in a way, "DC testing witha battery" is still valid, as long as your AC probe is not damaged.

Calibration is basically comparison to a "known-good" AC meter on the same source. If you were a Calibration Laboratory you would have a known-size AC source; but clearly you are not there yet.

yes, that is the universal way, works every time.

[PRR]

Originally Posted by multisync

...I saw VTVM and presumed it was similar to the VTVM multimeter I had.

Me too.

Originally Posted by JMFahey

Well, yes and no.
A classic VTVM is *always* a DC measuring instrument, just check classic schematics.
The trick they use to measure AC is thanks to a probe, which does the rectification and conversion.....

Did you look at the documentation?? This is an AC-ONLY amplifier meter. It will read to 0.03V FS which is well beyond an "RF probe" on a DC meter. It is a cheap version of the Ballantine 300 or the H-P AC-only meters. (Or multisync's Stark tube AC millivoltmeter.)