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Testing a VTVM?
Testing a VTVM?
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Old 1st December 2020, 10:40 PM   #1
tgreese is offline tgreese  United States
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Location: Medford MA USA
Default Testing a VTVM?

I obtained a free Simpson AC VTVM a few years ago. As I recall, it was completely dead - no light or anything. Tube complement is 2 12AT7s and a 6C4. On examination, it seems its only problem was the selenium half-wave rectifier went open. I replaced that with a 1N4007. The 6C4 also does not light up, but I will sub another tube to check that. Possible the 6C4 does not show a lot of glow - the 12AT7s glow fine.

Assuming it works now, how do I test it? Then if it works, how to calibrate? There's no adjustment to speak of other than the meter adjust screw on the front of the meter.

Last edited by tgreese; 1st December 2020 at 10:44 PM.
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Old 1st December 2020, 10:51 PM   #2
multisync is offline multisync  Canada
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On my old heathkit vtvm there was a battery holder for ohms scale. On one of the low voltage DC scales there was a mark at 1.5? Vdc. You got a fresh 1.5 volt not alkaline but old style dry cell and measured the battery and adjusted the meter to match the battery. The zero pot may be for the ohms scale or to 0 the meter as the tubes drift. For the A.c. scale you measured the heater, hopefully at 6.3 volts A.C.. Back when everything was tubes close was usually good enough.
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Old 1st December 2020, 10:52 PM   #3
rayma is online now rayma  United States
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Which model do you have? There should be some calibration adjustments inside.
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Old 1st December 2020, 11:07 PM   #4
tgreese is offline tgreese  United States
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Model 715.