Turning on VTVM for the first time

Patrologia

Active Member
I picked up an old VTVM (RCA WV 97A, actually it does ohms too), and wonder if all the same cautions apply as with other old tube stuff. Dim bulb (I don't have a variac), best to change the old caps, at least the electrolytics, etc? I haven't even had a chance to open it up yet, but anything else I should be careful of before I start tinkering?
 
I picked up an old VTVM (RCA WV 97A, actually it does ohms too), and wonder if all the same cautions apply as with other old tube stuff. Dim bulb (I don't have a variac), best to change the old caps, at least the electrolytics, etc? I haven't even had a chance to open it up yet, but anything else I should be careful of before I start tinkering?
Turn it on !

If problems occur, fix it!
 
Replace the battery inside first. Its used for bias on the amp section and it will not work correctly without it. Hopefully it doesn't still have one in there from 50 years ago, otherwise your batt holder is sure to be rotted away by now.

I would also change the caps, there should only be a couple of them. Some contact cleaner on the switches and re-seating the tubes won't hurt either.
 
Thanks, will do. Sounds like they aren't quite as risky as old amps. I guess because there are just few caps in the first place.
 
yeah not much inside a vtvm. That one is probably late enough that it has a solid state power supply. If its like my WV-77e, I think you'll have 2 electrolytic caps in there.
 
As Gadget noted, replace all of the capacitors. Every one of them is bad at this point. As he point out, the bias battery has likely corroded, as well.

A VTVM is a simple circuit, going up to a dual or four bridge configuration in the better ones, so there's not a lot to go wrong. But some things will have gone wrong in the past fifty or sixty years.

The carbon-composite resistors have all drifted, and should be replaced with precision metal film. (Also note that CC resistors have bad properties like Vcr which creates inaccuracies as the applied voltage changes.) Remember, the settings form a giant voltage divider, so the more accurate that is the more accurate the readings will be. Even when new these components were not what we would call precision. If you calculate the divider network you may find that approximations were used because the necessary values were not available and resistors were saved by using a value that was "good enough". You can now fix this to be perfectly accurate by adding a few resistors here and there.

A germanium diode (lower forward drop) may be added across the meter to protect against over-voltage conditions.

Any rectifier diodes should be replaced. If a tube, likely a 6AL5 isused, it always pays to add a small fuse (tens of mA) to the B+ secondary in case the rectifier tube shorts as this will take out the transformer. This is admittedly a rare event when the rectifier tube is not pushed.

A dim bulb tester is useful because things can be miswired.
 
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