2019 June 4       Electronics      How-To


Today we're going to look at repairing an RCA vacuum-tube voltmeter, the model WV-87A "Master Voltohmyst". 

WARNING:  Repairing line-powered electronics can be dangerous.  A mistake with high voltage can be fatal.  Do not work on electronics if you lack the proper skills and knowledge.  Disclaimer.

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In This Article


Primary Side

High Voltage Secondary

Low-Voltage Secondary

Bad Caps

Power Up?



First thing to do is check the power cord. 

The cord has more breaks elsewhere.  If you assumed the unit to be working, you might plug this in without ever looking at the cord... while leaning on a metal radiator with your other hand.  That could be fatal. 

This needs a new cord for sure.  But what it needs right now is to cut this cord and leave it off until I fix everything else, so nobody plugs it in and gets fried.

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Primary Side

This transformer primary circuit should have no continuity whatsoever with the chassis. 

So it should test "open line" from there to the chassis, unless one of two things has happened.  One, the cord may be shorted because of deterioration (see above...).  Two, the transformer primary could be shorted. 

Line to neutral should not test OL, but it shouldn't be zero ohms either.

This showed 0 ohms on the Simpson, but that's because the cord was shorted.

The primary actually reads about 180 ohms.

I don't know how many ohms the transformer primary should be.  Initial reading was 240-something on a DMM;  tried a few times until it settled down around 180.

The ring tester shows high Q, so I think the primary is good.  (Work with vintage inductors or transformers?  Get one, either in a kit that you build yourself, or one that's already put together.)

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High-Voltage Secondary

This says "RED" and "RED/GRN" on the schematic.  It has more turns and thinner wire than the primary.  Multimeter shows about 479 ohms.

It gives a moderate to low "Q" test.  This may be OK for this.  HV secondaries with fine wire tend to have low "Q".

It's too early to test this with live voltage, but so far the transformer seems alright.

Get your WV-87A here

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Low-Voltage Secondary

The pilot lamp and tube filaments are in parallel with the low-voltage secondary winding.  This winding should produce 6.3 volts AC (RMS).

Multimeter shows it as a short circuit (less than 2 ohms).  This might be OK for such a low number of turns.

Ring tester shows very low Q.  This, again, might be alright.  Low inductance (few turns) can lower the Q factor substantially.

Rarely a tube filament could be shorted to high-voltage in the tube, which would burn out the tube and also probably open the fuse in the primary circuit.  It could also burn up the transformer windings.  I don't think that happened here, though. 

Get your WV-87A here

Table of Contents

Bad Caps

The molded paper caps look sort of like oversized resistors.  These are C-3 and C-6 on the schematic;  both are 0.047 microfarad, 400-volt. 

So I located C-3: 

It's definitely shorted.  Here's the ohms reading on a Simpson 260, the scale set to x10K:

By the way, the distal lead (from V1) to the chassis reads halfway between 1.3 and 1.4 Megohm on the Simpson 260.  I think here's why:

These add up to 1.374 Megohm

So anyway, in-circuit test verifies C3 is bad;  schematic shows nothing in parallel with either cap that should read low ohms.  Molded paper caps are often bad.  Sometimes a shorted cap can take out a vacuum tube with it, but replacement 6AL5 tubes are common.

Capacitor C1 is a big waxed-paper cap that's 0.1 uF and 1600 volts.  It's not shorted but I'll replace it.  Stuff like this may be OK for crystal radios or tone caps, but here I'd rather use a new, in-spec capacitor. 

C5 is supposed to be a 10 uF 250V.  On this unit, it's two 16 uF caps in parallel.  Capacitance tests OK, but again these may leak current.

Get your WV-87A here

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It's too early to test with line voltage, but when it is ready... don't put full line voltage to it all at once. 

This will require a variable autotransformer to bring the AC voltage up slowly.  This is pretty much a core piece of equipment for this sort of work, so get a good one.  This seller is the place to go for refurbished or rebuilt US-made ones. 

That initial slow power-up might take an hour or more to get from zero to 120 volts AC.  During that time, you'd check for anything going wrong, maybe even use an IR thermometer to look for overheating. 

Get your WV-87A here

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The WV87A has a nice, big panel meter and the components are fairly easy to get to.  If you buy yours on-line, make sure to ask the seller some detailed questions before purchasing.  Some of these units can need quite a bit of work before they're usable. 

This was Part 1 of the repair, probably;  some of the resistors could be out of spec, but that's for another session.  The main thing now was to identify shorted components so they can be replaced. 

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