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May 16, 2012

RCA WV-87A Master VoltOhmyst, initial inspection

I’ve had a nagging desire for an analog voltmeter for several years now.  My small collection of digital multimeters has served me well, but the moving needle of old-school VTVMs has a certain romantic appeal.  Additionally, there are a few situations when seeing a moving needle is more useful then getting an accurate absolute value.  So I bought this thing on eBay for $19.99:

RCA WV-87A Master VoltOhmyst front view

Lots of companies made vacuum tube voltmeters (VTVMs) back in the day, but the RCA Master VoltOhmyst has the biggest panel meter I have ever seen — with one notable exception.  RCA had a whole family of multifunction VoltOhmyst meters — the Standard (non-specified), the Advanced, the Junior, the Senior, and the Master.  I don’t believe that every model was sold at the same time, but during the years in which it was manufactured, the Master VoltOhmyst was the top-end model.  The Master started out, probably in the late ’40s, as the WV-95A.  It was a 5-tube setup with the ability to measure capacitance.  That was replaced in 1951 by the WV-87A, which appears to be a complete redesign.  Functionally, the voltage and current ranges were increased, capacitance measurement dropped, and the front panel was redesigned for simpler operation and to allow for the big panel meter.  Inside, 5 tubes were reduced to 3, and 17 capacitors were reduced to just 7.  Somewhere around 1956, RCA introduced the WV-87B, which brought a few minor improvements.  Richard McWhorter (AllAmericanFiveRadio) showcases and repairs a WV-87B in a series of videos on his YouTube page.  I also discovered a manual, dated 1973, for the WV-510A, which is a transistorized Master VoltOhmyst.  Although I cannot say for certain that 1973 is when that subsequent model was introduced, I do suspect that the tube-based Master VoltOhmyst was discontinued before the transistor-based model was introduced, with the Senior model becoming the flagship in the interim period.

Enough chat; here’s some photos.  Click on the pictures for larger versions:

A closeup of the controls.  Note the unusual probe connector in the lower-right corner.  This connector was more common in the 1950′s, and was used in several brands of test equipment (Heathkit, Triplett) and microphone cables.  Nowadays it is best known as a “Switchcraft” connector, probably because Switchcraft is the only company still producing them.  Expect further postings on this topic.

The top surface.  The handle is broken, and the electrical cord has lost all pliability.  Any attempt to bend the cord produces another break in the insulation.  Clearly, something that will need to be replaced.

RCA WV-87A Master VoltOhmyst top view

The back side.  Serial number 573?

RCA WV-87A Master VoltOhmyst rear view

The bottom.  No rubber feet, just stamped metal.

RCA WV-87A Master VoltOhmyst bottom view

I removed the outer housing, finding this sticker inside:

RCA WV-87A Master VoltOhmyst inside cover

The chassis revealed:

RCA WV-87A Master VoltOhmyst chassis

Another view:

RCA WV-87A Master VoltOhmyst chassis view #2

And another view.  The battery and Sylvania-branded tubes are both indicators that the unit had been opened previously and had some measure of maintenance.  However the amount of corrosion on the battery would seem to indicate that it had not been used in probably 10-20 years.:

RCA WV-87A Master VoltOhmyst chassis view #3

Another view:

RCA WV-87A Master VoltOhmyst view #4

Yet another view.  In the foreground you see two towers with circular wafers.  These are ganged rotary switches which do most of the controlling.  Switching functions and ranges requires switching in and out a lot of resistors.  Most of the components in this photo — the green ones, the peach ones, and the striped ones — are all resistors:

RCA WV-87A Master VoltOhmyst view #5

A reverse-angle closeup of the larger gang.  Most of those resistors are non-standard values.  Additionally, many of them are very high (2+ Meg ohms) resistance, and should they be out-of-tolerance, replacing them will not be straightforward.  I will need to find some combination of standard sizes which will give me the required value.

RCA WV-87A Master VoltOhmyst circuitry closeup

A closeup of the back side of the control panel.  The ohms adjust and zero adjust functions are really just half-watt potentiometers, the same as what you might find in electronic equipment made 30 years later.

RCA WV-87A Master VoltOhmyst circuitry closeup #2

More resistors.  So many oddball values.  201K?  90R?  And worst of all, 18.9 Mega-ohms?  Cripes, RCA, what were you thinking?

RCA WV-87A Master VoltOhmyst circuitry closeup #3

Moving on, here’s a few photos of the included (although not original) probe.  There used to be a switch on the probe to select between AC and DC measurements (which switched out an internal resistor).  The switch is gone now, and I do not yet know if the internal resistor is in place.

The included probe is made by Triplett.  It has a funny musky smell which I find unpleasant.

That’s it for the initial set of photos.  From here, I’ll do a few basic diagnostics to determine whether this old unit is fixable, and post the results.

Since this restoration project, and not an assembly project like my amplifier was, there won’t be as many step-by-step photos.  Rather, I intend primarily to use this blog to keep some technical notes on the restoration, in the hopes of saving some time for anyone else inclined to fix up a WV-87.

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