Crawls Backward (When Alarmed)

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Servicing an RCA Model WV-98C Senior VoltOhmyst

There's a bit of a lull in the action in the Dungeon on the Vibro Champ project.  I ordered a new speaker from Weber and it takes a couple of weeks for them to make it and ship it.  I also got a new speaker for the Champ 600 while I was at it.  So the Vibro Champ is on temporary hiatus.

That doesn't mean there isn't anything to do.  I started this quick project servicing and recalibrating a RCA model WV-98C 'Senior VoltOhmyst' VTVM.  "VTVM," by the way, stands for Vacuum Tube Volt Meter. 

As it turns out, I actually have two of these things.  The one above is a recent acquistion.

I've had this other one for a few years.  It's not-quite-accurate and needs to be gone through.  Which generally means checking or replacing any out-of-tolerance precision resistors.  A fairly big undertaking.  When the opportunity arose to score a second one, I took it.

If you compare the two, you'll see the only differences are cosmetic.  The one I just got is actually older - early 1960s.  It has the old RCA 'meatball' logo and older font face.  The second one has the 'modern' RCA logo, and a different font on the control panel.  They both have different knobs as well. Other than that, they are electronically identical.

RCA probably made thousands of these things.  They're still a staple in the old-radio-and-amplifier repairing world.  They're reasonably accurate, and that large meter (about 5" or almost 13 centimeters) is easy to read. 

A VTVM is great for things like radio alignments, where it's easier to read the meter swing up or down versus a digital meter where the numbers are changing.  Not only are they functional, but they look really cool to boot.

You have to love a piece of gear that comes with a schematic and layout diagram.  Not to mention calibration instructions.

Here's what I'm doing first.  Replacing the power supply filter cap and the selenium diode rectifier.

The filter cap (the cylinder on the right) actually tested ok, but I replace these as a matter of course.  This one is about 50 years old, and it's not worth leaving it in and having it fail.

The rectifier is the square component with the 'fins' to the left of the capacitor.   You'll see these in radios and other gear made all over the world during the 1950s and 60s.  It's a diode that rectifies AC to DC.  They took the place of rectifier tubes in consumer gear - they were easier to manufacture, didn't require a special transformer tap to run them, and they don't produce heat.  However, 50 years later, they are prone to failure and are easily replaced with a three cent modern silicon diode.  Which is what I'm going to do here.

Here's the old selenium rectifier alongside its modern replacement.

In addition to the size difference, the modern diode offers far superior performance.

When replacing a selenium rectifier, you need to check the B+ voltage with the new diode in place.  Generally, there is much less voltage drop across the diode and you'll need a resistor in series with it to drop the voltage to the correct amount.

In this case, there was only a few volts' difference - in fact, the B+ is exactly spot on (88 volts) with the new diode.  What a bargain.

This VTVM was made using a circuit board instead of point-to-point wiring.  PC boards were just coming into use by the early 1960s.  So replacing the two components is just like you'd do on modern gear - unsolder the old, bend the leads on your new ones, and solder them in place.

Now I'm going to see how well the VTVM will calibrate.


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