RCA WV-97A Senior VoltOhmyst code 850 serial 8799 restoration

I promised ARFer John Stouffer a VTVM from my stash. I had also promised ARFer Hylaphone that I'd find good homes for a batch of three RCA WV-97A Senior VoltOhmysts that he had given to me, so one will be on its way to John. But first I have to restore it.

IMG 0923.jpg

These VTVMs are very interesting. They apparently came from some service shop, and had been repaired and modified. On two of them, including John’s, the RCA meatball logo had been neatly punched out and replaced by a pilot light.

IMG 0921.jpg

All three had Lectrocell battery eliminators installed in them. 

IMG 0924.jpg

I’ll write a separate post about the Lectrocell.

IMG 0925.jpg

The original banana jacks were replaced by binding posts.

IMG 5720.jpg

You have to remove range and function switches from the front panel to gain access to the components.

IMG 5721.jpg

I took lots of photos so I can get everything back together in the right place. 

IMG 5722.jpg

I have to admit to feeling some trepidation, though, every time I start dismantling a piece of gear.

IMG 5723.jpg

Will I be able to get all those wires back in the right place?

IMG 5724.jpg

Or will I spend hours tracing the schematic, trying to find a mis-wire?

IMG 5726.jpg

The pilot light has to be disconnected and removed from the front.

IMG 5729.jpg

Next step is to remove the Zero Adjust and Ohms Adjust knobs and the control nuts.

IMG 5731.jpg

At first I thought the chassis was spot-welded to the front panel ...

IMG 5732.jpg

… because the corrosion from a leaky battery had glued them together.

IMG 7128.jpg

But a little persuasion got the chassis free.

IMG 7129.jpg

The holes for the Ohms and Gnd connectors have been enlarged to accommodate the binding posts, so there’s no going back to the original banana jacks.

IMG 7131.jpg

Yes, it is indeed a Type WV-97A.

IMG 7157.jpg

I cleaned the corrosion off the back of the front panel and the front of the chassis using a wire wheel in a hand-held drill.

IMG 7158.jpg

John’s unit is from run 850, a fairly modern version. Note that the precision resistors are mounted on the range switch. The chassis for a run 350 version is visible (but out of focus) in the background to the upper right. 

IMG 7134.jpg

Note that in the run 350 variant, the precision resistors are mounted to a tag board attached to the chassis, rather than mounted on the range switch.

IMG 7159.jpg

The later unit also has teflon-insulated hookup wire, a very nice touch. Teflon is quite resistant to melting when being soldered. It’s also pretty pricey, so I was surprised to see RCA used it in a service-grade instrument.

IMG 7160.jpg

Even with the front panel removed, there wasn’t enough slack in the wires between the switches and the chassis to work, 

IMG 7161.jpg

so I started disconnecting the range and function switches from the chassis. I had to work up my nerve a bit before taking this step. Again, I took lots of photos to help get everything back together correctly. 

IMG 7163.jpg

Here’s a view of the chassis, minus the range and function switches. I don’t remember if I tested the mica caps. I’ll have to retest them when I get back home (we’re on vacation as I write this). I'll replace the paper capacitors as a matter of course.

IMG 7164.jpg

The 47KΩ 1/2 watt resistors have been replaced with 1 watt carbon composition resistors. The replacements are still in spec.

IMG 7165.jpg

Most of the rest of the resistors need to be replaced, so again I took lots of photos to help get the replacements in the right place. 

IMG 7166.jpg

I’m wishing I had some good wiring diagrams for the WV-97A.

IMG 7179.jpg

I’m replacing the Lectrocell with a modern battery eliminator based on an LM317, so I’ve got to remove the old Lectrocell wiring.

IMG 7180.jpg

The red and black wires terminate here.

IMG 7182.jpg

The yellow wire terminates on pin 9 of V3, the 12AU7. That’s the high side of the filament winding.

IMG 7170.jpg

I replaced the electrolytic with a 10 µF 450 volt capacitor. I didn’t have any 10 µF caps in stock with lower working voltages. I didn’t increase the size of the filter capacitor because the WV-97A uses a 6AL5 as the rectifier, and I didn’t want to overstress it. 

IMG 7307.jpg

I used a pre-fabbed LM317 regulator board I bought on eBay.

IMG 7308.jpg

I modify them by removing the input and output terminal strips, soldering in an input filter capacitor and diode, and putting a 51 ohm resistor across the trim-pot. I mounted this one with double-sided tape, because I got the wire just a bit to short to mount it using the old mounting hole (:oops:).

IMG 7291.jpg

I had to break out the Ungar iron with the #4039 tip to solder the chassis ground connection.

IMG 7292.jpg

I finally broke down and bought a PC board holder for my Panavise. It turns out to work well for holding the WV-97A chassis.

IMG 7301.jpg

Here I’ve replaced the bad resistors on the chassis. I’ve settled on using Vishay/Dale CCF60 series resistors, when I can get them in the values I need. I buy them from Mouser.

IMG 7302.jpg

The area around V3 was pretty crowded.

IMG 7303.jpg

V1 and V2 were roomier.

IMG 7304.jpg

Now comes the big challenge - the range switch. 

RCA WV-97A code 850 serial 8799

Most of the precision deposited carbon resistors are out of tolerance. The folklore on ARF says these resistors rarely drift out of tolerance, but not so in this case. My HP 410B had similar problems, though not as bad.

IMG 7306.jpg

Holding the shaft of the switch in a Panavise is handy for repositioning the switch as you work.

IMG 7311.jpg

Here I’ve replaced the 7MΩ with a 6.98MΩ resistor and the 2MΩ with a 2.00MΩ resistor.

IMG 7312.jpg

The 90 ohm resistor was close to spec, so I left it in rather than replace it with a 90.9Ω from my stock. This 90Ω resistor had clearly been replaced, since it’s a different style than the other precision resistors and the leads on the replacement weren't wrapped around the terminal, unlike the others.

IMG 7313.jpg

Working on the second deck of the range switch.

IMG 7314.jpg

The 20KΩ resistor was still in spec, so I left it in place.

IMG 7316.jpg

I don’t have 1.8 MΩ resistors in stock, and somehow missed ordering them when I ordered stock for this restoration. I’ll have to put in another Mouser order.

IMG 7317.jpg

Almost done. The 9KΩ resistor is also still within tolerance.

After I get back home from vacation, I’ll get to tackle the hardest part - putting everything back together!

© Steve Byan 2011-2016