The AM Forum

Wondering if someone out there has experience with RCA WV-98C voltohmyst characteristics. I recently lucked on a used one that had a few problems I fixed (leaky caps), but after calibrating it I noticed that the ac cal was ok but after removing probes from AC voltage calibration source the meter would slowly, veeery slowly wander back to zero setting, taking like a full minute to reach close to zero. Is this normal for this instrument? Or do I need to do more troubleshooting.

Thanks for any inputs. You can email to ralfowler@att.net.

Update: Here is schematic of the wv-98c if that helps. And here’s a jpeg of the front with switches identified.

It would be helpful to answer your question if you provided a schematic

We still had some of those when I was back in school in the seventies and from what I remember they would zero if you shorted the probe and other then that they would always be bouncing around, something to do with the high impedance high frequency input. Other then maybe looking at IF voltages or oscillator grids cannot imagine what else they would be good for. Back then we used crapy LabVolt meters at our workstations and when we started working on real world items used a Simpson 260 for everything because all those old VTVM were so squirrely.
Try hanging a 100K resistor on the probe and see if that fixes it.

Yeah, don’t know why anyone would choose to use it except to get that
“Vintage happy feeling” (which I admit I like). I’ve tried shorting the probes but that doesn’t make a difference at all. Very strange.

Did you try a different 12AU7? Think that's like a balanced bridge deal, not a lot to it. If it were only unstable in AC would think it can be the dual diode but if you changed the capacitors there is not a lot more to it. don't know, maybe I don't understand you question. is it that its unstable with out a constant input or that it just is slow to react?

It works fine on resistance and voltage measurements. It’s just that when I measure ac voltages the reading is accurate, but when I disconnect the probe from the ac source, the meter doesn’t go back to zero until it slooooly settles back to zero after a minute or so. I would expect the meter to settle back to zero within 1-2 seconds.

BTW, I  did replace the 12au7 and 6al5 but no difference.

I remember the one in the TV repair shop that I was a shop rat at doing the same random drifting when operating with the probe just floating and not connected to a circuit or component.

think maybe that part of its vintage charm!

Was the meter power switch ON or OFF when meter pointer drifting occurred?
What was the Range set to when the meter pointer drifted down?
Does it happen in all Range switch positions?
Has C1 and C2 been replaced?

Meter was powered on. Range was probably 50 vims for cal. Happened on all the ranges I used for AC cal. I replaced C1 but not trimmer C2.

Sounds like my “problem”. But I can live with it.

Yeah, I have quite a few “Charming” vintage instruments :)

Changed both tubes with no difference. Seems like some cap is holding the rectified ac voltage with no shunt resistance to discharge it.

I’m so charmed  ;)

THANKS TO ALL WHO HAVE REPLIED. I REALLY APPRECIATE IT. I WILL PROBABLY DO A BIT MORE POKING AROUND (BUT NOT ENOUGH TO DESTROY IT LIKE IVE DONE BEFORE) AND REPORT IF I FIND THE PROBLEM. BUT NOW INSUSPECT ITS PARTNOF ITS “CHARM). 73’S TO ALL.

If you ask the members for help or guidance, it's always good to provide the correct information.

Capacitor C1 is .05 mfd at 600V
Capacitor C2 is .02 mfd at 450V
NOTE: Info from the RCA WV-98C manual, schematic and parts list.

Also remember that if you ask the members for help or guidance, it's not always good to do what they say! I think that all high impedance, high frequency voltmeters are flakey at best. Neat test, it’s not ac but what the hell. On a vacuum tube receiver connect the meter to the AVC bus, that’s a place where the old analog meters would load them down and provide false readings and try feeding the receiver with a signal generator and see what happens when you vary the output??? Also, this would be like the NHRA Nitro test but in theory should work although I have never tried it. Connect the meter probe to the RF side of the detector, because the meter is both high impedance and RF you should be able to see the RF voltage on the output of the IF amplifier and on the detector, you can go up the IF chain and see the RF voltage gain of each stage, Woo Hoo! Try it with the AVC turned OFF and then ON and see the AVC action! Some may say you can do the same thing with a scope but that won’t include any RCA Analog fun.
It’s all like using a General Radio Bridge, people today do impedance and capacitance with stupid little digital meters that tell you to a decimal point but the only true reading is from using a bridge and your brain, and maybe a signal generator.
Remember, time spent working on vintage equipment is never wasted, like time at work.

When servicing and/or aligning boatanchor  equipment, I generally tend to use the same or similar type of VOM or VTVM that would have used back in the manufacturer's day. I also tend to follow their direction for generator and meter connections into the equipment. I never use digital meters to measure current or voltage on old equipment. They generally don't have the same +/- tolerances of the analog VOM and VTVM meters.

I also never/ever do an ad hoc replacement of all capacitors in equipment as one of my first steps in servicing old equipment. I tend to find the initial problem first, fix that, and then review and check all other parts and replace if necessary. I discovered early on that old doesn't necessarily mean "bad and replace".

Dear friends: Color me embarrassed and with egg all over. The model I have is an “A” and not a “C”. So my premise was wrong and caused a lot of confusion. For that I apologize for my carelessness.

Very true.

Oh darn!! Now we have to start over.  :o

Pull this schematic below off the web. Didn't feel like going to my file room to pull my WV-98A info.
Looks like whoever scanned this schematic did it with a $19 scanner  :o I diddled with it briefly to try and make it more presentable but I'm only working with one eye at this time so it is what it is.;D

Does anyone have a scan of the WV-98C meter face? The red scales on mine are totally faded, no way to enhance them with a computer/scan.
...or any spare '98C meter faces out there?

Thanks.

Those old timey engineers at RCA were way smarter then I am, after all they invented the Smith chart, I know its embarrassing for me to question there wisdom but look at the size of C4 @ 0.047 Its freaking huge compared to the load of the voltage divider for the AC scale.  Assume they needed a certain minimum size for correct readings at low frequencies but cant see any way you would have fast action on AC readings with a big capacitor in that circuit. I would have though something like a 0.01 would do it but maybe they needed that big a capacitor for correct readings at 60 cycles? But that’s just my speculation, and as my wife will tell you I am wrong at least three times a day and its not even noon yet.

And your wife would be correct.

Phillip Smith, an amateur radio operator (call 1ANB) working at Bell Labs, devised a chart (that bears his name) that even today can help engineers understand the behavior of transmission lines.

https://www.allaboutcircuits.com/news/phillip-smith-from-amateur-radio-operator-to-creator-of-the-smith-chart/

The chart is named for Phillip Smith, an engineer at Bell Telephone Laboratories, who devised and refined it between 1936 and 1939 while working on understanding transmission lines and standing waves at what was then considered “high frequencies” of up to 1 MHz.

Smith also invented the Cloverleaf antenna which has found favor in amateur radio antenna designs over the tears.
https://www.antenna-theory.com/antennas/cloverleaf.php

And it was still in use in mid 70’s when I worked for. Hewlett-Packard plant in Santa Rosa in their network analyzer instrumentation group. Once you understand how it works, it’s not hard to implement matching networks.

Just like the fish said when it hit the concrete wall, Dam. Looks like I was wrong somehow have memories of old RCA Broadcast publications where they included the Smith chart every time. Several years ago when I was actively involved with our microwave engineering program at work we still thought the Smith chart and its use. It was fun watching P/E students working out solutions to complex impedance problems and having no idea what it meant.
A couple of our faculty were obsessed with efficiency and power transfer, so we did lots of transmission line theory and the Smith chart goes right to the heart of that. I use to do the labs for slotted line and resonance and always made me happy to see the kid’s responses with the slotted line and effects of open and shorted circuits.
I only proctored the microwave lab, never smart enough to teach, suppose that why I was wrong about the Smith chart.
Now what about the size of C4 and its affect on duration of AC readings?

Which reminds me how caps have matured over the past couple decades or so. One of the caps I replaced was C1 in my WV-98A. It was a 0.1mF at 1kv paper cap about the diameter of a broom handle and at least 3 inches long - a real brute. I replaced it with same spec axial tubular (Russian surplus as I remember) less than the diameter of my little finger (and I have small hands) and maybe 1 inch long. I did meggar check another of same and it withstood 1kv with no discernible leakage. We’ve come a long way.