have to fix some Voltohmyst VTVMs - Videokarma.org TV - Video

DavGoodlin · #1 05-23-2016, 03:48 PM

The GO-TO meter to use on tube equipment, to measure the voltages found on our schematics, is a Vacuum Tube Voltmeter or VTVM.

Famed publisher John F. Rider developed a meter with Co-worker Jack Avins using a bridge circuit, calling it the Voltohmyst, then selling his business and a few patents to RCA in mid-1939.

Recently, a friend asked me if I had one for him, so I handed over a RCA-made, Viz-branded WV-98C with its manual, but minus the switchable probe. It needed a bit of work as do several others I have accumulated recently. So I thought a thread would be appropriate. I have four Heathkits, so maybe a separate thread is needed since I saw neither HK or RCAs, only some on Hickok VTVMs, in this forum since 2010.

I found a WV-97 in a pile of other junk that showed up, and the needle on the meter was stuck. I figured it was done for, but I took the meter out and I was able to free it up after backing off and re-tightening the front and back pivot nuts. Checking the movement coil on the 20K range of my Craftsman DMM, it supplied enough current to push the needle to 40% of full scale. Now to replace the battery with a 10 yr. lithium and the selenium rectifier, electrolytic and paper caps and see if I can calibrate it.

The second VTVM is a WV-77E I just picked up for $10 at Kutztown swap last week. It is in much nicer condition and a bit different. I also have a manual for it, which covers troubleshooting.

One thing I noticed without getting in deeper is that the WV-97 has two 6AL5 tubes and the WV-77 has one 6AL5.

How about your experiences with these or similar VTVM's?

davet753 · #2 05-23-2016, 08:51 PM

I've had several of the RCA Voltohymst meters, and have not had what you would call good luck with them. I just finished restoring a Heathkit IM-18, and I'm not that thrilled with it, either. The Heathkit seems to be a bit "jumpy" on the lower voltage scales.

I had an Eico one time that had a huge face, and was a very nice looking meter, but the accuracy was terrible.

My favorite is the B&K 177. I've had several of these, and found them to be the most dependable. Most VTVM's require a battery for the ohm function, but the B&K meters use an internal power supply to eliminate having to mess with changing batteries.

DavGoodlin · #3 05-25-2016, 02:37 PM

After getting used to putting my Fluke 177 on everything, I started to wonder why most voltages I was measuring were too high compared to the schematics for the tube sets.

The WV77e responded well to a new top-hat diode replacing the selenium and new B+ filter. Four .18 uf in series replace the .047 The 12AU7 was also gassed, likely the reason this VTVM was cast aside.
Attachment 191180

The older WV97a is on the left, has the second 6AL5 for a B+ rectifier and AC calibrate. It needs a .1uf @ 1.8kV, so three .33 uf @ 630 v in series make this value.

Those leaky batteries are pretty hard on these meters. It may be worth adding an internal bleeder supply using a resistor and 1.55 volt zener diode.

Both of these need a set of lead-probes. The Heathkits are really beat up and need them too.

***Kamakiri*** · #4 06-01-2016, 04:40 AM

Since we're talking VTVMs, I picked up this sexy lookin Jackson 709 Tele-Volter at a hamfest last weekend for $20. I was instantly attracted to it because of the big angled meter.

I've decided recently that I really have to get myself used to using analog meters as much as I can, and (thanks to Dave) have an RCA Senior VoltOhmyst to match my Master VoltOhmyst I got last month.

Anyway, back to the Jackson. I recapped it (which was a bit tricky with the layout inside), and adjusted it. It reads the ohm scale dead on, but wouldn't read DC. I read the manual, and it said to measure DC, switch the probe to DCV. Unfortunately the test leads were cut on the very end, so I just soldered on leads and alligator clips.

What sort of probe would the meter use? Something I can make up?

DavGoodlin · #5 06-15-2016, 02:53 PM

Quote:

Originally Posted by Kamakiri

Since we're talking VTVMs, I picked up this sexy lookin Jackson 709 Tele-Volter at a hamfest last weekend for $20. I was instantly attracted to it because of the big angled meter.

I've decided recently that I really have to get myself used to using analog meters as much as I can, and (thanks to Dave) have an RCA Senior VoltOhmyst to match my Master VoltOhmyst I got last month.

Anyway, back to the Jackson. I recapped it (which was a bit tricky with the layout inside), and adjusted it. It reads the ohm scale dead on, but wouldn't read DC. I read the manual, and it said to measure DC, switch the probe to DCV. Unfortunately the test leads were cut on the very end, so I just soldered on leads and alligator clips.

What sort of probe would the meter use? Something I can make up?

On almost all VTVM's I have seen, the DC switch on the test probe adds a 1 meg resistor in series. This is supposed to isolate any RF from capacitance of the shielded test lead.

DavGoodlin · #6 06-15-2016, 03:17 PM

Quote:

Originally Posted by DavGoodlin

After getting used to putting my Fluke 177 on everything, I started to wonder why most voltages I was measuring were too high compared to the schematics for the tube sets.

The WV77e responded well to a new top-hat diode replacing the selenium and new B+ filter. Four .18 uf in series replace the .047 The 12AU7 was also gassed, likely the reason this VTVM was cast aside.
Attachment 191180

The older WV97a is on the left, has the second 6AL5 for a B+ rectifier and AC calibrate. It needs a .1uf @ 1.8kV, so three .33 uf @ 630 v in series make this value.

Those leaky batteries are pretty hard on these meters. It may be worth adding an internal bleeder supply using a resistor and 1.55 volt zener diode.

Both of these need a set of lead-probes. The Heathkits are really beat up and need them too.

The WV-77 meter only needed a 22K dropping resistor in series with the rectifier diode and it calibrated as per the manual with good accuracy when compared to the Fluke.
The WV-77 was in production for many years and the WV-97 actually pre-dated it. The Senior Voltohmyst WV-98 has the larger meter and was more expensive.

The RCA WV-97 has some issues with DC measurement, due to several drifted resistors as it is over 60 years old. After repairing the meter initially, I decided it was not worth further troubleshooting after making this discovery. Anyone need some parts?