I picked up this little Trio VTVM last week - it says 115 V 400 CPS ONLY on the back - I know this means it was designed for use on aircraft power systems running at 400hz instead of 50/60...so I did some searching and found a few posts where people mention that it might work at a lower voltage - I think 18v - but don't see how this could possibly work with a tube based device! I have also seen where it's possible to use an amplifier, step up transformer and signal generator to generate a 115v 400hz supply...a lot of work just to get this little VTVM working...

So I figured it would be best to ask here if there are any alternatives that might work? Also if anyone has a schematic - I have not had any luck finding anything.

The reason it says "400 CPS ONLY" is because the transformer does not have enough impedance to work on 60 Hz. It will likely burn up if you try, and given that it is a hermetically sealed component, trying is not recommended. While it may be possible to operate it at lower voltages on 60-Hz without overheating it, reducing the primary voltage will also reduce the secondary voltages and the VTVM will not work as expected.

Best bet would be to replace the transformer with a 60-Hz one giving the same secondary voltages. It may be necessary to mount the transformer externally. You might also have to increase the sizes of the filter capacitors in the power supply. But before doing all that, I'd recommend testing the meter movement to make sure it is still good. Some of those hermetically sealed meters did not fare well over the years and if the meter is bad, it's not worth doing anything else.

Found a little background info on Trio: https://helfandretreats.com/memoirs/trio-laboratories/
And one of their products in Electronic Design, April 1954, page 16
-Chuck

Besides replacing the power transformer and filter capacitors to get it to work on 60 Hz, you also need to replace the chopper with a 60 Hz version. They are resonant and don't work well on the wrong frequency. You may also have to increase the value of the capacitors in the demodulator circuit.

The signal generator and amplifier solution is the best, in my opinion. I did that very thing when at a company that needed to test their design on a variety of power line voltages and frequencies. I used a broken Kenwood 300 W amplifier (one channel out so it was 150 W) and any audio oscillator, along with a 60 Hz transformer from the distributor catalog. Not so compact or efficient but it did the job for almost no money.

Finally getting around to finishing up some projects - had a few too many going on at the same time for a while. First - I wanted to thank everyone for the information! Also - K7MCG - really liked the link with the bit of Trio history - always nice to read about stuff like that!

When I bought this I was aware I may not be able to make it work but saw value in the enclosure/meter and other parts...considering I didn't pay much for it - but before I decide to do that I pulled the meter out and tested it and it is working fine, says ES100uA on it and has a 100uA swing each way. I also removed the transformer so I could read the side as it was too difficult to read in place.

I have an Eico 1030 HV power supply with 400v DC and dual 6.3v outputs - would it be possible to run this off of that at least to test? Since I don't have the actual schematic for it - how can I tell if it has a chopper circuit in it? I did a little research on choppers and can't find a lot of info but I thought they use lights?

The HP choppers used incandescent lights with a motor-driven slotted wheel to generate light pulses to CdS photocells, or else a neon bulb multivibrator illuminating the CdS photocells.

Most other manufacturers used a mechanical chopper. You can easily see one in your Trio VTVM. I've marked it on your photo.


On your photo of the bottom of the chassis, you can see the two semiconductor diodes that rectify the amplified chopped voltage for the meter, near the top center of the photo.

Ah - that's what that weird tube was - ok, so it's not worth trying to use - not a big loss though as I have several ideas for the enclosure already.

thanks!

Well, a chopper DC meter is very sensitive and quite stable with low offset, so it could be a good VTVM. But the chopper is probably designed to be driven by 400 Hz, so I think you are out of luck unless you try the oscillator driving a hifi amp driving a filament transformer to generate 115 VAC at400 Hz.

Or get a 60 Hz chopper.

So I guess I shouldn't have marked this one as solved - you guys have me thinking about trying the signal gen>amp>transformer thing - but does anyone have a link for an example of this that actually shows how things are connected and not just showing it working. I understand the concept and have a pretty good idea of how to do it, but I have never seen anyone document it and don't really want to wing it...

And where would I find a 60hz chopper...?

Generating 400 hz power as described above is pretty straightforward. You are just using a 400hz sine wave generator as the audio source to any medium power hifi amp or receiver. What comes out is 400 hz at whatever wattage you need within the limits of the amp. You can even listen to it if you want . Interestingly enough you can control your "generator" output with either the amp's volume control, or the signal generator's output level (if you don't clip the input to the amp). Exciting stuff huh.

Personally I’d sit that cute little meter on a shelf as a display piece, and get a usable VOM.

Yeah - I already picked up an RCA WV-98B that's working great.

How about a WTB in classifieds? Or one of the auction sites? Choppers were common in industrial equipment so there are probably still plenty around. I haven't looked, but there may be some company still making them (but that would be expensive).

You need to specify the dimensions (so it will fit in your instrument; choppers came in a wide variety of sizes). Also needed are the base type (7 pin or 9 pin; the one you have doesn't look like an octal), the pin connections, the coil voltage, and frequency.

They do wear out (or someone may have misused it) so if you can find a cheap source, it would be a good idea to get several.

If you need to measure low DC voltages, an RCA WV-98B doesn't come close.