Install a Battery Eliminator in a vintage VTVM

If you are tired of cleaning up corrosion and replacing your 1.5 Volt battery in your Vintage VTVM, this is a simple fix that will not only save you the hassle, it will prolong the life of your instrument and require less calibration! I will show you how to install a inexpensive mini 1.5V power supply in your VTVM. I will also walk through fabricating test leads and calibrating an RCA WV-77E VoltOhmyst Vacuum Tube Voltmeter (VTVM)

[@cwradio4571]

You could have gotten that for free, an built it from scrap parts laying around. Robert Sumption worked for Heathkit an has put the schematic an parts layout they used. There are other ones you could have found online. I hate that you paid anything for that kit. I am one that does my own an I would have liked to have seen you build it. Just my opinion.

[@antiqueradioarcheology-wil8878]

I agree, and I've built a lot of Heathkits in my day, including a computer monitor and an oscilloscope among many other kits. But what attracted me to this pre-built kit was the board, which was already laid out and the size was perfect for the replacement. I wanted this to be an example of how to do it the easy way for people who probably wouldn't do it because it would be too difficult.

[@mikehuedepohl7927]

@@antiqueradioarcheology-wil8878 I agree you do some great projects on your page I am restoring this meter and just finished the heathkit V7. keep up the video's you are very good I am fallowing you and hope to see more gear soon

[@youtuuba]

I have also been using this same eBay seller's VTVM battery eliminator circuit board package. Originally, I developed my own discrete parts version (no LM-317) that works equally well, and it turns out it is comparable to the design Heathkit engineers came up with earlier. But I still prefer the LM-317 version these days.
However, that little LM-317 kit from eBay is not really optimal:
- First, the eBay seller does not make the circuit board-he buys it from some Chines supplier, and it is a general purpose linear voltage regulator. The seller just adds the rectifier diode and the input electrolytic capacitor and the three wires.
- Second, the Chinese kit is designed to regulate over a variable range that is quite wide, up to 40V or something like that. The values for the two resistors are appropriate for such a wide range of adjustment, but actually pretty poor for a dedicated 1.5V regulator. The small potentiometer ends up being very touchy if you try to finely tweak the output voltage. Different resistor values, and ideally the addition of a third resistor, would make for a more optimal circuit and better low voltage adjustment.
- Third, there is no secret at all on the circuit or the resistor values. They are essentially right off the LM-317 datasheet.
- The circuit board has parts on it which do nothing of value in this particular application.
- The circuit board is missing a diode which should really be present to protect the LM-317 from damage if the assembly gets accidentally connected incorrectly.
- If somebody wanted to make one of these, and used only the parts that are needed for this particular application, they would certainly be obtainable for significantly less money than the circuit board costs on eBay. But there is still the convenience of having them on a circuit board and having the extra parts already wired up (which is why I still buy this battery eliminator from the eBay seller).
Really, the biggest issue with installing one of these in a VTVM is making sure that the ground and input wires are connected to the correct sides of the filament transformer secondary. Getting it wrong will still result in the battery eliminator producing 1.5V, but leaving the Ohms function not working correctly. It is hard to figure out the correct winding connections because everything in that part of the circuit is very low resistance components in parallel. It comes down to tracing the meter circuit ground to one side of the transformer secondary, and making sure that the battery eliminator ground wire goes to the same point in the filament circuit.
And boy, that RCA VTVM seems to be a very unruly design, looks like a 'dog's breakfast' compared to the typical Heathkit VTVM design.