Battery eliminator for Heathkit V-7A
All vacuum tube voltmeters (VTVM) use battery to measure resistance. This battery will leak and ruin your vintage meter. My Heathkit V-7A suffered from this same issue, but mine was lucky to survive with minor damage. I'll show you how to make a very simple 1,5 volt (or 1,55 volt) battery eliminator.
This battery eliminator is indeed very simple and basic LM317 circuit. If you look around the web, you'll find many very sophisticated versions, but there's no reason to over think this. In fact you can remove D2 and D3, as those are not reaaaally necessary. Component values are not critical either. R2 should be around 200Ω (220Ω is standard value), but other component values can be ±50% or so.
Wiring is simple. This circuit gets it’s power from the 6vac filament transformer. You can take it directly from the transformer or any other convenient place like pilot light. Heater winding is grounded on the other side, so make sure you wire it right way around. After you have adjusted the output voltage to 1,5 volts, connect wire to the battery positive terminal on the meter. Mount it securely to some convinient place and recalibrate your meter. That's it.
Two things to consider. VTVM power transformers are small. There is just enough heater power for tube heaters and pilot light. It might be necessary to replace pilot light with LED. Most 6,3 volt pilot lights draw 150mA of current (some even up to 300mA), but leds take less than 1/10 of that. This will help to compensate all the power the eliminator takes. Another thing is compatibility. This is only tested with Heathkit V-7A. Other meters might need a different solutions.