Thread revival! Ok, so thanks to
@george_tlc for paving the way on this. I installed a dual battery system and wanted to use the cheap Amazon dual voltmeter because I can stuff it inside a switch blank on the dash of my 60 for a more factory look.
I also ran into the problem where the dual voltmeter is always on since it’s directly connected to the second battery. I checked out the PC board and it’s different than the one dissected above - but the circuit was easy to break down. I’m leaving this write up below in case anyone else wants to this and ends up with a unit like mine.
In the main board each + input splits: they independently go to separate voltage sensing circuits (which have trimpots to adjust them!), and they also each pass through a diode and combine at a voltage regulator. The voltage regulator passes through a via to the backside of the board to a trace with a screen printed label that says “3.3V” next to it. This is the nominal B+ voltage to turn the displays on. All we have to do is get rid of the diode from the auxiliary battery to the 3.3V regulator and the display will only come on with the starter battery. In my application that + feed is switched with the ignition.
Here is the top side of the main board with the offending diode circled. Notice the diode above it - that’s for the topmost + input to power the 3.3V circuit. In my case the starter battery is connected to the + input labeled “VCC” and the house battery is on the + input labeled “OUT”. The word “OUT” is a strange way of labeling this - it’s not like it can be used as a pass through due to the diodes. Chinese manufacturing misnomer or typo I suspect.
Here’s the backside of the main board with the 3.3V trace labeled.
And here’s the top side again with the diode removed.
After that I used a meter to test actual voltage and then calibrated this thing as best as I could with the trimpots. It operates just as expected now. Thanks George!
FYI, I use a piece of dense foam with a slit in it so it slips around the PC board to hold it to the switch blank. Like this: