I went ahead and adjusted it back to where I started which was 120V = 142V.
That's good. Wrong, but back to a known position.
This schematic may be helpful at finding proper leads. I will be able to test this in a few minutes. I am assuming because of d3 d4 d5 d6 it is a rectifier. So I will be testing an AC signal on the input pins?
I think the AC IN is a low voltage AC from which the supply rails fo rthe meter itself are derived.
The actual input appears to be on the top right corner, and these are wired to one of the connectors on the board.
My impression is that this meter is reading DC.
That would explain the rest of the board as a precision rectifier (although I'm not sure you need to go to all that effort).
Did you mention if you had tried this unit with an output load?
It would be interesting to get 2 low wattage lamps, connect one to the mains and one to the output of this and adjust the variac until both have the same brightness -- then measure the voltage and note what the built-in meter says.
The huge transformer inside this unit is way too large to just power the electronics, so I wonder if it is not also an isolating transformer for the variac. Having these 2 inductors connected together could cause unusual things to happen to the AC waveform under no-load conditions.
I'd suggest you look at the output waveform (I've thought of asking this before) but:
1) we need to know if the output is isolated
2) you need to have appropriate probes for one of your scopes
3) you *REALLY* need to know what you're doing.
A mistake in any one of these three can cause a large bang and the loss of your scope (or worse)