Sanity check on power circuit on unknown PCB

[tobiasbp]

I'm playing around with a non-working, unidentified Z80 based game: https://photos.app.goo.gl/5KbQuS547RRUmeQz9

A previous owner has hooked up 5V and ground to the edge connector (I believe they are hooked up to the correct pins). When I apply 5V to the board, I meassure 1.25V at the ICs.

I have traced the area around the power pin on the edge connector. The result makes little sense to me. The regulator UA78MG is hooked up in a way that is not what I would expect (Pin OUT feeding the ICs).
The power pin of the op amp in the audio circuit is connected to pin IN of the regulator?? UPDATE: Is it possible, that the board expects 12V at the power to the op amp, and that voltage feeds the regulator? That would make the regulators connections make sense..

The regulator (UA78MG) and the transistor (BD535) shows sign of having been replaced (Wrong parts?). Diodes are not equal, and the soldering is horrible (Someone replaced them).

1: Does the way the regulator is connected make any sense? UPDATE: Regulator makes sense, if 12V is connected to pin IN
2: The (power) diodes in the circuit are connected to pins on the edge connector. What could the purpose of those pins be? Is the orientation correct? UPDATE: Reverse polarity protection?
3: Would it be a bad idea to supply 5V to the part of the schematic were I currently measure 1.25V, and see if the CPU springs to life?

schematic_bitmap.png

Schematic as PDF:

unknown-board.pdf

Any thoughts much appreciated. Thanks for looking.

[yosai]

Is it possible, that the board expects 12V at the power to the op amp

Yes. 12v is generally used for the audio circuit

Your best bet is to dump the EPROMs on the board and run them through ROMident. If successful you'll probably be able to find the pinout for the edge connector.

[tobiasbp]

Yes. 12v is generally used for the audio circuit

Is it also common for the whole game to rely on the 12V supply like this? I was expecting to be able to run the game (Without sound) on 5V only.

As far as I understand the schematic, the 12V power basically enables the 5V supply. Maybe it's a feature to make sure both power pins are hooked up before running the game?