Cabinet vs. Test Rig Boot Problems - Bump n Jump

wxforecaster

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I just finished building a JAMMA test harness to troubleshoot some boards that have been driving me nuts working on the garage floor. Last year I posted about a Bump n Jump that I picked up. When fired up, I hear the famous "BOOOOOOOP of that game, Burgertime, etc. but yet the monitor either shows a deep blue color or blue with a bunch of static green characters. I later picked up a "tested working" board online to help troubleshoot, but I didn't want to put it in the cabinet not knowing what the issues were.

Here's the odd thing. BOTH games boot and play perfectly in my test rig, although one has music that is "off key" for lack of a better description.

So I put each back in the cabinet. The "tested working" second purchase boots right up, nice clean picture, plays fine. The original board does its thing (either a dark blue screen or a blue screen with static green characters).

I'm baffled as to how this is possible. The only thing I can thing of is that one of the wiring harness connectors isn't making connection on the original board? I've got 12V on the money for 12V and 5.2V for the +5V.
 
Actually, the "bad" board, the one that doesn't boot in the machine, but does (with low "flat" offkey music) on my test rig, does not otherwise work cleanly on the test rig. I mean it boots and plays, but I noticed that I have 5.2V coming into the board, and about 3V at the far end. Obviously I've got a problem somewhere on one of the two boards, with a bad chip or perhaps a short somewhere? What is the most likely culprit for this kind of voltage drop?
 
FIXED.

RESET circuit and CPU tested out fine, which seemed bizarre. Actually two separate problems here.

Weird off key sounds/music was due to 2 of the 8910 sound chips being partially bad. Board not booting properly was due to 2 bad game ROM chips.
 
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