NBA Jam low volume?

GPD1444

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Picked up a Jam last week and was told it had really low volume but when I got it home it had no sound. I'm not the most technical person so I figured it would be easier to buy a known working board to replace it. Thinking it had a bad sound amp.

Replaced the board today and got really low volume with the new board. Tried the old board again and got low volume with it also. Next step was to jump pins 2-4 (should bypass the sound pot correct?) giving it max volume but it remained low. Replaced the ribbon cable with a known working one and still low volume.

Any ideas? I think the odds of both birds having bad sound amps is slim considering the replacement was sold as known working.

I'm really just learning how to repair machines so please treat me like an idiot when explaining what to check.
Thanks
Tim
 
well, first tech tip: the ribbon cable has nothing to do with it, that merely interfaces the CPU and sound boards so it knows what sounds to play. :)

if you're not familiar with power supplies, I assembled the PSU guide in my signature for that very purpose. my advice is to get a new Happ Power Pro if you're intent on keeping the game for a long time. if you have some cheap off-brand unit then buckle up, it's going to get interesting.

there's a wiring harness that goes between the CPU board to the sound board; this is how the sound board gets power and how it interfaces the audio output back to the CPU so it goes through the JAMMA harness and finally to the speakers.

for NBA Jam there's only 2 voltages that power the sound board: +5 (grey wire) for the logic components and +12 (grey-yellow) for the amplifier. use your meter to measure the grey-yellow against the black ground wiring -- I'm willing to bet it's low.

so that's your homework assignment. +12 isn't adjustable, so if it's low the power supply is junk. I would just replace it regardless and see if anything changes.

good luck
 
Mecha, thank you for the dumbed down explanation.

I tested everything and found that the amp was only getting 5 volts. Tracked it back and whoever replaced the power supply had the 12 volts hooked to the +5 volt.
 
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