Take one ordinary NAMCO Galaxian PCB pre converted to DC....
Fault: Sound had ceased working.
Prevailing Symptoms: It had been going for a while when it got warm and finally gave up the ghost.
On the bench, sound not working at all, scope for signals at the amp, nothing going in.
I suspected the sound amp - on this board, an MB3712.
Obtained and popped in a new one but the problem persisted - I can hear audio very faintly through the speaker. I hook up to my JAMMA test rig and if I crank up the volume on everything its sort of there indicating that its not amplifying properly.
I decided to get on the multimeter and the scope and check the inputs to the amp chip.
I've got sound signal out of the LM324 at 7T on pins 1 and 7 as expected according to the schematics. Followed this down to the input pin - pin 6 on the MB3712 but nothing getting there.
Traced it back to a bad 22k resistor at R42.
If you look at the schematics for a Midway board you won't find R42 as its a different sound circuit, I couldn't find the namco schematics online so had to just trace it out the hard way.
After replacing the 22k resistor i've now got sound at pin 6 but still nothing on the speaker.
After a little tracing I find that the output from pin 1 goes through a 470uf/16v to pin 18 on the underside of the board, however, there's no standard return from the speaker, normally this would be pin V you'd think and you wouldn't ground reference the amplifier chip.
On this occasion though pin V goes absolutely nowhere, checking around a few other pins on the chip they go direct to ground so I move the jumper on my adaptor and ......
..... i've got sound.
Fault: Sound had ceased working.
Prevailing Symptoms: It had been going for a while when it got warm and finally gave up the ghost.
On the bench, sound not working at all, scope for signals at the amp, nothing going in.
I suspected the sound amp - on this board, an MB3712.
Obtained and popped in a new one but the problem persisted - I can hear audio very faintly through the speaker. I hook up to my JAMMA test rig and if I crank up the volume on everything its sort of there indicating that its not amplifying properly.
I decided to get on the multimeter and the scope and check the inputs to the amp chip.
I've got sound signal out of the LM324 at 7T on pins 1 and 7 as expected according to the schematics. Followed this down to the input pin - pin 6 on the MB3712 but nothing getting there.
Traced it back to a bad 22k resistor at R42.
If you look at the schematics for a Midway board you won't find R42 as its a different sound circuit, I couldn't find the namco schematics online so had to just trace it out the hard way.
After replacing the 22k resistor i've now got sound at pin 6 but still nothing on the speaker.
After a little tracing I find that the output from pin 1 goes through a 470uf/16v to pin 18 on the underside of the board, however, there's no standard return from the speaker, normally this would be pin V you'd think and you wouldn't ground reference the amplifier chip.
On this occasion though pin V goes absolutely nowhere, checking around a few other pins on the chip they go direct to ground so I move the jumper on my adaptor and ......
..... i've got sound.
