Ms. Pac PCB loud buzzing sound

keithsarcade

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Before I throw a bunch of caps on this thing, just wondering if theres a typical culprit for this. Put this PCB in a working game, working except I'm missing some color, but the main problem is it makes a loud buzzing noise, so loud that it even rattles the cabinet. Haven't tried anything yet but I will clean and reseat all the chips and I have some of the 470uf caps (although radial) that I can replace. I did play with the volume pot briefly with no difference.
 
Leaky D3/D4 or a bad C29 will cause all kinds of noise on the power supply line feeding the amp chip. This in turn gets sent to the speaker.
 
Yep, comparing it to my good board it looks like D3 and D4 have bulged out. Manual is listing them as standard 1N4004's, is this correct?

Also noticed the large ceramic resistor above the heat sink has been replaced with a smaller one, a 4 ohm 15 watt. Manual says there should be a 10w there, does this matter?
 
If those diodes are bulging you have an issue. You can replace them with a standard 1n4004 diode. If you still have noise its probably the amp going bad. Unlike alot of amp chips the lm1877 usually starts showing symptoms before it fails. The 2200uf and 330uf caps may be going bad too which can cause humming.
Dont worry about the ceramic resistor being a higher wattage, it wont hurt anything.
 
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