Space Invader audio problem

MacPac80

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I replaced the 76477n chip on my Midway Space Invaders board to fix the missing Space Ship sound. It worked great for a week and now when I turn on the machine it has a real high pitch but then after it warms up it start lowering and going up and down but not really like it should sound. I did have the machine running without the speaker connected while I was testing out the monitor, would this have messed that up? All the other sounds work great.
 
Ok, so I noticed that as it got a little warmer it worked more like it should. I took a hair dryer to it and held it for a minute or two and now it sounds like it should. What the heck, is this normal?
 
...and now after I wait a hour or two it's back to being a high pitch only :(
 
Hmmm.... seems reasonable to expect that the new IC (@H2) is good. I'd perhaps replace the tantalum caps connected to that IC (C35 & C36), and check the values of the resistors connected to it.
 
I'd check the caps, and meter out the legs of the chip. You may have a cold solder joint.

You could also heat the board, and then freeze each cap, to see if you get the problem back. That would allow you to localize which one of the little buggers is causing your noisy headache.
 
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