Asteroids PCB issue bad M4 ram even after replacing.

danpiraino1971

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Asteroids PCB issue bad M4 ram even after replacing.

One of my working Asteroids PCBs went bad. A pulsing noise comes through the speakers. Player one start button blinks with this pulse and there are random vector lines being drawn on screen in a diagonal inconsistent pattern. Initially it gave me two high tones and two low tones in test which is confusing since I thought the tones are supposed to stop when it gets to the bad ram. Well I changed R4 and M4. I then got two high tones and one low tone. That would indicate M4 being bad. I replaced it twice no change. I replaced all ram and still no change. I cut pin one at L6 and it went dead. No idea other than its not in the VSM.
 
You've got either a bad bus buffer, or it's something bad in the address decoding logic. But you'll need a logic probe, and to dig into the schematics to get further. (See those respective areas of the schematics for which chips to focus on.)
 
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