Strange Galaga PCB issue

gregatron

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I'm repairing a Galaga board for a friend. It would work occasionally, but would cut out.
He removed it, recapped it and cleaned the PROM legs. After reinstalling them, all it would do is show a blank screen. He realized he had a few of the proms on upside down. (not sure which ones). After he flipped them it would show a stationary starfield and that's it. I received it in this condition. I found the 06xx (2L) and the 08xx (2J) swapped. After I placed them in the correct locations the starfield now eventually starts to twinkle then scroll downward but that is it. No self test, no sprites/characters. I checked all of the ROMs ( except the bipolars) and they all seem OK. I have also swapped out all the custom chips, Swapped out all Z80s (yes 4M is still a Z80A) verified continuity of the data and address lines to each ROM from each corresponding CPU (only on the CPU board). Made sure the Voltage was 5.2 at the chips, visually inspected for bad traces. Verified clock pulse at all the Z80s. And several more things I'm sure I'm forgetting. Am I missing something? Has anyone seen this before? I'm in the process of emulating bad ROM images in MAME one at a time to see if I can recreate the same problem but haven't had a chance to do more than two of them. I know it's got to be something I must have over looked. Anybody have an idea what's going on with this thing?
 
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Tell your friend to never tough a PCB again!

Have you checked or swapped the bi-polar PROMs? Since you know (at minimum...a few chips were plugged in backwards)....all socketed chips are now suspect.

Edward
 
Tell your friend to never tough a PCB again!

Have you checked or swapped the bi-polar PROMs? Since you know (at minimum...a few chips were plugged in backwards)....all socketed chips are now suspect.

Edward

The crappy thing is I don't have any way to read/verify the Bipolar PROMs. And I don't have spares so I guess I'm screwed there. I have verified just about every other socketed IC though. I'm sure it's a fried IC because it's the most logical thing but would a bad bipolar cause it to skip the self test and boot sequence?
 
Success!! I emulated corrupt PROMs in locations 1C and 2N in MAME and bam! Exactly the same problem. I guess I'll be needing to order some pre-programmed Bipolar PROMs ASAP.
 
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