Asteroids PCB repair help

Long time lurker, first time poster. I picked up an Asteroids cabaret a while back, and the PCB died on me after a few weeks. The spotkiller kicked in mid-game and it started making the small UFO sound over and over. No test mode or anything. So of course rather than get it fixed, I bought two more "untested" (i.e. broken) PCBs. I have a multimeter, logic probe, solder sucker, schematics, and the belief that owning 30 year old electronics without knowing how to fix them is crazy. I have no problem performing repairs, but haven't quite honed my ability to diagnose what needs repairing. I've successfully repaired a few monitors (vector and raster), power supplies, and AR1/2 boards, but these are my first attempts at PCB repair.

So here is where I am.

PCB #1 (rev 02), the board that came in my Asteroids: No clock. Replaced Y1 crystal, LS04 at B5, no change. Traced the lack of clock to a failed pullup resistor R29. Board will now boot and play for a few minutes, then start drawing scrambled vectors and reset. Appears to be the watchdog, since I see RST on the CPU activating. Test mode is the same. I'm not sure how to approach this one. Vector timer?

PCB #2 (rev 05): Booted with scrambled vectors, then quit booting. Test mode said RAM #4 was bad. Socketed and replaced, board booted and played okay. Started resetting and behaving badly when I left it running, self-test came up with 4 FF / 5 FF PROM errors. Swapped out J2 PROM and it reported 5 AE / 4 9C. Swapped F/H2 PROM, and the board now boots and plays.

Remaining issues: I turned X & Y gain to max, but it doesn't fill the whole screen. It's playable, but smaller than it should be. Just wondering if anyone has seen this before I start flailing around in the video output section. Doubt it's the pots themselves, since they do adjust the screen size, just that the max adjustment doesn't fill the screen. Looks like there are a couple resistors / caps that should be tested. However, both X and Y axes exhibit this problem, so I might be wrong.

PCB #3 (rev 05): Booted up with some jagged vectors, then started resetting, flipping between test and attract modes, throwing PROM errors. Reseated everything and it indicates RAM #2 failure @M2. Socketed and replaced, board now boots and plays with the same jagged vectors.

Remaining issues: Diagonals are not straight, and as vectors cross the middle of the monitor horizontally, they collapse slightly on the X axis. I checked the position counters / rate multipliers and they seem okay. DACs? Also, cannibalized the PROMs from this board for PCB #2 above, so I don't know what I can do until I get new set - or maybe it was just a matter of cleaning / reseating.

Any advice would be greatly appreciated.
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Received on Sun Aug 19 19:32:35 2012