Re: Asteroids PCB repair help

> 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?

Make sure processor is a 6502A.

Make sure +5 is right AT GAME BOARD.

If still bad, cool EPROMS/PROMS one at a time to see if one is heat
sensitive.

> 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.

Check reference voltage pin of DACs. Low voltage=smaller picture.

>
> 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

DAC failure.

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Received on Sun Aug 19 20:03:18 2012