Re: Asteroids PCB repair help

Been busy with other projects, circling back to this stuff.

On Aug 19, 2012, at 5:03 PM, Rodger Boots wrote:

>
> > 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.
>
It is a 6502A.

> Make sure +5 is right AT GAME BOARD.
>
It's correct, reading just over 5v on the far side of the PCB.

> If still bad, cool EPROMS/PROMS one at a time to see if one is heat sensitive.
>
Will report back. I let it run for 5-10m and the problem didn't recur, so I don't know. I'll just let it go for a few hours when I have more time.

> > 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.
>
Both look fine. Pin 14 reads +5.01v, and pin 3 reads -15.44v. Identical readings on X and Y DACs.

Comparing against a board without this issue gives me +5.09 and -15.53v.

I checked the pots, and they're both giving me .03-4Ω at minimum and around 9kΩ at max, which seems okay. Thinking something in the output stage around C12/A12. Can I measure X/YOUT on a normal multimeter?

> >
> > 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.
>
Is there some way to test this to make absolutely sure? I don't have a tool which would allow me to remove the DAC without clipping the legs, and I'd be disappointed to do that and find my problem unsolved.

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Received on Sat Nov 10 01:53:53 2012