Asteroids PCB repair log

ieure

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Donor 2014-2015
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Hot on the heels of this PCB repair, I tackled one of the other two Asteroids boards I have. This is the original one that quit on me.

Symptoms:
- Doesn't boot into test or game modes
- No video output, spotkiller on
- Constantly makes little UFO noise

Because the UFO noise was what I heard, I started off digging around the SAUCRSNDEN line. Sure enough, it was high, but I wasn't sure where else to trace from here, so I started over where I should have in the first place, at the clock.

Probing the RESET line on the CPU, I saw that it was not pulsing, so the repeating of the sound wasn't caused by the game stuck resetting. Checking the clock input to the CPU showed no activity, which pointed to the clock circuit as the source of the problem.

C4 generates the clock for the CPU, but probing its outputs shows them stuck low. C4 is driven by a clock on pin 2, which was missing. This clock is generated by the inverter at B5, which is further driven by the 12mhz crystal Y1.

Testing B5, pin 8 wasn't generating clock, and the rest were stuck high or low. Socketed and replaced B5. Problem persisted, though I now saw clock on pin 8. I expected to see other pins pulsing on B5, but didn't; replaced the 12mhz crystal. This was a red herring, because the crystal doesn't seem to generate TTL levels, so measuring it gave me the wrong idea. The symptoms persisted unchanged.

Switching back to C4, it was now getting the correct clock on pin 2, but all outputs were low. Looking at the datasheet, I noticed that pin 1 should be high, but was low. Pin 1 is the clear pin, and unless it is high, the counter will not count. Going back to the schematics, pin 1 feeds through 1K pullup resistor R29. I tested continuity from R29 to C4, and the traces were all fine. Testing R29 itself showed 250 ohms of resistance instead of 1K. Replaced R29, and the board is now fixed.

This was my first attempt at repairing a PCB, and there were some definite missteps. I'm glad I kept at it and got it working — I learned a ton that will help me the next time around.
 
Thanks. I really like reading these, too. It gives you a great look into how people work through these problems. As someone who hasn't done many of these repairs, I find that invaluable.
 
Wow, not sure if I would have honed in on the resistor as quickly as you did. I mean, how often does a resistor fail? Good reminder to be methodical. Congrats!
 
Wow, not sure if I would have honed in on the resistor as quickly as you did. I mean, how often does a resistor fail? Good reminder to be methodical. Congrats!

Thanks, but it wasn't quick at all. This all took place over a period of weeks, hooking the board up in the evening and spending an hour or so alternating between probing it and reading the schematics and datasheets.

I'll definitely be quicker the next time around. Having the other board working so I could compare against it was invaluable.
 
Shhhheeeit man, you did great! Thanks for posting the log, I need to do more of these myself.

Unfortunately most of my logs will end in "shelved for later date,and bought working PCB from X". :)
 
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