Asteroids video issues

mogrinz

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I'd appreciate any thoughts on where to get started.. there are a lot of Asteroids repair logs and videos on the web, but none of them seems to describe a problem like mine.

My machine doesn't indicate any failures in self test, but the pics shows otherwise. During gameplay, I get random horizontal lines, crooked text, and when I rotate the ship it makes a jumpy loop instead of a smooth circle.

Thanks!
Mike
 

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Bad chip in the vector generator section, specifically the vector timer. Could be one of several, but that behavior is typically one of the chips around the LS175 latch at M7 (either that one, or one of the ones connected to it).
 
Hmm.. no luck after replacing the M7. Disclaimer: I'm totally green to Asteroids board repair.

Looking at the schematics, it looks like the vector timer is F6, M7, M6, E7,D7, C7, and B7. Should I systematically replace each of them? Is there any way to test beforehand to narrow it down?
 
You'd need a logic probe to check the inputs and outputs of each chip. Sometimes you can get lucky and find dead pins, or sometimes the pins are obviously stuck high or low (which requires knowledge of how each chip functions, so you can compare the inputs and outputs).

However sometimes with some chips like counters, the inputs and outputs will be toggling, which can give the appearance of working correctly, but the chip may not be counting properly, which requires more sophisticated tools to find. So it's not always an exact science, and the better tools you have, the more likely you can be to find the offending chips.

Shotgun replacing chips is generally not advised with these boards, as it is very easy to introduce new problems when replacing chips (particularly causing shorts under chips/sockets), as the traces can be very tight under some of the chips. And Atari did not use solder mask on the tops of the PCB's, so the metal traces are exposed, and solder will stick to them very easily. And when you introduce a new issue on top of the one you're trying to find, it gets exponentially harder to troubleshoot. So you can try replacing more chips, but be very careful with your work.
 
You could try carefully piggybacking a new chip on top of the old one while it's still installed on the board. You must do this carefully so none of the incoming pins short out any of the existing pins. Make sure you have the incoming chip pointing in the right direction. Also the legs must be spread wide enough to fit over the installed chip but close enough together to make good contact with its legs.

Warning: if you slip and do short out pins you could make things worse. Usually it will just reset the board, but....

If you do it properly, you may see the problem clear up as the new chip does the job of the failing chip, or at least see a change in appearance as the new chip contends with the old one to try to do the right thing. Or, it may be unable to overcome the fault and make no difference.

It's a quick process that may help narrow down the hard-to-test chips that andrewb mentioned.

I had almost the exact same (looking) problem on an Asteroids Deluxe board at it turned out to be the LS161 at D6 (the first chip in the vector timer chain). The equivalent chip on Asteroids is D7.
 
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