Asteroids - board or monitor issue?

modessitt

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I'll be doing some research on it tomorrow, but figured I'd get a jump start tonight and see if anyone had an idea.

Picked up an Asteroids today for repair and it appears to be playing, but the screen is awfully funky.

Here is the gameplay screen:

Photo-0023.jpg


Here is the test screen:

Photo-0024.jpg


The only beeps in test mode are when I push buttons (as expected). Can't see if an error is being given on the screen or not.

I'm guessing a problem in the X/Y section of the board?
 
Anyone care to point me in the right direction? DAC's? Something else? I seem to have gameplay, but the X/Y ouput is screwy...
 
Confirmed as a board issue. Asteroids Encyclopedia seems to indicate it is a problem in the Y-counters, so I'll start there....
 
This issue has been fixed.

Replaced the three 74LS191 multiplexers in the Y circuit and now have a perfect picture...
 
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