Atari Asteroids - Graphic issues

Flocke28

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hi all. What could cause these issue?


I checked all 74LS191, LS157, LS273x swapped both A/D converter and both to81

I have no ideas anymore.

Please help
 
Yes looks similar… i piggybacked all ls191 in those area for x and y but no change.. or maybe piggybacking won't work for ls191?
 
Piggybacking doesn't 'always' work - regardless of the chip.

If an output pin is dead (like a blown fuse) - then it will likely give you a clue.
If an output pin is internally shorted to ground - then it does nothing..
If the chip has scrambled up the data - it won't help.
If an input pin is shorted to ground - it won't help..
etc.
etc.
At best - piggybacking is a diagnostic helper..
If piggybacking worked 100% of the time.. there would be no repair guys.. (in theory)..
 
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I also had something similar look because of a bad 157 or 174 just a little downriver of the counters. piggy backing can not override a pin that is stuck in a gnd state but it can gnd a pin that is stuck in a high state on logic gates but if it is doing an operation like an adder or a register all you might see is something just as bad but different then if it wasn't there so something like a test pattern on the screen is easier to pickup on a change in the pattern. Piggybacking looks to be so good because you only see the 10 out of 100 that it works for the other 90 people don't hardly say anything
 
B/C10 was damaged. DACY8 was always low.

But seems some more issues are on the pcb.not a perfect grid yet

IMG_1649.jpeg
 
looks like something from the dac to the output. you are going to have to use the scope in xy mode to see if you can locate where it is going wrong. do xy at pin 7 on the tl082 then move out to the tl081 if you don't see at its earliest stage, if you do see it then think bad dac or bad tl082

ps piggybacking would not have shown b/c10 problem since the output was gnd and it would have dragged a good chip output to gnd too
 
your glitch is not on the digital side. I said to test analog side from dac out. swapping chips without data is possibly bad if you do damage swapping them.
 
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