Asteroids troubleshooting

Hi all,

I've gotten a dead Asteroids board most of the way back to life; game is playing but some objects are missing (and an incomplete diamond field in self-test). Self-test passes and all signatures in Peter Fyfe's asteroids_sa guide appear as they should. Since the vectors that are drawn appear good (right size, place and brightness) I thing the vector timer section is good. I may have a problem in the X/Y position counters but it feels more like the VG is just skipping some drawing. So I checked the program counter section and found ADMA11 is stuck low (but not grounded). That comes from J5 pin 6 and goes to J4 (LS670) pin 2 and K3 pin 3 - so how can I determine which one is dragging down the level? I piggybacked another LS193 on J5 with pin 6 bent out, figuring the new chip would leech power and inputs and give me a good output on dangling pin 6... but it's still low. Is there a flaw in my reasoning or method? Any ideas on other approaches or causes?

-Douglas

PS. Have a scope, 5004A and have been using the parallel port on an old PC as a down & dirty logic analyzer but am thinking of getting a Tek 1241 or HP 1650. Any thoughts on a good LA would be appreciated (newest game in my collection is from 1986).---------------------------------------------------------------------------
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Received on Wed Dec 14 09:34:23 2011