GORF graphics issue (no yellow, stars or sparkle)

i86time

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I'm working on a spare GORF set and need some help. The first screen after boot (scores/text, 1/2 player ships, etc.) displays normally and the first level screen is mostly normal; the only issue is the shield is solid white and not sparkling. However all subsequent screens are missing any yellow, displaying only dark red or blue, are missing stars and do not show sparkling (shields/flag ship reactor). Using my good set, I narrowed it down to the game board (bad board in good set has issues while good board in spare set does not have issues). Using the same method I tested the two custom chips and they are fine.

I found a short on the board which I tracked down to a bad 74LS74 @ U14 and replaced it but it had no effect. Trying to diagnose the yellow, I tested Q6, Q7 & Q8 which all tested fine, but I went ahead and switched their positions which also had no effect. I then went ahead and switched out U20, a 74LS257, which feeds the VIDEO signal, again without any improvement. Since I wasn't getting anywhere with the yellow, I decided to move on to the stars/sparkle.

Looking at the schematic, the only chips I see dealing with the sparkle/stars are U5, 74LS153, and U6, MC10499/CD4099, which I replaced again without improvement. Since I didn't have any new CD4099 or 74LS257 I used pulls from my boards, but I know at least the 4099 is good as I put it back in the main GORF and it plays fine.

So, I'm open to ideas.
 
Time to close this out. In the time between I picked up an HP 10529A. I tested all the ICs for which I had spares (which wasn't much), but nothing was jumping out. The only thing I noticed was the remaining two 74LS74's (U17 & U22) were not agreeing with a 74S74 I had, but I figured since the HP manual states ICs must be exactly the same, I figured maybe that was the cause.

Since that time I received my parts kit for the Omega Race repro, so I had more ICs to compare. When I got to the LS367 @ U24, I saw a lot of weird activity. I pulled the chip, but it tested fine with one of those cheapo IC testers. I replaced it anyway but no improvement. Then I got to the 74LS74s again, and again they showed differences with the same activity as before. I went ahead and tested them with a logic probe and only U22 seemed bad. I pulled it, it tested bad, but still no improvement upon replacement. I then pulled U17, it also tested bad, and after replacement the yellow, stars and sparkle all returned. Finally tally- all three 74LS74s on the game board were bad.

Now on to my spare pattern board which is completely dead.

tl;dr version- trust your tools
 
Time to close this out. In the time between I picked up an HP 10529A. I tested all the ICs for which I had spares (which wasn't much), but nothing was jumping out. The only thing I noticed was the remaining two 74LS74's (U17 & U22) were not agreeing with a 74S74 I had, but I figured since the HP manual states ICs must be exactly the same, I figured maybe that was the cause.

Since that time I received my parts kit for the Omega Race repro, so I had more ICs to compare. When I got to the LS367 @ U24, I saw a lot of weird activity. I pulled the chip, but it tested fine with one of those cheapo IC testers. I replaced it anyway but no improvement. Then I got to the 74LS74s again, and again they showed differences with the same activity as before. I went ahead and tested them with a logic probe and only U22 seemed bad. I pulled it, it tested bad, but still no improvement upon replacement. I then pulled U17, it also tested bad, and after replacement the yellow, stars and sparkle all returned. Finally tally- all three 74LS74s on the game board were bad.

Now on to my spare pattern board which is completely dead.

tl;dr version- trust your tools
Hello. I came across this post when I googled about no stars on Gorf. How did you test those 74ls74 chips?
 
I first tested with a logic comparator and verified they were bad with an IC tester. If you have a logic probe, you can probe the outputs to see if an output is stuck hi or lo.
 
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