i86time
Well-known member
I picked up an spare OR set a while back and am getting around to repairing it. It was sold as having issues. It had previously been repaired, worked for a while, then did what it is doing now. I've made some repair to it, but it is still displaying these weird vector issues and I'm spinning my wheels.
tl;dr version
When I got the board, it was displaying these issues and also had a VRAM error.
The previous repairs done had included socketing (and likely replacing) the 3 LS670 @ K4/L4/M4, the 393 @ R5 and the LS245 @ S1.
I started by switching out both of the possible RAM, no change, then both sockets, still no change. I tracked it down to a bad LS32 @ R1, switched it out and VRAM error was gone but vector issue still remained. Here's where it gets fun.
Using a scope, I saw that the errors were present @ the output of the TL082s. I wasn't sure if there was a way to test directly out of the DAC to eliminate the TL082s themselves (is there?), so I started as near there as I could.
Iterating between logic probe and comparator, at various times I found possible issues with many chips, I know that comparators can show disagreement for reasons other than bad ICs, so I had to do my best with what I was seeing. The first IC with a possible issue I found was the LS157 @ F4. I pulled it, but it tested OK w/ a TTL tester. I socketed it and put it back.
Moving back and figuring it was related to Y9/Y10/Y11 signals, I followed this back along the SDY lines, came to the LS367 @ G4 and found an issue. I pulled it, but it tested OK, so I socketed and put it back. From there I switched over to SSETA lines and came to the LS193 @ K5. I pulled it but it too tested OK, so I socketed and put it back.
So, it pretty much goes on like this, moving from chip to chip and pulling and testing when I find an issue. In total I ended up also pulling/testing/socketing:
LS273 @ E2, LS139 @ F2 & S04 @ S6 (I have no idea what led to this last one, can't find anything in my notes) and all tested OK.
At one point I checked all ROMS and all were good.
So I just said screw it and I took a working board and my comparator and tested all ICs. I noted if the chip tested OK and if not, which pins were different. I then used this info to compare the ICs on the non-working board. If any chip showed a difference to what the working board showed (even if there was a disagreement between the referenc chip) I looked at it further. This led me to pull/test/socket the following:
LS74 @ E7/8, LS20 @ G2 and LS374 @ D5
but all tested OK. The 393 @ R5 tested different, but as it is part of the RESET line, I ignored it. Also the LS374 @ A5 also showed differences but after testing the one @ D5 I didn't bother pulling it. The LS161s @ L2 and M2 showed differences, but only for the unused pins 11-15. I was unable to test the LS74 @ H9 and LS367 @ J/K9 as the clip wouldn't fit. Other than that all other chips/pins showed the same activity between my good and non-working board. Then just for fun I replaced/socketed the DACs and TL082s, still no change.
So, to double check there was nothing wrong with the TTL tester I was using, I swapped all the socketed chips on the non-working board (including ROM/RAM/CPU, DACs/TL082s and all my pulls) to my OR repro, in which all ICs are socketed, and it played fine.
So here I remain stuck. About the only thing I have not done is to cap the board (but I did replace the 10 uF @ C108 as it tested bad, all others tested OK). Any pointers are appreciated.
tl;dr version
When I got the board, it was displaying these issues and also had a VRAM error.
The previous repairs done had included socketing (and likely replacing) the 3 LS670 @ K4/L4/M4, the 393 @ R5 and the LS245 @ S1.
I started by switching out both of the possible RAM, no change, then both sockets, still no change. I tracked it down to a bad LS32 @ R1, switched it out and VRAM error was gone but vector issue still remained. Here's where it gets fun.
Using a scope, I saw that the errors were present @ the output of the TL082s. I wasn't sure if there was a way to test directly out of the DAC to eliminate the TL082s themselves (is there?), so I started as near there as I could.
Iterating between logic probe and comparator, at various times I found possible issues with many chips, I know that comparators can show disagreement for reasons other than bad ICs, so I had to do my best with what I was seeing. The first IC with a possible issue I found was the LS157 @ F4. I pulled it, but it tested OK w/ a TTL tester. I socketed it and put it back.
Moving back and figuring it was related to Y9/Y10/Y11 signals, I followed this back along the SDY lines, came to the LS367 @ G4 and found an issue. I pulled it, but it tested OK, so I socketed and put it back. From there I switched over to SSETA lines and came to the LS193 @ K5. I pulled it but it too tested OK, so I socketed and put it back.
So, it pretty much goes on like this, moving from chip to chip and pulling and testing when I find an issue. In total I ended up also pulling/testing/socketing:
LS273 @ E2, LS139 @ F2 & S04 @ S6 (I have no idea what led to this last one, can't find anything in my notes) and all tested OK.
At one point I checked all ROMS and all were good.
So I just said screw it and I took a working board and my comparator and tested all ICs. I noted if the chip tested OK and if not, which pins were different. I then used this info to compare the ICs on the non-working board. If any chip showed a difference to what the working board showed (even if there was a disagreement between the referenc chip) I looked at it further. This led me to pull/test/socket the following:
LS74 @ E7/8, LS20 @ G2 and LS374 @ D5
but all tested OK. The 393 @ R5 tested different, but as it is part of the RESET line, I ignored it. Also the LS374 @ A5 also showed differences but after testing the one @ D5 I didn't bother pulling it. The LS161s @ L2 and M2 showed differences, but only for the unused pins 11-15. I was unable to test the LS74 @ H9 and LS367 @ J/K9 as the clip wouldn't fit. Other than that all other chips/pins showed the same activity between my good and non-working board. Then just for fun I replaced/socketed the DACs and TL082s, still no change.
So, to double check there was nothing wrong with the TTL tester I was using, I swapped all the socketed chips on the non-working board (including ROM/RAM/CPU, DACs/TL082s and all my pulls) to my OR repro, in which all ICs are socketed, and it played fine.
So here I remain stuck. About the only thing I have not done is to cap the board (but I did replace the 10 uF @ C108 as it tested bad, all others tested OK). Any pointers are appreciated.
