DistantStar001
Experienced Member
- Joined
- May 8, 2019
- Messages
- 178
As the title says, I'm trying to repair a 250425 board that's giving me a single flash on the dead test.
I've conducted extensive repairs to the board, removing and eventually socketing almost every single IC on the board (and at this point, all the ICs that went in them). The only exceptions are the Kernel and BASIC ROMs as I ran out of sockets. However, all the traces are intact and connected to what they're supposed to be.
I've checked and rechecked the sockets. As far as I can tell, everything that should be connected is connected, and there are no unexpected shorts on the board. Even so, I'm getting a single flash RAM error.
Sadly, I don't have a working oscilloscope, but my logic probe reads activity on the data lines of U9, U21, and U22. The others seem to be held high. However, my logic probe isn't that good (on is off, red is high).
I then tried removing RAM to check the errors on the dead test. Below is what I got:
U9: 7 7 √
U10: 5 5 √
U11: 3 3 √
U12: 1 1 √
U21: 8 8 √
U22: 6 6 √
U23: 1 4 X
U24: 2 2 √
In short, with U23 removed, I still get 1 flash when I should get 4.
As I understand it, that could be an issue with data bits 0, 1, 2, or 7. However, since removing U9 and U21 results in the expected flashes, that would seem to indicate that 0 and 1 are fine? I can't rule out 7, but 2 seem to be the most likely suspect.
I can also eliminate the CIAs, SID, and ROMs as likely suspects as they're not installed. I've tried two CPUs (6510 and 8500) with the same results. The PLA is brand new (PLAnkton) and the VIC II, as well as the 8701 have been tested as good in another machine. Also, as mentioned, all the support logic is brand new as well.
Anyway, what am I missing here? Any help would be appreciated.
I've conducted extensive repairs to the board, removing and eventually socketing almost every single IC on the board (and at this point, all the ICs that went in them). The only exceptions are the Kernel and BASIC ROMs as I ran out of sockets. However, all the traces are intact and connected to what they're supposed to be.
I've checked and rechecked the sockets. As far as I can tell, everything that should be connected is connected, and there are no unexpected shorts on the board. Even so, I'm getting a single flash RAM error.
Sadly, I don't have a working oscilloscope, but my logic probe reads activity on the data lines of U9, U21, and U22. The others seem to be held high. However, my logic probe isn't that good (on is off, red is high).
I then tried removing RAM to check the errors on the dead test. Below is what I got:
U9: 7 7 √
U10: 5 5 √
U11: 3 3 √
U12: 1 1 √
U21: 8 8 √
U22: 6 6 √
U23: 1 4 X
U24: 2 2 √
In short, with U23 removed, I still get 1 flash when I should get 4.
As I understand it, that could be an issue with data bits 0, 1, 2, or 7. However, since removing U9 and U21 results in the expected flashes, that would seem to indicate that 0 and 1 are fine? I can't rule out 7, but 2 seem to be the most likely suspect.
I can also eliminate the CIAs, SID, and ROMs as likely suspects as they're not installed. I've tried two CPUs (6510 and 8500) with the same results. The PLA is brand new (PLAnkton) and the VIC II, as well as the 8701 have been tested as good in another machine. Also, as mentioned, all the support logic is brand new as well.
Anyway, what am I missing here? Any help would be appreciated.
