I am restoring a Mac Classic. Recapped the logic board, fixed a trace or two. Washed everything thoroughly, so no more residue. Overall, the board looks pretty clean now. However, a boot test results in clean, unwavering black and white checkerboard.
5v and 12v power tests fine, CRT is nice and bright, crisp, no shimmering—basically no evidence of issues (yet) on the analog board which looks very clean.
SO, research is guiding me to the ROM. Started by double-checking orientation and position (40 pin chip in a 42-pin socket. . . you know the drill). Tried a little Deoxit. ROM is properly installed position, but still no joy.
I have no knowledge of this ROM chip—might be good, might be bad, might have been installed improperly by previous owner at some point. Who knows. I also don’t have any spare Classic parts to swap.
I thought maybe I had a rotten via or a bad trace on the address bus, so I started testing continuity between the ROM and the CPU using Bomarc schematics. The schematics are not right--I know that much. There is definitely a pattern. I can’t remember now whether it was the address lines or the data lines, but there is some interleaving where testing the bus lines in order skip every other pin on the ROM as you move through the sequentially. . . In any case, I was able to confirm continuity on all of them, even though they don’t perfectly match expectations from Bomarc. I have seen others write about this too, so I assume it is a known discrepancy.
Where does that leave me? I cannot find a fault in the board, but I am suspicious of the ROM chip itself. Coincidentally, I just watched @Mac84 create his own Mac Classic ROM using an EPROM burner. Fascinating, and reassuring, but I am hoping he, or someone else who has done this maybe has a spare or two they would be willing to sell to me. SO, what say you? DOES ANYONE HAVE A SPARE ROM?
If there is a good source or better place to ask, I can do that. Also, I would entertain additional troubleshooting ideas if you have any.
Thanks!
5v and 12v power tests fine, CRT is nice and bright, crisp, no shimmering—basically no evidence of issues (yet) on the analog board which looks very clean.
SO, research is guiding me to the ROM. Started by double-checking orientation and position (40 pin chip in a 42-pin socket. . . you know the drill). Tried a little Deoxit. ROM is properly installed position, but still no joy.
I have no knowledge of this ROM chip—might be good, might be bad, might have been installed improperly by previous owner at some point. Who knows. I also don’t have any spare Classic parts to swap.
I thought maybe I had a rotten via or a bad trace on the address bus, so I started testing continuity between the ROM and the CPU using Bomarc schematics. The schematics are not right--I know that much. There is definitely a pattern. I can’t remember now whether it was the address lines or the data lines, but there is some interleaving where testing the bus lines in order skip every other pin on the ROM as you move through the sequentially. . . In any case, I was able to confirm continuity on all of them, even though they don’t perfectly match expectations from Bomarc. I have seen others write about this too, so I assume it is a known discrepancy.
Where does that leave me? I cannot find a fault in the board, but I am suspicious of the ROM chip itself. Coincidentally, I just watched @Mac84 create his own Mac Classic ROM using an EPROM burner. Fascinating, and reassuring, but I am hoping he, or someone else who has done this maybe has a spare or two they would be willing to sell to me. SO, what say you? DOES ANYONE HAVE A SPARE ROM?
If there is a good source or better place to ask, I can do that. Also, I would entertain additional troubleshooting ideas if you have any.
Thanks!
