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Unhappy Macintosh 128k, Thick vertical lines▸
Unhappy Macintosh 128k, Thick vertical lines
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Unhappy Macintosh 128k, Thick vertical lines
Just checked again, and it looks like the bad ROMs are 342-0220-A and 342-0221-A. The working ROMs are 342-0220-B and 342-0220-B.
H3NRY, thanks for the information about the 27C256's. Jumpering pin 1 to 28 seems like it wouldn't be too difficult.
With two different sets of ROMs, I guess I can't pinpoint the bad chip by replacing the low or high rom individually. But I do have a ROM reader circuit that I used for some Apple II+ ROMs around somewhere, I might be able to resurrect that to read off the contents of the chips and see which one is bad.
H3NRY, thanks for the information about the 27C256's. Jumpering pin 1 to 28 seems like it wouldn't be too difficult.
With two different sets of ROMs, I guess I can't pinpoint the bad chip by replacing the low or high rom individually. But I do have a ROM reader circuit that I used for some Apple II+ ROMs around somewhere, I might be able to resurrect that to read off the contents of the chips and see which one is bad.
I find looking in my box of old ROMs, a set of 342-0220/21 (no letter), a set of -As and a set of -Bs, so I guess I have both versions plus a possible third version. There's also a set of Beck-Tech 64K upgrade ROMS which support large memory up to 4MB. Question: will a set of -A ROMs work in a Mac that shipped with the -B version? I don't remember what the differences are (if I ever knew to begin with.)
I seem to recall that one set of ROMs revised only one of the two chips, though both got new letters. I believe that was one of the 128K (Mac Plus) ROM sets, though, and the fix had to do with booting SCSI. When I have time, I'll try to dump these ROMs assuming my ROM burner for the Apple II still works. It's been in the closet for at least 20 years. If I can get a good read, I'll post the raw files for the curious. I imagine the ROM files are already on the 'net someplace, since emulators need them. I'm not inclined to disassemble and trace the code myself. Did that long ago and it was a lot of work.
I seem to recall that one set of ROMs revised only one of the two chips, though both got new letters. I believe that was one of the 128K (Mac Plus) ROM sets, though, and the fix had to do with booting SCSI. When I have time, I'll try to dump these ROMs assuming my ROM burner for the Apple II still works. It's been in the closet for at least 20 years. If I can get a good read, I'll post the raw files for the curious. I imagine the ROM files are already on the 'net someplace, since emulators need them. I'm not inclined to disassemble and trace the code myself. Did that long ago and it was a lot of work.
I too have a 128K which has a 342-0220-A/0221 with no letter - however, it does say "REV A" next to the copyright date. It happens to be on my early 13th week, completely original, unaltered 128K. So this is either a mixed ROM set, reflecting an early revision in the product cycle, or an early method to represent the "-A". Do yours have the words "REV A" next to the copyright date?
As for the ROM dump, I sent a -B disassembly file to Paul Pratt to compare to his -A ROM set, and he indicated minor changes. I did not think to send him the 0221 REV A ROM, however. Nevertheless, the 1984 copyright date relates to the Sony drive stepper motor update. Both ROMs will work in either MAC. However, since the fix pertains to the newer 400K drive revision, newer drives are incompatible with the -A chips. The -B chips are backward compatible.
As for the ROM dump, I sent a -B disassembly file to Paul Pratt to compare to his -A ROM set, and he indicated minor changes. I did not think to send him the 0221 REV A ROM, however. Nevertheless, the 1984 copyright date relates to the Sony drive stepper motor update. Both ROMs will work in either MAC. However, since the fix pertains to the newer 400K drive revision, newer drives are incompatible with the -A chips. The -B chips are backward compatible.