WarriorRocker
Member
Howdy 68kmla'rs!
I was given a Macintosh 512K that was in very rough shape. As is common, the battery had leaked and the whole chassis was covered in rust.
Phase 1
The computer actually showed signs of life even in this rough shape. After wiggling the cable between the logic board and analog board I was finally able to see a sad mac screen with a lot of artifacts.
First thing was to stabilize the analog board with some new capacitors and cleaning of the corroded traces. The logic board was also cleaned up and besides for a few places like the rear ports and bracket bar practically looks brand new.
Now the computer powers on properly and all voltage rails from the analog board look great.
Turning the computer on and many times the speaker would simply pop, occasionally it would make it to the sad mac screen.
Phase 2
Even though the sad mac screen had heavily distortion and artifacts I was able to look up the sad mac codes and determine the failure was in the ROM check.
Sure enough, verifying the ROM chips against known good ROM images pointed to one of the chips having many read errors.
After some fiddling with the TL866II Plus programmer and some fresh EEPROMS I was able to flash the replacement ROMs.
With the new ROMS inserted I was presented with yet another sad mac code. Interestingly since swapping the ROMs it has ceased making the normal startup beep.
The results are generally the same when trying the original 64K ROM for the 512K and even with various versions of the Plus 128K ROMS.
Phase 3
The new sad mac codes pointed to RAM issues, which seemed obvious enough with the amount of artifacts on the screen.
First I went ahead and socketed all 16 RAM chips, the artifacts were bad enough that the sad mac codes did not fully translate to particular Bits/Chips. In the end I was able to identify a total of 3 bad RAM chips. Replacing these with something similar would clear the error in that Bit/Chip.
However... It's still not behaving as it should. Once the new chips are all inserted and the Mac is able to successfully check the ROM and RAM. It goes to a grey screen, no mouse cursor, though no mouse is connected, I am not sure if this is normal.
It stays on the grey screen for a couple of seconds before I hear distorted and popping sounds from the speaker until the screen goes black entirely.
If I pull one of the RAM chips it goes back to showing the sad mac screen with the obvious error about that chip being removed. With all chips inserted I would have expected either another error code or to finally see the happy mac or question floppy icon.
Brick Wall
Looking at the schematics it appears the RAM is directly connected to the CPU with signals coming from the BMU0/BMU1 chips for things like chip select and/or address decoding. I am not seeing anything strange on the oscilloscope however.
With the ROM and RAM fully replaced it just seems like something else on the board is misbehaving. Something with activating the other peripherals like the IWM and Serial controller that there may be a bus conflict.
I am at the point where the only choice would be to start socketing other chips like the 74 LS logic. Unfortunately it doesn't appear the equations are known to burn new PAL/HAL chips, so if one of these is the problem it may simply be a lost cause.
Hoping that someone can recognize the symptoms and point me in the right direction and save this machine from the great scrapyard in the sky.
I dont have a floppy drive, but even using the Rominator it still never makes it to any sort of working state.
I was given a Macintosh 512K that was in very rough shape. As is common, the battery had leaked and the whole chassis was covered in rust.
Phase 1
The computer actually showed signs of life even in this rough shape. After wiggling the cable between the logic board and analog board I was finally able to see a sad mac screen with a lot of artifacts.
First thing was to stabilize the analog board with some new capacitors and cleaning of the corroded traces. The logic board was also cleaned up and besides for a few places like the rear ports and bracket bar practically looks brand new.
Now the computer powers on properly and all voltage rails from the analog board look great.
Turning the computer on and many times the speaker would simply pop, occasionally it would make it to the sad mac screen.
Phase 2
Even though the sad mac screen had heavily distortion and artifacts I was able to look up the sad mac codes and determine the failure was in the ROM check.
Sure enough, verifying the ROM chips against known good ROM images pointed to one of the chips having many read errors.
After some fiddling with the TL866II Plus programmer and some fresh EEPROMS I was able to flash the replacement ROMs.
With the new ROMS inserted I was presented with yet another sad mac code. Interestingly since swapping the ROMs it has ceased making the normal startup beep.
The results are generally the same when trying the original 64K ROM for the 512K and even with various versions of the Plus 128K ROMS.
Phase 3
The new sad mac codes pointed to RAM issues, which seemed obvious enough with the amount of artifacts on the screen.
First I went ahead and socketed all 16 RAM chips, the artifacts were bad enough that the sad mac codes did not fully translate to particular Bits/Chips. In the end I was able to identify a total of 3 bad RAM chips. Replacing these with something similar would clear the error in that Bit/Chip.
However... It's still not behaving as it should. Once the new chips are all inserted and the Mac is able to successfully check the ROM and RAM. It goes to a grey screen, no mouse cursor, though no mouse is connected, I am not sure if this is normal.
It stays on the grey screen for a couple of seconds before I hear distorted and popping sounds from the speaker until the screen goes black entirely.
If I pull one of the RAM chips it goes back to showing the sad mac screen with the obvious error about that chip being removed. With all chips inserted I would have expected either another error code or to finally see the happy mac or question floppy icon.
Brick Wall
Looking at the schematics it appears the RAM is directly connected to the CPU with signals coming from the BMU0/BMU1 chips for things like chip select and/or address decoding. I am not seeing anything strange on the oscilloscope however.
With the ROM and RAM fully replaced it just seems like something else on the board is misbehaving. Something with activating the other peripherals like the IWM and Serial controller that there may be a bus conflict.
I am at the point where the only choice would be to start socketing other chips like the 74 LS logic. Unfortunately it doesn't appear the equations are known to burn new PAL/HAL chips, so if one of these is the problem it may simply be a lost cause.
Hoping that someone can recognize the symptoms and point me in the right direction and save this machine from the great scrapyard in the sky.
I dont have a floppy drive, but even using the Rominator it still never makes it to any sort of working state.