Macintosh 128K (JG 1)

This computer belongs to a customer, it mostly worked when I received it, but the floppy drive was not functioning.

Work done:

• Rifa capacitors replaced on analogue board
• ROMs upgraded on logic board to support Sony OA-D34V-22 400K floppy drive
• 400K floppy drive cleaned and lubricated

Analog board

There was a large RIFA paper capacitors installed on the logic board, these tend to go bad and explode, so it had to be replaced first.

Logic board

The logic board that came with this computer had a RAM upgrade to 512KB, to restore the machine completely back to a 128K model the customer asked to swap it with this computer instead. The text below is about the board which ended up in this computer.

Initially the logic board seemed to work, but the next day it failed to show any image on the screen.

After testing the logic board in another computer I found that it's showing the same issue, with no video displayed at all. I attempted the following to try and resolve this:

• Resoldered all the pins on J7 (logic board power connector)
• Checked pins 1 (video), 3 (hsync) and 5 vsync on J7 with a scope, all three seem to show a similar trace to a working logic board.
• Checked pin 16 (video out) on BMU0, seems to show a regular signal
• Checked Pin 18 (video in) on BMU0 with a logic analyser, looks like the data coming in is just a constant pattern of 1, 0, 1, 0, etc separated by a longer low. There doesn't appear to be any variation in this.
• Tested all the LS393, LS257 and AS253 logic ICs off board

After further analyses of the LAG chip using a logic analyser, I found that it wasn't addressing the memory correctly, resulting in a blank screen.

When I swapped the LAG from a donor board, the issue was resolved and the computer could boot normally. Unfortunately, these PAL ICs (early FGPA like devices) were programmed at the factory and no source code is available for them. This means that they are pretty much unobtainium and can only be replaced from a donor board.

Logic board ROM

While testing the 400K floppy drives, I found it impossible to get them to track properly, and they couldn't read or write any floppies.

After a hint from the owner and some research, I found that the issue was caused by the ROM revision on this board not being compatible with these drives.

The drives were Sony OA-D34V-02 and OA-D34V-22 models, which are not compatible with the ROMs originally installed on this logic board: 342-0220-A (High) / 242-0221-A (Low). Only Sony OA-D34V models are compatible with this ROM.

After upgrading the ROMs to 342-0220-B (High) / 242-0221-B (Low) the drive worked perfectly.

Here is the completed board with the upgraded roms:

Original 400K floppy drive

The original 400K floppy drive for this machine had completely seized and was missing its felt pressure pad:

I cleaned and lubricated the mechanism, so it could move freely again and swapped the felt pressure pad from another drive, but even with that it still wouldn't read or write any floppies. When I ran SonyTest it initially showed an inability to find the correct track locations, but this turned out to be caused by the ROM (see above). After swapping the ROM the drive still couldn't write and would read random data.

Replacement 400K floppy drive

Since I couldn't repair the original drive, I used a replacement instead, which was originally meant for another computer. Like the original, it was fully seized and needed a full cleaning and lubrication. After upgrading the ROMs on the logic board, the drive functioned perfectly.

The cover of this drive was missing, so I used the one from the faulty drive