That elusive Commodore 128 80-column mode

Hardware

The 80-column mode on my Commodore 128 from the imitable Screenbeard continues to produce no output, regardless of whether its involed with the 40/80 switch, or using GRAPHIC 5 in the functional 40-column mode. But I think I’ve made progress since blogging about it last year.

So far I’ve:

  • Replaced my MGA/CGA to VGA converter with one using a different FPGA, because I like lots of initialisms. Somewhat redundant given I confirmed I wasn’t getting any monochrome composite output either from the RS 232 connector, but tried just in case.

  • Replaced the MOS 8563 R9 with two other known-good chips; I got an extra from a gentleman in Germany who sadly couldn’t revive his spare 128D.

  • Added a 64 K RAM upgrade daughter board, which overrides the dedicated RAM for the 8563 in case any of that was fried. I also have a more modern SaRuMan replacement that uses less power to try.

All the voltages across the machine look fine, though running the 128 diagnostic cart recently showed RAM errors. The 80-column mode has its own memory, but maybe it still can’t initialise correctly. I have a SaRuMan 128 RAM replacement on its way, so we’ll see if that makes a difference.

The other possibility is a faulty timing crystal. The 8563 has its own discrete crystal separate from the rest of the system, but I’ve seen a few YouTubers report issues with the 80-column display that disappeared when they replaced it. I’ve got a new one in the post for that too.

Now that I have a dedicated Commodore 64C, my hope was to transform the 128 into my primary CP/M and DesTerm128 tinkering machine, as well as learning more about the Z80. Getting 80-column working on it would be absolutely glorious!

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