Adventures in C64 repair

My C64 is still banjaxed, but now in a way that makes me feel it's closer to being fixed.

I had thought it was just a dead PSU, so I ordered a new one.  When it arrived, I plugged it in and.....the same problems happened.  So that's the first potential fault ruled out.

So I ordered a C64 dead test cartridge.  When this arrived, I plugged it in and powered on...and it gave me all sorts of crap on the display.  Still recognisable characters and border, but utter random garbage.  There was some screen flashing, which is the way the cart indicates dead-ram chips, but it couldn't decide which one - every time I switched the machine on it gave a different number of flashes.

The internet had already suggested that garbage text meant a duff multiplexer, and with the cart encountering seemingly random errors across multiple chips, I decided to order a couple of spare multiplexers (there are two in the C64).

So today I cautiously desoldered the U13 multiplexer and soldered in a replacement.  This C64 is the actual machine I grew up with, so I was petrified of totally bricking it.  I decided I would test it out before trying to replace the second chip.

...and it made the dead test cartridge give a consistent single red-white flash every time I switched it on.  So that feels like progress!

The dead test manual seems to suggest that for my Rev-B machine, a single flash means RAM chip U12 is duff.  So now I've ordered a couple of RAM chips and when they arrive, I can set about replacing the chip (although I'm sure I read somewhere that it was possible to 'piggy-back' a working chip on top of a potentially faulty chip to test if it's faulty without having to desolder it first  - but that's for another day).

 

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