Four busted C64s


First some C64 testing advice

  1. read Ray Carlsen
  2. test suspect chips from dead 64 (or vic 20 or apple or whatev) in known working board, one a time

Fixing thangs

I own (at the time of writing) six Commodore 64s. Three are breadbins, two are C64c longboards and 1 is a C64c shortboard, thankfully working as it's schematically quite different from the others. Four of the other five were dead or dying.

A couple of these had bung 6510s (cpu) which is easy enough to diagnose. Before having some spare 6510s arrive from ebay I took a punt and unsoldered one from a working C64 to use it as a tester. This also had the bonus effect of making the donor board a useful for testing other 6510s by virtue of putting a socket in where the 6510 sits.

One of my bung 64's had its 8701 clock fried. This was found by simply poking around with the CRO. I think the symptom was no video signal (not just blank screen). I found that my old analogue CRO and my lack of skills limiting: but just about any mug can use a CRO to see if a clock signal exists. 8701 easily swapped with another and problem solved.

The 2nd last C64 to fix was the one that was most satisfying: "out of memory in 0" came up on boot. Goggle search and lemon64 forums suggested RAM fail and directed me to World of Jani. World of Jani ( suggested looking for flashing or flickering characters on the screen: and I had the occasional " (double quote) appearing here and there. Take the ascii of " which is 34, subtract the ascii for space (which is what is meant to fill the screen, not ") which is 32 and you get 2. That is bit 1 which, according to Jani's table, is U9 in an old 8-RAM board (since those RAM chips are only responsible for 1 bit in the entire address-space). I pulled out the old RAM, socketed it, and put in the only spare 4164 I had, lifted from my other 8-RAM-chip C64 which had one of its RAM conveniently socketed (ie replaced way back when). Bob was my uncle: C64 fixed. Thank you World of Jani.

The final C64 I fixed was a blank screen with video (called a white screen?). VIC2 was good. I suspected memory fail after testing every other chip liable to give a blank screen, so I bought a Dead Test cart rev 781220. Plugging it in gave 3 flashes in a sequence: meaning RAM U11 was toast. I bought some 4164 off ebay and 4 weeks later the dead RAM chip was replaced and this 64 was resurrected.

I have enough parts to have five working C64s, tho only two SIDs work fully (a couple half work). I'm down a 8701, a 6510 and three SIDs.

See also: resurrecting a piece of junk vic 20

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