Commodore 64 ‘breadbin’ repair log #3

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Aug 062011

Number 3 of the 5 I brought back from RCM.
This one ended up being a bit of a pain.
Black screen at power up. Removed PLA but didn’t have an original spare I could swap out so it was time to use the CPLD based replacements I have.

Now the 64 boots up but sometimes gave garbage on the screen and sometimes gave the normal boot screen but with 6336 bytes of RAM.
My initial thinking was a RAM fault so went round all the RAMs and doing a thorough soak test on them but it yielded absolutely nothing.
I plugged in my diagnostic cart and it would flag up a different RAM as being at fault each time. This suggested that the 74LS257 data selector for these RAMs was shot. I removed U13 first and it failed its tests. I replaced this with one from another C64 I had and all is well with this one too.

As Ive been using a couple of these 64’s as donors I now find myself unable to do anything with the remaining 2. Ive got some RAM chips coming but will have to keep an eye out for some VIC replacements.

Commodore 64 ‘breadbin’ repair log #2

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Aug 062011

Had black screen on power up.
Tested the PLA, this was fine. The VIC has getting really hot, replaced and system powered up but only showed 796 bytes of RAM free.

Using a homemade diagnostic cartridge, I found that a RAM at U11 was to blame. I pulled this and tested it but it passed. I set the tester to “PASS LOOP” which will rerun the RAM tests as long as the outcome is a PASS. This chip was now showing failures after a few cycles.

I replaced the RAM and retested, all RAM is available once again.

Commodore 64 ‘breadbin’ repair log #1

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Aug 032011

Brought 5 broken C64’s back from the RCM to try and ressurect.
This one just booted to a black screen.

On my initial checks I found that the 6510 CPU did not have a clock. The clock for this is outputted from pin 17 on the VIC chip. Although this was getting a clock input, it was dead on the output and was getting very hot. I replaced this with a known good one and fired it up again. Still got a black screen but clocks were now present.

A black screen is commonly associated with a dodgy PLA. I removed the PLA and replaced it with a known good one and the C64 now loaded up.
Next problem, there was no cursor and the keyboard would not type anything. These are handled by IC U1 which is a CIA. Again, I removed this chip and fitted a known good one.

The Commodore now fires up and works fine.
One down, 4 more to go (and a VIC20 but dont have a PSU for it)

Commodore 64c repair log

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May 282011

The data cassette player would not power up and there was no sound outputted.
There is a big 1.5A fuse inside labelled F1. This supplies the +9v to pin 28 of the SID chip and is also used in powering the cassette unit.
Replacing this fuse brought everything back to life.

The C64c boots up fine without this fuse. I have been using an SD2IEC adapter to load games and didn’t realise at the time that the cassette unit was not working either.