Commodore 64 assy no. 250407 Rev. B repair log with a broken PLA chip at U17

Assy 250407 Rev. B #3

This machine is an Assy No. 250407 Rev. B long board.

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When I got this on my workbench nothing worked! Black screen and no light in the power LED at startup. Using a multimeter I tested the voltage on the two voltage regulators (7812 & 7805). No voltage here. I then measured the voltages at the power plug. The board was getting power, but somehow it did not get through to the PCB. I then applied a ton og contact cleaner to the power switch while working it up and down like 50 times. I also cleaned the rest of the plugs with the same contact cleaner (serial port, AV port, joystick ports, user port and expansion port). I removed all socketed chips (SID, VIC-II and the small chips surrounding the VIC-II chip) and used contact cleaaner on them as well. After drying all of the remaining cleaner with a blow dryer, I reassembled everything and turned the machine on. I now got to the BASIC screen but all the letters and numbers were in rainbow colors.

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Commodore 64 guru, Ray Carlsen suggests replacing U29 (74LS74 logic Divider) as this chip may cause the startup screen to show normal characters in multicolored “rainbow”.

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Replacing the chip did not change the rainbow colors. So I took a closer look a the PLA chip at U17 once again.

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I decided to remove the PLA, soldered in a socket and replaced the PLA with a more modern one I got on Ebay.

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It turned out that it once again was the PLA chip that were causing trouble!  The machine had been brought back to life!

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