Assy 250469 Rev. 4 #2

Still a black screen. I then tried loading the cartridge game Jupiter Lander as this particular game bypasses the Kernal ROM. If it works, something may be wrong with the Kernal.

Assy 250469 Rev. 4 #2 repair log

Still a black screen so the Kernal seemed to be fine. I then went over my previous solderings and I spotted two shorted pins. If looking closely at this previously shown image, the fault was created while replacing the first RAM IC at U11. However, the second RAM IC (U10) was still getting really hot, so I’m pretty sure it was burnt in the first place so removing it had to be done anyways.

Commodore 64 Assy 250469 Rev. 4 with two shorted pins on a RAM module.

The mistake was easily fixed with a soldering iron.

Commodore 64 Assy 250469 Rev. 4 with two shorted pins on a RAM module.

…and back came the sweet blue screen. So the MPU may not have been broken after all. Note to self, always check everything before moving on as faults may otherwise be created unintentionally…

Commodore 64 Assy 250469 Rev. 4 working but with no keyboard support

However, when testing the machine with a keyboard nothing happened. I tried another keyboard that I knew worked. Same deal – nothing.

Commodore 64 Assy 250469 Rev. 4 working but with no keyboard support

Ray Carlsen suggests that if cartridges work (Jupiter Lander did actually work at this point) but there is no keyboard access, the CIA (MOS 6526) at U1 may be the problem. I therefore removed the CIA IC using the same approach as above.

Commodore 64 Assy 250469 Rev. 4 working but with no keyboard support. Changing the CIA at U1 fixed it.

Commodore 64 Assy 250469 Rev. 4 working but with no keyboard support. Changing the CIA at U1 fixed it.

Commodore 64 Assy 250469 Rev. 4 working but with no keyboard support. Changing the CIA at U1 fixed it.

I soldered in another ZIF socket.

Commodore 64 Assy 250469 Rev. 4 working but with no keyboard support. Changing the CIA at U1 fixed it.

And the keyboard worked again. Thanks Ray!

Assy 250469 Rev. 4 #2 repair log

As a final step I tested everything with a ’64 Doctor diagnostic computer test’ cartridge.

Commodore 64 Assy 250469 Rev. 4 repaired. 64 Doctor test cartridge

Commodore 64 Assy 250469 Rev. 4 repaired.

Everything worked and the machine had been brought back to life.

NOTE: ZIF sockets were installed during the repair job as this board was later used for ‘The ZIF Socket Mod’ as described here (link).

© breadbox64.com 2016

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