Commodore 64 Assy No. 250469 Rev. 4 repair log with a broken CIA (MOS 6526) at U1, two broken RAm IC's and a broken power switch. Read more on breadbox64.com

Assy 250469 Rev. 4 #2

This is an Assy no. 250469 Rev. 4 shortboard. The machine would start up with a completely black screen.

Assy 250469 Rev. 4 with two broken RMA IC's, a failing power switch and a broken CIA

Flipping the power switch was far from being smooth. When the switch was actually turned on, it would make the screen flicker when being touched. I therefore started by replacing it with a new one.

Assy 250469 Rev. 4 with a broken power switch. How to replace the power switch can be seen here.

The easiest way to remove a broken power switch is by cutting all its legs and then remove them one by one.

Assy 250469 Rev. 4 with a broken power switch. How to replace the power switch can be seen here.

I usually use soldering wick to remove broken parts and clean up the mounting holes.

Assy 250469 Rev. 4 with a broken power switch.

A fresh new power switch was soldered in to get the board a more stable supply of power. However, the the black screen remained…

Assy 250469 Rev. 4 with a broken power switch.

I then removed the SID (MOS 8580R5) from its socket,  swapped the VIC-II (MOS 8565R2) and the 8701 with known working ones to rule those ones out as well. Still a black screen. I then started touching the different IC’s on the board and noticed that the two RAM IC’s (M41464) at U10 and U11 would get really hot right after the machine had been turned on. I therefore started by removing the RAM chip at U11.

Assy 250469 Rev. 4 #2 repair log

Again the legs were cut and then removed one by one.

Assy 250469 Rev. 4 #2 repair log

Assy 250469 Rev. 4. The RAM IC at U11 was removed by cutting its legs off.

The capacitor at C48 had to be re-located on the underside of the board in order to make room for the ZIF (zero insertion force) socket that I decided to use instead of a standard IC socket on this particular board.

Assy 250469 Rev. 4. A capacitor at C48 had to be removed to make room for the ZIF socket.

Assy 250469 Rev. 4 with a broken RAM IC. New ZIF socket mounted.

The black screen remained, so I removed the second RAM IC at U10. I did not have an extra socket, so the IC was soldered directly into the board (I usually never do this, and do not recommend it either, but I had other plans with the board when it was working again).

Assy 250469 Rev. 4 with a broken RAM IC. New ZIF socket mounted.

Assy 250469 Rev. 4 with a broken RAM IC. New ZIF socket mounted.

Still a black screen. Next IC that was on my short list was the MPU (MOS 8500) at U6.Commodore 64 Assy 250469 Rev. 4 with a possible broken MPU (MOS8500).

Again, I started by cutting the legs, removed them and installed another ZIF socket. The ZIF sockets (MPU and RAM) had to be trimmed a little in order to fit.

Commodore 64 Assy 250469 Rev. 4 with a possible broken MPU (MOS8500).

Commodore 64 Assy 250469 Rev. 4 with a possible broken MPU (MOS8500).

Commodore 64 Assy 250469 Rev. 4 with a possible broken MPU (MOS8500). ZIF socket installed.

As I did not have a spare MOS 8500 at hand I temporarily swapped it with a working MOS 6510 MPU. I do not know if leaving in the older generation 6510 chip is something I would do permanetly, but for faultfinding it should be fine.

Commodore 64 Assy 250469 Rev. 4 with a possible broken MPU (MOS8500). ZIF socket installed.

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