Commodore 16 Repair

Taking a break from pinball restoring to fix some old computers (next update a Vic20). I have had this C16 in the to fix pile for over 2 years, I suspected the CPU was bad but I didn't have a spare to confirm this. I recently got a Commodore Plus/4 in a job lot so was able to take the CPU from that and confirm my suspicion.

As nothing is ever easy this didn't quite fix the C16, it would only boot to basic about 1 in 5 times of power cycling the system. It would usually show a black screen or a screen of garbage. Even when it did boot to basic it didn't behave quite right, for one the reset button did nothing and loading from a cassette wasn't working as it should.

I followed the trace from the reset button to the other end of the board to a LM555 @ U10 but this turned out to be fine. Next to it is a 7406 @ U9, after replacing this everything worked as it should.