eight088
Experienced Member
Hi all
I'm currently repairing a c64, and the problem it has is when you turn it on, a normal screen is shown but a '2' appears as if it was pressed. Thinking the keyboard may be faulty I disconnected the keyboard but still have the same problem. The only other keys that work are 'HOME' '-' '0' '8' '6' '4' and 'F7'. Whenever you press one of these you get the correct response but it is immediately followed by '2' again. I'm thinking the CIA is bad, and using the scope I have found that PB3 (pin 13 of U1) is stuck low, but I can see that it is trying to go high but is stuck low. The above keys that work, are all on the PB3 part of the keyboard matrix.
Is this suspicion enough to suspect the CIA, or is it possible there is a short somewhere else and if so any tips on where to look? I would simply bend the pin out of the socket to test further, but this CIA is soldered direct to the motherboard, so just want to have a strong theory that it is in fact faulty before removing.
The board is a 251469, assy number 250425. Schematics 1 of 2 and 2 of 2.
Thanks
I'm currently repairing a c64, and the problem it has is when you turn it on, a normal screen is shown but a '2' appears as if it was pressed. Thinking the keyboard may be faulty I disconnected the keyboard but still have the same problem. The only other keys that work are 'HOME' '-' '0' '8' '6' '4' and 'F7'. Whenever you press one of these you get the correct response but it is immediately followed by '2' again. I'm thinking the CIA is bad, and using the scope I have found that PB3 (pin 13 of U1) is stuck low, but I can see that it is trying to go high but is stuck low. The above keys that work, are all on the PB3 part of the keyboard matrix.
Is this suspicion enough to suspect the CIA, or is it possible there is a short somewhere else and if so any tips on where to look? I would simply bend the pin out of the socket to test further, but this CIA is soldered direct to the motherboard, so just want to have a strong theory that it is in fact faulty before removing.
The board is a 251469, assy number 250425. Schematics 1 of 2 and 2 of 2.
Thanks