C64 Keyboard Help

[eight088]

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

 

[DistantStar001]

Checking continuity on one of my own C64s, and given that the schematic shows a direct connection between pin 13 on U1 and pin 8 on the keyboard connector, I'd say you've found your culprit. So I think you're going to have to remove it. That said if you have a mouse (even an Amiga one will work), you could connect it to either of the joystick ports and see if moving it makes any characters appear on the screen. Pin 13 also connects to pin 4 on port 1. In fact, U1 connects to pins 1-4 and 6 on both ports. So you may look for shorts there. I would also look to see what happens on Pin 13 when you press any of the functional keys. Does it stay low or go high?

The other (I'm assuming unresponsive) keys could be due to deterioration of the carbon contacts. You could use a multimeter to check resistance/continuity across the pads, but then, that wouldn't pull your pin low. Even so, if you do find bad pads, a No. 2 pencil can bring them back to life.

Unfortunately, a faulty CIA will mask other problems with the keyboard, and the only other diagnosis I know is to swap CIA locations. But that would mean removing both chips.

 

[Timo W.]

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.

You can also cut the pin. If the CIA is faulty, you have to desolder it in the end anyway. If not, a blob of solder is enough to re-attach the pin.

 

[DrAM19]

One time recently, rather than cutting the pin, I desoldered it and then pushed it slightly so it was in the center of the through-hole and not touching anything. Verified no continuity, and then carried on with my testing. Saved me from cutting the leg on a good chip.

 

[eight088]

Thanks for the suggestions guys. I was able to replace the CIA and it is now working! The keyboard has some keys that aren't working very well, so if you have suggestions on how to clean that I'm all ears! The main key not working, and rather annoyingly, is the enter key. I've cleaned the PCB with IPA, and tried cleaning the carbon contacts with paper (which I've done in the past on a PET with good success) but still no go. Do those pads get to a point where they're just worn, and can't be revived?

Cheers

 

[DrAM19]

I've used a pink pencil eraser on the PCB for the keyboard with good success where IPA was not sufficient. Very very very fine sandpaper if that doesn't work. You can also get a conductive paint for the key plunger. I've never done that but have seen videos of people doing that.

 

[DistantStar001]

One time recently, rather than cutting the pin, I desoldered it and then pushed it slightly so it was in the center of the through-hole and not touching anything. Verified no continuity, and then carried on with my testing. Saved me from cutting the leg on a good chip.

I would start by checking the resistance of the plungers for the keys that work against the keys that don't. If they're different, then that's likely your problem. Generally, the non-working keys will have a higher resistance than the working ones. A decent coat of graphite from a no. 2 pencil should bring them back to life.

Be careful about abrasives on the PCB. Some boards have simple metal contacts, while others are coated in a carbon material. Abrasives like erasers and sand paper can remove this coat and make your problem worse. IPA and other solvents should also be avoided on these boards for much the same reason.