IBM Model F XT Keyboard - Hardware Troubleshooting

[hupshall]

Are there any good troubleshooting steps for the Model F Keyboard's electronics? I've been given one with unknown history that appears to be broken.

I've disassembled it to deal with corrosion on the top and bottom plates and right now, i've got the bare pcb on the bench. I've plugged it into an arduino running a program which is outputting the keyboard data line to the serial port. I know this program works because I have plugged my functioning model F and that does what I expect (passes BAT and sits quiet unless I press keys and then I get the keycodes)

This particular board, the BAT is always failing with a different byte code and there is always data on the data line like many keys are constantly depressed.

Any pointers or should I just give up?

The keyboard would clean up nice otherwise.

 

[ibmapc]

Make sure the grounds are sound! One is attached via a screw on the pcb and the other is at the berg connector. IIRC, it won't function at all without the ground at the berg connector, but will display odd behavior if the screw terminal is disconnected.

 

[hupshall]

I have continuity of ground from the arduino through the din and header connector into the pcb and then also through the screw that holds the pcb to back plate. There is no difference if I leave the cable shield ground disconnected or connected to ground through the screw.

 

[ibmapc]

... i've got the bare pcb on the bench....

Have you tried it assembled? I read another thread somewhere someone hooked up a bare model F PCB and got strange results. The model F is a capacitive keyboard and relies on the "flippers" to establish a baseline capacitance to work from. The capacitance changes when the flipper at a particular location is moved and that is what the controller senses to indicate a key press. So, if you run it disassembled, it might be seeing "stuck" keys or other anomalies. Just taking stabs in the dark here.

 

[hupshall]

Yeah, unfortunately I have - and it behaved the same way - and I took it apart again to see if I could figure it out. I'll clean all the flippers and pcb with isopropyl alcohol and reassemble it and try again - but I do have a feeling there is something electronically wrong

 

[hupshall]

Keyboard reassembled and it's still behaving the same way -

 

[evildragon]

Cleaning the flippers destroys them. They are coated with something for the flippers to be capacitive. By cleaning it with isopropyl alcohol, you've probably wiped it off.

 

[hupshall]

The flippers are impregnated with carbon so cleaning them IPA (or at least this version of the flippers) didn't do any harm. I ended up finding a great project which had plans for a DIY controller that works with the model F capacitive pcb. I built one of those and at least have it running via USB now.