1200XL Select, Option, Start issue

[JGRAHAM2]

I originally had issues will all keys, but did the keyboard fix. All keys now work except for Start, Option and Select. Reset works. Could this be the GTIA?

[JGRAHAM2]

I found a spare GTIA and replaced it. No difference. I doubt it is the Mylar connections to the PCB because the reset works and it uses the same connector. What they have in common is they connect back to the GTIA. I tested connectivity between the carbon pads and the connector strips and it looks good all the way through, so it shouldn't be a broken trace.

[ACML]

Yes, START, OPTION and SELECT are on the same mylar node as RESET, but there is a few inches of trace down stream of RESET. The other three are on the dead leg of the trace. You could easily have a break between the RESET key and the other three,

[yorgle]

Use a short bit of wire to momentarily jump pin 14 with pin 11,12 or13 at the keyboard connector. That will tell you if it's the keyboard or not.

[JGRAHAM2]

Yorgle, that worked. It has to be the keyboard. Doing the RAM test now. Looking for broken traces.

[JGRAHAM2]

The keys do work according to the multimeter, but the resistance must be too high to be read by GTIA. Any idea what it needs to be?

Edited October 11, 2016 by JGRAHAM2

[yorgle]

Measured at the apprropriate pins at the connector, I would expect that resistance should be at or near zero ohms when the keys are pressed. The pads on the mylar can be stubborn. I've had good luck cleaning them with a pencil eraser. Another trick I've learned is to put a strip of tape over the top of the Mylar where the lines connect to the pcb so when the plastic key assembly is screwed on the little bit of extra thickness sandwiches the connector together with a bit more force.

[ACML]

Measured at the apprropriate pins at the connector, I would expect that resistance should be at or near zero ohms when the keys are pressed. The pads on the mylar can be stubborn. I've had good luck cleaning them with a pencil eraser. Another trick I've learned is to put a strip of tape over the top of the Mylar where the lines connect to the pcb so when the plastic key assembly is screwed on the little bit of extra thickness sandwiches the connector together with a bit more force.

I bet if you peel back the white silicon spacer on the mylar you will find a break on the mylar side downstream of the RESET pad going to the START, SELECT and OPTION conductive pads. Be careful, only remove as little of the white casket as possible as you may damage more traces. It is important to return the white spacer back where you found it after repairs.

Edited October 11, 2016 by ACML

[JGRAHAM2]

I used the eraser to clean the 3 pads, added a little of the conductive paint to the 2 pads and added the tape as recommended by yorgle. At this point I have a working 1200XL! Thanks to all who responded with suggestions.