Monday, July 6, 2009

Repairing a wireless keyboard

Last night, I sat down to use my home theater (a PC connected to a DLP projector) and had some problems typing in the name of a movie into Netflix. I thought I had mistyped the word, but I soon realized that my wireless keyboard had some non-functional keys on it. The letter 't', numbers 1 and 3 among other keys were not working. Since the failure affected more than one key, and pressing harder did not help the situation, I reasoned the problem was one of the matrix lines in the keyboard's circuit and not a bad connection in the button itself. The rest of the keys worked fine, so the microprocessor and transmitter were probably OK.

I took the keyboard apart and located the matrix lines between the microprocessor and the array of buttons. I used an oscilloscope to see which lines were pulsed (outgoing from the microprocessor) and which were switched (incoming to the microprocessor). I also compared the signals from the working "2" key to the broken "1" key. The signal was present but weak for "1". I tested the total circuit resistance for "1" and "2". "2" (working) was a couple hundred ohms, but "1" was 10k ohms. The circuit had a bad connection somewhere.
I used my meter to trace the bad connection to this spot on the flexible circuit board beneath the keys. It looks fine visually, but there is an electrical discontinuity right in the very center of this photo where the trace becomes narrow and passes above the middle rectangular cutout.

I used some conductive paint (marketed for repairing automotive defrost grids) to cover the bad area of the trace.

I was tempted to use conductive epoxy for the repair, but it can be very brittle and has low adhesion qualities. We'll see how the defroster repair product holds up.

1 comment:

  1. Do you know of a repair center where I can send my keyboard for repair?