Fixing a Roland PC-200 MIDI keyboard controller
Continuing in my series of keyboard repair blogs, I recently bailed an old Roland PC-200 keyboard out of storage. This old guy is just a 4 octave MIDI controller (no sound generator), but it fit the bill for having a reasonably sized polyphonic keyboard for a NYC apartment. Unfortunately it had long ago developed a problem with an F# key no longer working.
While there are newer controllers with many more buttons and polyphonic aftertouch, it seemed such a waste to replace it just because one key didn't work. After a little net research, a problem with many keyboards is that the carbon contact pads glued onto the rubber deteriorates. Replacements for these are available, but this was not a solution to the problem in this case.
I opened the case up and removed an octave worth of keys. The keys are ingeniously held by a spring, which is removed with a pair of needle nose pliers, and the keys lift off. The PCB has a molded rubber conductor strip which sits into holes on the circuit board, one strip covering an octave. To lift the rubber strip requires removing an octave set of keys. The white keys are numbered both by the chromatic name (A, B, C etc), and also the order in which they should be removed and replaced.
Lifting the rubber contact strip revealed the cause of the problem.
In manufacture, a pad of carbon is deposited onto the printed circuit board (PCB). This carbon pad contacts with the small carbon dot embedded into the rubber. There are four pads per key, the time difference between the rear pads conducting and the front pads conducting allows calculation of key velocity. Testing each pad with a multimeter showed a resistance of 60 Ohms, except the failed key, which no longer conducted.
To fix the deterioration of the carbon pad, I found an online supplier of conductive carbon paint. This comes as a 30g bottle with a paint brush.
The carbon paint was tested on an old circuit board first, painting between two metal contacts that I scraped bare. This requires painting a coat, then measuring the resistance with a digital multimeter, and repeating. Of course, as the carbon builds up, the resistance will drop. After several coats, the resistance of the damaged pad dropped to 94 Ohms, which when I tested with the key reinstalled, worked perfectly.