Been recently having trouble with one of my own keys on my Indigo, it only played faintly compared to all the others. Opened it up several times, cleaned the contacts but no improvement.
Opened it up yet again, and on closer inspection when manually handling it it appears the rubber contact for that key had a tear around the contact itself. Consequently once the key is pressed the rubber contact won't return back down into position afterwards properly as it should, instead the contact remains partially up, as you can see:-
Flipping the rubber contact strip over and very gently spreading the rubber with my finger, you can clearly see the tear:
The contact was barely connected to the rubber strip to be honest, a consequence of the age of the keyboard and the amount of use I think.
Looked at other contacts and noticed a few others showing signs they were going the same way, so I decided to replace the whole 37-note key range with new strips.
The Indigo uses rubber strips grouped into 12 keys (one octave) per strip, with the right-most octave being a 13-key strip as it includes the top C.
They're seemingly hard to source! I tried
http://SynthesizerService.de (Access' hardware service company) but they didn't get back to me.
The easiest that I've otherwise found, that appear to match the keyboard contacts I needed, was from a
guy on Ebay from Turkey, of all places. So ordered 2x 12-note strips, and 1x 13-note strip.
Still currently awaiting delivery from Turkey/eBay.
Will let you know how I get on.