A colleague of mine has an Aria Sinsonido silent bass guitar. For those who have never seen these things this is what they look like: http://www.ariauk.com/aria-as-691b-bass-guitar/ The pickup is a described as "Original Stereo Mic in Tube", and there is a volume and tone control.
In this case the tone control does not work. A local guitar shop took a look, and passed it to their electronics person. He looked at it and concluded the circuit board was damaged, but was unable to source a replacement or repair it. It's a small circuit board with what look like two ICs, a bunch of caps and resistors, and the pots soldered directly onto the board. If I can figure out how to post a picture I've got a decent pic which shows the components.
It's a good travel bass, still playable, and would be worth trying to repair, the question is where to start? Do we try to see if the whole board is dead, is there a suitable design that can be copied and fitted in? I'm long out of practice with electronics (like 30 years), but would be happy to have a go at building a new board. In fact I would prefer that to trying to repair the old one, as the old one still allows the guitar to be played

Many thanks as ever,
Rich