Some of the symptoms I have experience are the following, and, these occurred even after I use contact cleaner on the potentiometers and push-buttons:-
- one channel cutout and remain silent after some initial low level crackling upon powerup
- crackling sound on one channel which seem to get louder and occur more often as time goes by
- volume on one channel which seem to get lower and lower over time
One option is to use contact cleaners. Spray some onto a cotton-type cloth of sorts and use it to rub against the contacts. Unfortunately I find the results does not last long. You would need to repeat the procedure about every 3-4weeks due to the effects of the hot and humid climate.
After some experimentation I now use a different procedure. A good quality pencil eraser is recommended. Give these copper contacts a good rubbing with the eraser. I normally hold the board in one hand and ensure only the contact end of the board are say 1cm are inside the edge of a table. Then use the other hand to deploy the eraser. This will ensure minimising the possibility of damaging the board, Once completed, please ensure no eraser leftovers are on the contacts before reinsertion into the unit,
My Quad 33 boards seem to require such maintenance every few months. One of my clients with a Quad 33 has the same issue and was quite surprise by the effectiveness of such a simple solution.
So save yourself a costly trip to the shop by giving the above a go before that.