I know it sounds dumb, but getting something like DeOxit here in rural Panama (Bocas del Toro) is not easy. For other electrical things I have occasionally used WD-40 with some success. I have one more place to look for DeOxit.
In the mean time, I have some test results that would suggest it is not contacts, but I simply cannot explain them totally. With the amp assembled as it was from the factory, the R channel plays beautifully and the L is great for the first 10-15 secs and then fades to nothing (actually I can barely hear the music - badly distorted - at full volume). If I switch the wires going from the line level boards to output boards, the R sounds good, but there is a faint scratchy sound in the background that wasn't there before. The L plays very briefly (<5 sec) and then fades out. I have repeated this several times with consistent results.
From this, I would conclude that there is a problem with the L output board. It really does sound great for the few seconds it plays. What would come on like that and fade out? Maybe a capacitor? That is not the whole story, however, as with the wires switched, the good R channel has some background noise that was not there before, so is there maybe something going on with the L line level that damaged the L output??
