I have been looking around here for a quick fix for an amp but found none so I tackled it myself.here goes.
I purchased a fairly nice DC300a on ebay (where else!) that was advertised as working.I was working but only one channel.The other was heavily distorted but no DC at the output,maybe 50mv.I removed the face to see what was up.Set up on the bench with a tuner as input .channel 1 was ok but channel two was no go. The 61061 pre-driver was really hot and bias was only .150 volt at the neg outputs and .080 on the pos.Offset controls had practically no effect.I was headed to the bootstrap circuit as most posts suggested but the working channel had me pause.voltage checks looked good for this area so i headed elsewhere.checking actives with the good ol sencore super cricket revealed a problem with lo gain at the positive driver and sure enough after pulling it the leakage BtoE was the prob.Subbing something on hand ( RCA 2N3442) fixed the prob.While i was in there i changed the other driver too.Still heavily distorted.Changed the uA739 (had three of them in the parts drawers)and it cleaned up a bit and offset now had some range on the neg half but very little on the pos.Looking around,a small signal transistor appeared to have been changed at some time in the recent past.It looked very shiny and not dulled as many years in an amp will do. I checked it out and it read very good for gain but showed up on the super cricket as a PNP.I still didn't suspect anything at this point so I kept that in the back of my mind and kept going.Brushing a finger against two traces near the 1st transistor following the IC cleaned the signal up completely.I looked to see if i could figure out what was going on. If I turned the gain up it distorted again.Got the schem from this site and downloaded it for a look.The new kid was supposed to be the pos. drive limiter and also was supposed to be an NPN.Idecided to remove it and power up the amp with no load just a scope on the output.It was fine.At this point i was able to get bias in the range of .300 on both pos and neg halves.Sticking in a 417-801 salvaged from the heathkit days finished it off.I checked with the dummy load and had decent limiting but I think the heath trans has too much gain for this job so i will look for the correct sub and finish the job right.The two traces mentioned before had small inset pins on the component side and are for the mono strapping i will assume so I was probably just injecting signal at a point where it really wasn't supposed to be.Just goes to show you,It appears that someone was on the right track but was hesitant to unsolder ,unscrew and test the driver to be sure what was going on.Thinking that the limiter may have an effect on the hot pre-driver it was swapped out but with the wrong type.I have to say this was kind of a tough dog as it had been FIXED already and was supposed to work.I can't *beep* as i really got it for peanuts.Getting it back to it's old self was rewarding too.It's playing now as I type.WFUV from here in the Bronx.NO........ we all don't do crack and shoot each other!