Hello All! Thanks for all the help I've had so far on this great forum. This has been a great resource for my amp for the last decade!
My friend recently got a 112 65 with the PI tube for cheap, but it was distorting badly at low volumes (sounded ugly, maybe crossover distortion). I replaced the tubes, same story. While in there, I noticed that the electrolytic caps were all old, except for one which was way out of spec. I replaced all the electrolytics, and the distortion improved dramatically.
But the amp is still crunchier than what I'd expect. I have a 112 65 model without the PI tube, and I'd expect these two amps to sound pretty similar, since their preamp circuits are identical. But my friend's amp seems to break up more easily, and also has a lower output volume at the same settings. I ordered replacement LM307Hs, and I'm going to try to replace those in case one got fried from the bad caps I replaced.
Any other thoughts on why this amp would be so crunchy? It sounds like it's in the preamp, because it increases when I turn up the channel volume, but the master volume seems to provide clean gain.
There's one other thing I noticed while working on this amp. When it's powered on (with standby on and with it off) the chassis buzzes. Not the speaker, but the chassis. It kind of sounds like it's coming from the output driver board. In addition, when I grab the power switch on the front when it's on, it vibrates a little. This seems really bad! I'm gonna hold off on working on this until I get some input, because it seems kind of dangerous! I've been really safe working on these amps, discharging all the caps, checking them, one hand deep in my pocket whenever there's power, but I'm too young to die!
Thanks in advance for the help!