I've got a 1990's Fender Blues Deville 410 that's been in the family for a while. As part of a quest to make the volume a little more manageable, I've been wanting to put a little volume "pedal" in the effects loop. However, that's where I started running into problems. It's been a bit of a journey but I think I've addressed some issues. I'll try to put everything in a succinct format: When I first got the amp from my dad some months ago, I was unable to get any sound when patching a volume pedal from the preamp-out into the power-in jacks Soon after, the amp started having issues with it switching to the gain channel on it's own and no sound coming out. Eventually, this increased in frequency until it basically sat in the no-volume-gain-channel state whenever turned on. I'm not sure if it's helpful, but I'd typically get it to work by flipping the standby switch back and forth a bunch, but eventually, that didn't even help. I did a bunch of research and figured out the reverb, channel switching, and Effects loop were all connected by a set of sandblock resistors that have a history of going bad from heat. My plan was to replace these. Before I tackled the amp, I practiced working on PCB by making some guitar pedals. Not too important but the amp sat for a long time Recently I got the amp on the bench, checked the caps, and opened it up. The two resistors I thought were going to be bad looked immaculate and the solder looked good. Just in case I reflowed those connections (looked a little dull) and noticed that the only "bad" looking soldering was for a resistor and a cap near the reverb ICs (C18 & R35). It looked chunky and away from the board, so I solder sucked it away and resoldered them. I turned the amp on and it slowly, but increasingly, created a loud farting sound. I turned it on and off a few times but it just got worse. The next day I noticed the second Preamp tube wiggled a lot more than the other, so I pushed it back up The amp turned on fine. 'Success!' I thought. Unfortunately the FX loop still didn't work. At this point I did some testing to get as much info as I could: Plugging my guitar directly into the Poweramp-in produces no sound. I was thinking this meant a faulty jack, but: Bridging the effects loop with one cable makes a very quiet distorted sound when playing. Not popping but like a distorted signal. However, it is so quiet that you have to turn the volume up all the way to hear anything. Turning on the drive channel with the volume all the way up attenuates the signal even more - basically unhearable Patching the preamp-out into the input of another amp creates sound like normal. The presence knob seems to have [roughly] its normal effect when bridging the effects loop Not sure if it's related, but the treble knob (normal playing, not using effects loop) seems to greatly change the volume of the amp in a way the other parts of the tonestack don't. Turning treble up turns the volume down. Now, this may have been true before but I don't remember it working that way I'm not sure where to go from here, and I'm hoping you all can at least point me in the correct direction. Thanks!