5150 combo issue

[metalhead_harvey]

ok so it started with kind of a random pink noise in the background that varied in volume, I replaced the power tubes, no change, then the pre amp tubes, no change. So I opened it up and started checking connections, solder joints etc. I saw some questionable joints so i touched up all the tube joints re-assembeled and tested. The random noise appears to be gone... but now for the strange part.
When playing through the lead channel, the low EQ acts backwards meaning no bass at 10 and full bass at 0, but the clean channel is opposite (acting normally)... so I went back and inspected all of my work, touched up some more solder joints and still the same issue. any ideas? Thanks in advance!

[Alex R]

ok here's the tone stack schematic. As you can see the tone stack is used for both channels (I'm assuming it's not one of the later ones with separate EQ controls for each channel). But the clean post and lead post controls are switched by the relay K2b. If you have made a permanent connection between pins 5 and seven of K2b then when you switch the lead channel in, both post controls will be connected. That, or something similar, might have an effect like the one you describe.

You could check whether the post controls work correctly, or whether the one that should be switched out still has an effect - that would confirm it without taking the chassis out.

Basically I am saying, check the connections on relay K2.

[metalhead_harvey]

Alex thanks for your quick response, and yes it is an older model with a single EQ control for both channels. I checked the post controls, they appear to be functioning properly, although my Rhythm channel is a bit weaker than it used to be before any of my problems started. I pulled the chassis out again for another look, inspected the relay in question, tapped on it a few times to make sure it wasn't stuck for some reason, reassemled and tested again, still no change. this may be a stupid question but, is it possible that one of the capacitors in the EQ section(or anything else for that matter) may be acting up with all the gain of the lead channel? or any other ideas?

[Alex R]

you see it's a very unusual symptom, not the sort of thing caps usually do. I was wondering whether the slightly unusual connection to the lead post pot, through that 47K resistor, might be behind it. I was thinking also, the lead/clean post controls are the only thing that changes there when you change channels. So I'd be checking everything around those two controls and their connections to the eq circuit. I know you did this already!

There won't be too much difference in signal levels going through the eq from either channel. High gain circuits overdrive tubes but then throttle back the signal.

[Enzo]

Be aware metalhead has a 5150 Combo, not a 5150. They are different amps.

Due to the construction, I'd be thinking you might have snapped off a jumper wire. The board is divided into three main sections, and there are a bunch of short bare wire jumpers joining the sections across the gaps.

[Alex R]

Be aware metalhead has a 5150 Combo, not a 5150. They are different amps.

(blush) how true. Correct schematic for Peavey 5150 COMBO attached... I note that it doesn't have that 47K resistor on the ground side of the bass pot. Ah well...