Try including the preamp board part number "206-0251-A" in quotes when searching.

Another Gallien-Kruger amp uses the same preamp, evidently:

https://elektrotanya.com/gallien-krueger_700rb-ii_sch.pdf/download.html

Assuming that is correct, I can see that, as I suspected, the Return is a switched jack. If you are getting signal through the amp with nothing plugged into Return, that means signal is flowing through the connection in the jack. If you can't insert signal by plugging into the jack, something is wrong there. 

Like if the Return jack's switch is somehow not opening as it should, then your input has a low impedance path back to the output of the U5-A op-amp that drives Send.

Or do you mean that there is no sound out of the amp: but the last confirmed externally observable output where signal is seen is the Send jack?  I.e. the Return jack worked before, but you only touched volume pots; so the jack is probably fine.

I had to change the volume and master volume pots. The old ones were difficult to remove and I may have nicked the pcb doing so. I can get sound out of the fx send jack, but nothing when I plug into the fx return. I would like to verify signal passes thru the master volume to the fx return. But although I can find plenty of schematics for the power amp, I can't find anything for the preamp.
Can anyone point me in the right direction?

Quote from: b-slayer on August 10, 2026, 02:13:31 AMI replaced it by the same values component so it'll be OK I think

Still, you are sensing that something is different; amp volume is not where you remember it to be for the same amp control settings.  Double check the work: all the solder joints. If anything looks cold, reflow it.

When we have a 0.1 Ω resistor, and a cold joint adds another 0.1 Ω, we now have 0.2 Ω which is a big deal. Whereas if it was on a 1 kΩ resistor, it wouldn't make a diff.

So I went back and looked at my signal generator remembering that it has a DC offset control

I set everything up using that and like clockwork using the DC offset I was easily able to make "the AC go up and down"

(Provided I had my scope on DC coupling and also the DC movement was kinda crappy only about 5V each way)

I remembered having trouble with this and sure enough I was told I had the scope coupling wrong

https://www.ssguitar.com/index.php?topic=5281.msg41307#msg41307

Back then those settings didn't mean too much even if I learned what they did it wasn't the same as experiencing it 

Nor did I imagine like now that probably when your scope is on AC coupling it runs the signal through some capacitors and the DC can't make it through (ya think)

Which is another thing oh yeah noobs read "DC can't go through capacitors"
So yeah we write that down and remember it and swear in blood yeah it's a rule  "
But until I had real stuff on the bench it was just a bunch of concepts that well these other guys they know all this stuff and so should I

So now when I see the switch on the bottom of the scope probe BNC it actually means something     

But hey all good in the hood  I'm gonna be able to set some more stuff up and then see what I get with a meter set to AC then DC and then on the scope and get this sorted out 

Another thing yeah the rich get richer it all goes hand in hand because maybe now I could take the lid off my signal generator and see a 741 and a DC source and so on and possibly take notice of the different sections of the terrain 

I would never take the lid off my scope but maybe look at a schematic and see the inputs and where the capacitors are for the AC coupling 

(I know what you're thinking.....yeah brah that's awesome only took ya three years to figure it out)

I replaced it by the same values component so it'll be OK I think

Btw I'm still in awe that that worked 

It's gonna be a while me trying to figure that out   

Reminds me of the time I was in a pickle 🥒 because I need negative voltage on a pin of a DIP I was freaked out and you were like "dude..what's the problem?!?!"     

Holy cow  wow ok

Yeah I can understand your concern that does sound pretty messed up like I connected it to the power cord 

Btw this raggedy a$$ house is all jacked up

I don't think any of the three prong outlets are actually grounded 

One time I looked it up and I think it said measure AC from the ground socket and the neutral/common
If you see a difference there's a problem 

Bias doesn't change the volume of the amplifier. The amplification level of the power amp is set by feedback resistors R165, R100 and R101.  There are two on the bottom (R100 and R101) because of the mixed-mode current feedback: the speaker current returns to ground through the 0.1Ω R104 current-sensing resistor, which generates a small voltage corresponding to how much current is flowing through the speaker. This is mixed into the feedback.

Ah, speaking of which, current feedback has a big effect on the gain of the power amp; it is sensitive to the value of the current sensor R104.

You mentioned above that you replaced this!

If the original R104 is 0.1Ω but you happen to substitute a higher value like 0.47Ω, that will drop the volume, mainly by scooping the mids more, due to feeding a larger current feedback signal. More negative feedback means less gain.

Great.  Just to clarify, what concerned me was the use of the word 'neutral'.  That is a very specific term which is only used for one side of the mains AC line.  As soon as you use that word, it raises concerns about electrocution as it is one side of the 'live' AC coming out of the wall.

So I think you probably meant the (-) or 'common' terminal of PACO.  'Common' is the best word to use for a reference point that is not necessarily tied to ground.  Like the one you just made that is floating on DC.

Dude IT WORKS  

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At first it "didn't work" because I had the scope on AC but then I switched it and

LIFT OFF

sorry about the flickering scope display it literally drives me BANANAS


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R1 value (22k)  randomly selected