Fender Rumble 500 Power Supply Issue

[g1]

First I would try disconnecting the jfet Q3. Either of the legs connected to R42.

[nevetslab]

I don’t recall their pot details. On larger dia pots that have the mechanical terminals riveted to the resistance wafer, I have had to generally tap the rivet between a suitable center punch pin mounted in a vice and further flatten the other side of the terminal revit to restore solid mounting and restoring conductivity.

[Bryan]

I will try disconnecting Q3. Is it just a switch shorting out R42?

I know what you are saying about the rivets. These pots were split cleanly right at the wafer. They must have been hit somehow.

[g1]

Q3 shorts R42 but brings the signal line much closer to ground, so may be acting as a mute.

[x-pro]

... I get nothing at Pin 8 on U3

????
This is the power supply pin U3.

Is there a reason I only get signal on one side of C18? I get signal between R37 and C18 but not C18 and R43.

What signal is being input? If the frequency is below 480 Hz, it will be attenuated. The normal signal level should be just over 100mV.

[g1]

????
This is the power supply pin U3.

U3 is TL074 (quad), pin 8 is an output.

[Bryan]

I'm using 100 hz. That's what it says in the schematic.
Anyway, I'm good on the Overdrive circuit now too. All is working. My bad resoldering on U4 pin 11.
I still get the balance shift on the rails when I plug in the fan. I get -12v instead of -10v. Maybe it is a bad zener D26. But it is now making much better sounds than when it arrived.

[Bryan]

Thanks for all of the help. The amp is up and running. I still have a question about the grounds. Gnd-Comp and Gnd-C both equal Chassis Ground. PGrnd Is about 50 ohms to Chassis Ground. It looks to me that PGnd is strapped to Gnd-C. Is there a reason for the difference?

[x-pro]

R254

[g1]

R254

But P103 pin 2 is shown as connected to chassis at 125ASX2 end. Or are we talking about GND-O ?

[Helmholtz]

Check if there's a voltage drop across R254 with everything (including fan) connected.

[Bryan]

You just solved all of my problems. I had the board out and didn't connect the chassis grounds......always had an offset voltage reading because of R254. Everything is perfect now. Main problems were broken R23, R51 and the input jack. All had taken a hit. The rest was me chasing my tail. Thank you to everyone.......................I hope I do better with the Acoustasonic 150 that I am about to look at. Same friend different amps.