Gretsch G6162 help

[Jazz P Bass]

I really am not sure what to make of the measurements.
If the no signal input is truly 0.050Vac, that is 50 millivolts.
That is 10 times too much acceptable hum.
The '1KHz input at full volume= .8Vac' has me confused. Is that a misplaced decimal point?
0.8Vac indicates what? The amp is not working.

[davohilts]

I really am not sure what to make of the measurements.
If the no signal input is truly 0.050Vac, that is 50 millivolts.
That is 10 times too much acceptable hum.
The '1KHz input at full volume= .8Vac' has me confused. Is that a misplaced decimal point?
0.8Vac indicates what? The amp is not working.

Okay, I've retested using my fluke dmm instead of an old led student model that I thought was accurate measuring voltages.
I got much different results.
No input voltage is .037 VAC (still too high)
1 KHz Input with volume at 0 is .06 VAC
1 KHz Input with volume dimed on Standard Channel is 11.35 VAC
1 KHZ Input with volume dimed on Reverb/Trem Channel (both verb/trem switched off) is 5.5 VAC
This lower reading supports the lower gain of the Reverb/Trem Channel.
So it seems I either have a bad ground or cap somewhere and I'll bet it is in the Rebverb/Trem side which would account for the lower gain on that channel?
The work I did on the tone network really increased the gain for that channel, I'll poke around the tone stack and see if I can see anything there.
Sorry for the bad readings, I guess I should toss the old student model meter.
Dave

[davohilts]

I've been working on trying to reduce the hum in the amp by replacing the feeds to the board from the inputs using shielded cable grounded on one end. this has helped a little and maybe that is what this is going to take a bunch of 'little' improvements until the hums drops to an acceptable level. The tremolo has been problematic in that is works then stops. Right now it is not working. I can hear the trem oscillating very slightly and have the voltage changes happening on V1 pins 1-3. I can follow these thru the caps and resistors to the footswitch and then on pin 3 of the output tubes I get a changing voltage but without the volume change thru the speakers or on the scope.
I've changed tubes around to see if it might be a tube to no effect.
Anyone have a suggestion on how to trouble shoot this further??
Thanks
Davo

[52 Bill]

I can hear the trem oscillating very slightly and have the voltage changes happening on V1 pins 1-3. I can follow these thru the caps and resistors to the footswitch and then on pin 3 of the output tubes I get a changing voltage but without the volume change thru the speakers or on the scope.
I've changed tubes around to see if it might be a tube to no effect.
Anyone have a suggestion on how to trouble shoot this further??
Thanks
Davo

The trem is spread across halves of 3 tubes. Half of V1 is the oscillator, half of V2 is the modulator and half of V4 where the signal is supposed to be modulated. If the oscillator is running ok and you can follow the oscillator output to the input grid of V2, see if the cathode of V2 is reacting to the oscillator signal. If it is, check to be sure that the cathodes of V2 and V4 are connected together. I could be a loose wire, or a dirty tube socket, etc.

[davohilts]

Well, I have made some progress!
I got the trem circuit working without any issues. What I discovered was the footswitch was grounding out the tremolo. When I removed the footswitch from the chassis the tremolo began working. New switch and bingo, works perfectly!
Now on the hum, I have gone thru the circuit resoldering grounds, replacing any suspect resistors thinking maybe they were contributing to the noise, looking at lead dress, moving wires around ect... all without any result. I then took a 500pf cap clipped onto some alligator clip leads about 24" long and clipped one end to ground and the other end I started touching various locations to see if I could quiet the amp. There are alot of these 500pf caps sprinkled around the circuit, some shown on the schematic some not. Well, when I got to a shielded cable leading from the #2 pin of V5, it runs from the tube under a couple of circuit boards and connects to a 100K resistor going to ground and a 270k resistor going to the #6 pin of V4 through a .02 mfd cap. When I took the 500pf cap from the intersection of the 100K resistor and the #2 pin of V5 and took it to ground the background hum dropped markedly. I connected up my voltmeter to the load resistor on the output and the voltage dropped to .01 VAC. So, I thought I had it solved and soldered in the 500pf cap. I turned on the amp and the hum was back to .05 VAC. I took the cap out, soldered it to 24" of twisted 20 ga wire, protected it with shrink tube and resoldered the wires back into the circuit. The voltage at the output dropped back to .01 VAC.
Now what the heck would cause this to work? The one unknown on the schematic is the rating of the cap between the pin 6 of V4 and the 270k resistor, it shows a cap but not its value. What was in the amp was a .02 mfd ceramic cap, I replaced this with a new mallory cap thinking that is was leaking but this had no effect on the hum. I replaced the 100k resistor going to ground from the intersection of the shielded cable from pin 2 of V5 thinking it was a noisey resistor but this had no effect on the hum.
Anyone have any ideas???
Dave

[52 Bill]

If grounding the cap only works through the wire, you are probably dealing with ground loops. Too many grounding points throughout the amp creating multiple return paths.

Due to the construction method of this amp, it will not be easy to fix. I would start by trying to identify all of the places that the different sections of the amp connect to the chassis ground. Also check any shielded cables that are grounded at both ends. Unless the shield is meant to be the ground connection between two points, the shield should only connect to ground on one end.

[davohilts]

Yes, there are about 30 places where the individual break out boards ground to the chassis, this is going to to a monumental task to eliminate them but I will give it a try. None of the shielded cables are grounded at both ends.
I sure thought it was weird that the long wires on the cap dropped the hum but mounting the cap directly to the board didn't help.
I'm getting closer and closer, everytime I do something inside the amp I run across another issue, but hey, I'm learning a lot!
Thanks for all your advice and help.
Dave