Gretsch G6162 help

[davohilts]

Hello,
I've been working on a G6162 amp for over a year trying to bring it back to life. This amp is vertically laid out with 2-10" speakers and the power/preamp chassis above, not the one with the power section in the bottom and the preamp above. The tube layout is 5U4, 2-6973, 5-12AX7. I've done the following:
1. replaced the 3 cap cans
2. replaced the electrolytic caps(35/50v)
3. replaced out of tolerance caps and resistors
4. resoldered most connections new input jacks
5. new tubes, came without any

I have the amp up and running but have very weak channel 2 (reverb/trem).

Connected to a scope I have the following:
1. Channel 1: nice strong sine wave with slight drop between high and low gain inputs
2. Channel 2: very weak sine wave with almost no wave on the low gain input.
3. When I turn on the tremolo at the pot switch I get no oscillation at all.
4. When I turn on the reverb at the pot switch I see the weak sine wave drop to almos a flat line as I turn up the reverb.

At this point I've reached a dead end and don't know how to try and troubleshoot this issue. Any help would save me from pulling the rest of my hair out.
I'll attach a schematic with my voltage reading written in and a gut shot of the chassis.
Thanks in advance!
Davegretsch g6162001.pdf

[52 Bill]

Where are you scoping the signal? If the plain channel works, everything from the second half of V4 to the speaker is okay.

If you inject a signal at the top of the volume control of the FX channel (just in front of the tone circuit) does it sound loud and clear?

Unless there is a mistake on the schematic, the reverb circuit is very strange as the control at the front of the reverb driver controls both clean signal and the reverb drive level. How are the panel controls marked? With the reverb switched on, when you shake the tank do you hear it?

The trem works by modulating the signal at the cathode of the first half of V4. You could try temporarily disconnecting the connection from the trem circuit to see if it is killing the audio signal.

[davohilts]

Where are you scoping the signal? If the plain channel works, everything from the second half of V4 to the speaker is okay.

I'm putting a 1 kHz signal from my cell phone app into the input of each channel, I don't have a signal generator.

If you inject a signal at the top of the volume control of the FX channel (just in front of the tone circuit) does it sound loud and clear?

I will see if I can figure out how to do this. If I take a cable out of the phone (my signal generator) would I ground one lead and connect the other to the center terminal of the volume pot?

Unless there is a mistake on the schematic, the reverb circuit is very strange as the control at the front of the reverb driver controls both clean signal and the reverb drive level.

I agree it is strange, there are some many connections and tag boards it is really hard to trace a circuit, they run all over the place.

How are the panel controls marked? With the reverb switched on, when you shake the tank do you hear it?

The controls read from left to right: Standard Channel (high and low inputs), Tremolo-Reverb Channel (high low inputs), Tremolo Speed pot and Intensity pot (also on off switch), Reverb intensity pot (also on/off switch). I can hear the tank if I shake it.

The trem works by modulating the signal at the cathode of the first half of V4. You could try temporarily disconnecting the connection from the trem circuit to see if it is killing the audio signal.

Would I do this by disconnecting at the cathode pin 3 of V4?
Thanks for the help. This is such a complicated circuit, there are tons of 500pf caps going to ground that are not shown on the schematic.

[52 Bill]

Would I do this by disconnecting at the cathode pin 3 of V4?
Thanks for the help. This is such a complicated circuit, there are tons of 500pf caps going to ground that are not shown on the schematic.

If you hear the reverb tank when you shake it, it means that the circuit at V4 is passing signal, so you are back to either V2 at the input or the wiring at that first control before the reverb driver.

All of the 500pf caps may be there to stop any oscillations due to the rat's nest construction.

Please list all of the controls or post a photo, because I still want to know what that first control is supposed to be. It may not even be a panel control. Are there any non-panel mounted controls in there?

[davohilts]

got the trem oscillating but speed control doesn't seem to have any effect, could be bad pot or connection somewhere.
I'll attach a pick of the face of the chassis. there is a footswitch jack for each effect.
I tried tracing the signal from input and I get a very strong signal at pins 1 & 6 of V3 but loose it from there.
Also the volume control for the effect circuit is a double pot with two blue/white wires going off to different places. Only one goes to the 270K/68K direction as shown on the schematic.

[davohilts]

here is a detail of the two volume control wires and where the go.
I injected a signal at these points and got signal on the scope from the wire closest to the volume pot, not a strong signal but it matched the signal on the standard channel.

I'm beginning to suspect the reverb transformer may be weak or something associated there is not right.

[davohilts]

I have my scope connected to the load resistor I have installed on the output transformer wires that would go to the speaker. I'm injecting the signal thru the input jacks on each channel. I injected a signal at the center tap of the volume pots and got equal sine waves on the scope from that point in the circuit. I don't know if that is something that should tell me anything. Should I be connecting the scope somewhere else. I don't want to fry my scope with high voltage so I'm staying at the output point of the transformer.

[52 Bill]

Okay, a double ganged volume pot. That makes sense now. Having the scope on the output load resistor limits the information you can get from the circuit. If your scope doesn't have one, you can always use a high voltage rating cap to protect your scope input. You're reading ac signal not dc voltage, so a blocking cap will keep the dc from getting to the input.

Interesting that the footswitch for the reverb opens the line to the reverb tank, leaving the transformer with no load. Test the drive transformer if you think it's bad.

If you get a strong signal at pins 1&6 of V3 then your first stage amp at V2 is working and you get reverb noise when you shake the tank, that leaves the problem to the wiring around the reverb pot going to V4.

[davohilts]

I just couldn't find an issue with the reverb circuit so I thought I would go in and replace all the ceramic disc caps in the 'tone circuit'. I replaced them and checked the values of the resistors out of circuit and put it all back together and tested it and the gain out of the channel has increased markedly. It is still not has high as the normal channel but I think that is as good as I'm gonna get.
I appreciate all your help in guiding me through this. I have a question about my scope, how can I tell if I have a high voltage cap protecting it from high dc voltage? could I just alligator clip one in from the test lead to where I'm connecting to the circuit?
Thanks
Dave

[davohilts]

Well as they say, 'hold the weddin'. I put the chassis back into the cab and plugged it in and I have a constant hum/hiss which is unaffected by any controls. It isn't terribly bad but I would say that with the new filter caps that it should be much quieter. I connected up my scope again to the load resistor I have across the speaker leads and cranked it up to enlarge the waveform of the hum/hiss without anything in an input jack. I took a pic of the waveform without any input and a photo with the 1 kHz signal adjusted down to fit the screen. I've checked all the grounds with out any change, I've paralleled in a 20mfd 500v cap with all the filter cap connections looking for a leaking cap without any change. Does anyone have a suggestion as to what to do to try and trouble shoot this issue.
thanks for any help.
Dave

[52 Bill]

And if you pull the chassis does the hum and noise go away? It may be that you have something that has come loose or is touching something and needs to be found. Or it could be a ground loop caused by metal shielding in the case.

[davohilts]

No the hum is there in or out of the case. The sine wave is made up of several lines which form a wave when I get it dialed in. I wasn't listening to the amp, just using the scope so I didn't realize the hum was so loud until I installed the chassis. It isn't horrible but would not cut it as a recording amp. It is stuff like this that makes my lack of overall engineering knowledge very frustrating. I don't know really where to go from here.
Dave

[Jazz P Bass]

I usually have a DVM hooked up to the dummy load, along with the scope.
The DVM set to read Vac will 'show' any hum. The scope is usually set to read (div/volt) a much higher voltage so unless you crank up the div/volt it will be missed.
And a final test with a speaker is always good.

[davohilts]

Thanks! I've just started using a scope. I thought oh boy, no more issues, no need to listen, just scope it and I'm good to go! I guess there is no substitute to listening since that is what you do everytime you use the amp!
Dave

[davohilts]

Okay, I connected my dmm to the dummy load (not my head) and have the following to report:
Scope set to 2V/div
DMM set to lowest AC range (2)
with no input I read .05 VAC
with 1 KHz input and volume full on amp I read .8 VAC
with 1 KHz input and volume at zero on amp I read .08 VAC

So does this seem normal or should I get 0 VAC without any input signal?