Fender Stage 112SE - Offset on input jacks causing pop

[Mars Amp Repair]

Hey guys,
I have this Fender Stage 112SE amp when you pull out the input jack plug the amp makes a really loud obnoxious pop. I replaced the high gain input jack as it was typically loose given the stripped threads & missing metal insert. This was an aftermarket jack. I subsequently replaced both input jacks with the originals to be certain there wasn't some weird incompatibility.
I've traced it down to what appears to be the voltage that appears on one of the SPDT internal jack switches that feeds the 'Mute' line which ultimately feeds muting FET Q9. There is a 1M resistor at Q9 to the -16Vdc supply and on the other end of the mute system at the Power amp jack it is connected thru a 1Mohm resistor to the +16Vdc line.
When a jack plug is inserted, there is a resultant 2VDC that appears on the signal line on the jack side of the input cap C1 to the 1st amp IC.
If I unsolder the mute lines from the jacks, the 2Vdc imposed on the input line is not present anymore. the amp is muted at this point but when scoping the signal at the speaker you can see that there is no 'pop' anymore w/removing the input plug.
Interestingly, there is no pop on the low gain input jack w/removing the plug (guitar connected or not). It actually acts like there is leakage in the jack itself from the muting line switch to the input signal line but replacing the jacks creates no change in the effect. I did not use the hi-gain input before replacing the jacks to see if it was doing this originally.
Anyway, see attached schemo. Unfortunately, this is the only source for this schemo I could find and wouldn't ya know it is the upper left hand corner that apparently did not copy well when the originator scanned it. Still you can pretty much figure out where the connections go.
Any ideas?
Thanx, Glen

[Enzo]

Stage 112SE.pdf

Perhaps a little better.

The voltage in the mute circuit should not get on the signal path at the input. Only way to do that seems to me to be if the jack solderwork or tracery gets shorted somehow.

[Mars Amp Repair]

[ATTACH]n945414[/ATTACH]

Perhaps a little better.

The voltage in the mute circuit should not get on the signal path at the input. Only way to do that seems to me to be if the jack solderwork or tracery gets shorted somehow.

Hello Enzo,
Thanx for the quick response. Yes I agree the voltage SHOULDN'T be getting to the input, however HOWEVER, it is. My first thought was the somehow the after market jack was weird somehow but moving one of the original jacks to the input position proved that wrong (was a goofy idea anyway, but ya know you get to that point after exhausting every other possibility.
I also removed the jacks & scrubbed the board underneath the jacks just in case there were something conducting a high resistance path to the input lead to no avail. It's interesting that the resultant voltage that gets to the input jack side of C1 is exactly 2Vdc and doesn't vary.
I can see where the resultant voltage that reaches Q9 the muting FET flips from +16V to -16V depending on the jack in or out. Perhaps those 2 supply voltages need to be perfectly balanced so there is no resultant voltage somehow insanely leaking to the input cap?!!! It just doesn't make any sense to me.
I have worked on these amps before & never encountered this symptom before. I'm at a loss. I even tacked in a 1meg resistor from C-1 to ground in case it were a very high resistance on the board or in the jack that was bleeding to C-1 to no avail.
Anyway I know it's always difficult to diagnose a unit when (as a car mechanic told me when I was younger before cell phones 'Well maybe I could diagnose it if you held the car up to the phone!
Thanx....glen
BTW, I was working for an appliance repair place just to make it to 70 and pretty much hating it. I had for the last 3yrs been doing repairs on the side at home and also for a dealer named '12th Fret' in Boise, ID (I live in a suburb now called Nampa. The owner of 12th offered me a position along with his son who is a younger tech and I took it. Better pay as well as no freakin' heavy appliances to lug around & crawl on the floor to repair!!
Funny how things work out. take care, g

[g1]

So you have 2V at C1, is there any difference at the other side of R3 or R4? This may show if the voltage is getting in via J1 or J2.
You said you unsoldered the 'mute lines' but there is only one called mute. The other goes to the PA In jack. If you unsolder those connections one at a time, what happens?

[J M Fahey]

This is the +16V mute voltage path around the input jacks,J1 and J2.

* With nothing plugged in, you MUST have +16V ONLY at pins 4 & 5 at both J1 and J2
YES/NO/what else?

With a plug on J2 you MUST have +16V ONLY at J2 pin 5 and nowhere else.
YES/NO/what else?

With a plug on J1 you MUST have +16V ONLY at at J1 pin 5 and J2 pins 4 & 5
YES/NO/what else?

If not, check tracks, wiring and solder blobs with a loupe.

OR:

actual Mute circuit is bad,

Check you have 0V on any FET leg (a sensitive meter might read some negative voltage on Gate), and that voltage at left of CR50 switch between about 0V (mute) and -16V (unmute).

[J M Fahey]

Somehow circuit around J1 J2 didnīt load, here it is, above post comments apply: