Fender red knob twin pops when using push pull pots...

[Metalmaster06]

My 1988 red knob twin is popping very loudly, even when at low volume, when any of the push pull pots are switched. There is some mild crackling when I twist the knobs, but its probably just dirty pots.

I picked up the amp broken while working at a guitar shop, and had a tech get it running for me as a trade for some guitar repairs I did for him.

My setup has my digital floorboard w/ preamp running into the power amp in on the back panel, so I don't typically use the preamp on the twin.

The amp has been moved quite a bit and i have a decent knowledge of electronics, as well as the schematic for this twin.

I plan on replacing the tubes soon, but I have a working set, biased properly, with no microphonic tubes.

Any help is greatly appreciated.

[Metalmaster06]

so i'm getting my small guitar repair shop off the ground, and it's time to clean house with my equipment. still having the same issues with this amp. I've been using my 1973 Earth G-2000 as my main amp and would like to get this one rolling again so i can use it at the shop.

Any suggestions?

[pdf64]

'popping very loudly, even when at low volume, when any of the push pull pots are switched'
Are you sure, ie does the mid cut on chn1 pop?
The pull treb chn 1 could pop, as it's missing a 10M bypass resistor to keep the bright cap charged.

The cause of each switch popping will need to be addressed individually, so we need to identify if it really is all of them. As a switch which already has a 10M bypass resistor won't need another one fitting, rather the one that's in there should be checked, if the switch pops.
So, best to check the amp out and list each control that has a problem, and exactly what the problem is.
Are you prepared to go to all the trouble of taking the control pcb out in order to fit each resistors / service/replace pots to fix this?
Peter.

[Metalmaster06]

i love this amp, and i'm pretty handy when it comes to fixing things. i have plenty of time and patienc. thanks for pointing me in a direction. will post back w/ more diagnostics.

[Metalmaster06]

ok, back w/ more

starting at the front end

input jacks all sound clean, no scratchiness when rotating a plug or removing it.
ch 1

treb does pop when switchin the push/pull, i'll look into adding the resistor you mentioned, 10Mohm 1/2w?
mid pops a small bit when switching the push pull, but significantly less. similar fix, but i'm guessing at that one.
bass has almost no noticible pop on switching the push/pull, will leave it as is

ch 2 here's where i've had the most trouble
gain has no push pull, but will cut out if the knob pressed laterally
treb small pop when switched, but minimal, also cuts out when pressed laterally
mid no pop, but also cuts out when pressed laterally
bass no pop, but also cuts out when pressed laterally
presence cuts out when pressed laterally
volume push/pull acts as channel switch, no problems here

with all of these pots cutting out w/ lateral pressure, i'm inclined to think it's a dirt/mechanical problem. I'll hit them with contact cleaner and remove the knobs to see if the shafts are bent from a fall, drop etc. If no improvement from cleaning, then on to replacing the pots across the board, couldn't hurt to swap them all if i'm going to pull the chassis for it.

Having done some heavy reading over the last few days i also looked into a few other things.

I have noticed some hum/crackling through the speakers.
Cut the problem in half by isolating the power stage. I plugged a jumper cable into the power amp in to disable the preamp and i was left with only a mild hum, no crackling/hiss.
I'm assuming from this that the problem lies in the preamp.

Current tubes are
5 x 12AX7 groove tube red labels
1 x 12AT7 electro harmonix
1 x 12AT7 japanese no name
4 x 6L6GC groove tube red labels

+ 1 spare 12AX7 mesa just in case

Next course of action:

1) take current tubes to local technician for testing, he agreed to give me specs on them for free (great guy)
2) replace any bad/microphonic tubes to achieve a full working set
3) consider a full retube
4) full cap job. Figured the amp is 20 years old, and judging by it's appearance, it has been gigged hard
5) spec and replace resistors as necessary
6) consider adjusting the values of the caps/resistors to tweak tone

I also read somewhere that one of the tubes in the preamp has both sides wired together in one gain stage. The poster called this "wasting a tube". He also mentions rivera wasting half a tube later in the amp. If these assets are truly being wasted, would there be a way to put them to use? (ie add a new gain stage using these wasted tubes or reorganize the preamp components to make use of them?)

also gonna email the Torres amps guys to see if they have a kit w/ caps/res/tubes that would be a fantastic package for what i'm looking to do. gonna have a closer look at their tube rectifier kit too maybe w/ a switch to go from solid state to tube.

even then, the focus is on repair first, customization second.

[pdf64]

Treble boost switches on both channels should have a 10M 1/4W+ resistor fitting across the switch contacts.
The mid boost switch on the normal channel (and overdrive channel) already has a 10M across it - maybe put another one across it to see if that helps.
All those controls on the overdrive channel which cut out when pressure applied, I think there's probably a problem with something loose or shorting out for the whole circuit board. They won't all be exhibiting the same fault for no reason. Check out the ribbon cable / connectors between the control board and main board, stray solder blobs, bits of tin foil hanging down from the underside of the cab etc.
Re the hum/crackle, I'd assume that there are a couple of noisey pre-amp tubes in there. One strategy would be to take them all out and then replace 1 by 1, starting with phase splitter and working backward through the circuit. Sub out any which seem to add excessive noise with known good ones.

[Metalmaster06]

ok, i've got her open. this amp has definitely seen some abuse. i see a couple obvious scorch marks under r128 and r129, but they are stood up on their leads away from the board. it looks like the board was basted in flux before soldering, burnt flux everywhere. they also used a fair amount of black rtv silicone to hold parts in place. gonna be a pain to deal with if it comes to that.

got some new preamp tubes, noticed an immediate decrease in noise after swapping a few.

i was weirded out by the "wrinkled" traces i'm seeing. they're still conductive and firmly attached to the board, just looks kinda...offputting.

i've got a series of pics i'll post for you guys to see after i get back from the day job.

[Don Moose]

DON'T use contact cleaner on pots.
Caig makes a product called Fader Lube that is a much safer choice.