1977 Fender twin reverb tone controls not working

Joe Shelby · From: Walnut Creek, California, USA

I recently dug out my twin and hooked it up. The volume control works, but no matter how I turn the tone controls it seems to make no difference.
Any guesses as to what is causing this?
Thanks in advance for any suggestions you might have.

Joe

Larry Dering · From: Missouri, USA

If memory serves me everything starts through the bass pot. Make sure you have some bass on and maybe rotate all the pots back and forth full sweep a number of times. There are possibilities that are internal but if it worked prior and has set unused for a long period this may be the culprit.

Joe Shelby · From: Walnut Creek, California, USA

Hi Larry. I hit all the pots with contact cleaner and turned the bass control up to 10 and it made no difference. I haven't used the amp in 4 plus years, but the tone controls were working then.

Tony Prior · From: Charlotte NC

Fender amps such as the 1977 TR use TONE STACK circuits, they are all interactive. If setting them all at "5" does nothing there is an issue which requires service. It could be as simple as ONE 12AX7 tube , a tube socket or as serious as a power supply issue.

Knowing the amp has been sitting for 4 years, a good tech would 1st- pull each tube individually, clean the tube sockets and put the tubes back in WHERE they came from. If it doesn't cure the issue, then one at a time, remove and replace the preamp tubes, ONE at a time. A 1977 amp that has been sitting 4 years needs to have this done, regardless. If cleaning sockets and replacing tubes ONE AT AT TIME , the amp needs a visit to a tech for full circuit examination. A quality tech will not SHOTGUN the fix.

Before you open it up to look at the chassis/components inside, remember, there is +465 VDC running around in there.

Great amp though !
_________________
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jobless- but not homeless- now retired 9 years

CURRENT MUSIC TRACKS AT > https://tprior2241.wixsite.com/website

J Fletcher · From: London,Ont,Canada

So do both channels have the same problem?

Joe Shelby · From: Walnut Creek, California, USA

J, the first channel (normal) the tone controls work on it.
I hadn't thought about trying that...

Charlie Powell · From: Kingsport, Tennessee, USA

i would pull V2. Then pulll V1 and put in V2 socket. You can then turn it on with nothing in the V1 socket and then see if Vib channel (2)works. If so, then the tube that was in V2 has a short in it. BTW the Normal channel (1) will not work at all while you are doing this with no tube in the V1 slot...If that finds it you can simply get you a new 12ax7. If that doesn't work, I would take it to someone who is comfortable working in the chassis with the high voltages present.
_________________
Charlie Powell

Bruce Derr · From: Lee, New Hampshire, USA

I suggest taking off the knobs and using a nutdriver to tighten all the pot nuts. The tone stack is grounded to the chassis via the casing of the Mid pot, which relies on a good connection through the lockwasher or bushing. A bad connection here has the effect of lifting the entire tone stack out of the circuit. An amp I was servicing had this problem, and I'm aware of one other instance as well. It may not be the cause of your amp's problem but it's an easy fix to try. The Volume, Reverb, Speed, and Intensity pots also have ground connections on their casings. I usually tighten all the pot nuts and jack nuts as a preventive measure. (The speaker jack nuts are especially important to tighten. Because of the way most of these old Fenders are wired, a loose speaker jack can cost you your output transformer.)

Joe Shelby · From: Walnut Creek, California, USA

Charlie, your suggestion did the trick.
Thanks to everyone who replied and now I've got a functioning amp again.

Jerry Overstreet · From: Louisville Ky

Cool Charlie. So much knowledge and spirit of sharing here Joe. Great to hear you don't have a major expensive and long term teardown issue.

Charlie Powell · From: Kingsport, Tennessee, USA

Glad that worked.
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Charlie Powell

Tony Prior · From: Charlotte NC

Glad it worked out Very Happy

- but do keep in mind that this amp is now 45 years old and has not been used in 4 years- it needs some basic attention.

At minimum, clean the tube sockets and do what Bruce suggested, pull the knobs and tighten the pots.

When you swapped tubes, did you take the suspected BAD tube and place it in the other position to confirm it was bad ? Asking for a friend ! Very Happy

77' TR, a great amp Very Happy

_________________
Emmons L-II , Fender Telecasters, B-Benders , Eastman Mandolin ,
Pro Tools 12 on WIN 7 !
jobless- but not homeless- now retired 9 years

CURRENT MUSIC TRACKS AT > https://tprior2241.wixsite.com/website

ajm · From: Los Angeles

Regarding Bruce Derr's suggestion about retightening the pots: Yep.

I had an old Stratocaster with the full aluminum shielding plate that went wonky one time.
It started being noisy, scratchy, etc.
The pots were all grounded to each other and the rest of the circuitry through contact with the plate.
It had been probably 30 years since they had been last touched/installed.
There was enough gunk/oxidation/etc that had been built up over time to give some flaky ground connections to the plate.

Redoing those nuts (in addition to adding a ground wore connection the pot cases) solved the problem.

Joe Shelby · From: Walnut Creek, California, USA

Tony, I'm going to do those basic maintenance things you mentioned.
The tone stack seems out of whack; I've got the controls set at treble (1), middle (10) and bass (10). This is with a Jackson Maverick HD (single coil pickup).
I used to play my Zum D-10 through it and the treble was (4), middle (1) and bass (6). I can't believe that these two steels are so different as to warrant such different tone settings.
I tried to remove the knobs to access the pot nuts and they
won't budge at all. The speed and intensity knobs come off fine, but the rest seem like they're glued to the pot shafts.
Any ideas as to how remove the knobs? I don't want to break off the pot shafts.

I didn't try the bad 12ax7 in other sockets (yet).

Joe

Tony Prior · From: Charlotte NC

Joe, 1st, to be certain that the tube you pulled was the culprit you need to put it in the position that the suspected GOOD tube came from or even put it back in the same location. Many times when we pull a tube and replace it, we score (burnish ) the tube socket just enough to allow a tube to work again. Give it a try to confirm the bad tube. I do recommend pulling each tube one at a time and cleaning the sockets with some sort of spray cleaner. Then put the tube in and out a few times. A dirty socket connection can and will cause all sorts of crazy issues.

Pulling the knobs when servicing we use a thin FORK style tool , thin enough to go under the knob and gently prying the knob UP while turning it slowly. I've broken a few over the years.

These Fender Twins are great amps and deserve to be serviced from time to time, they will last and perform for decades. Iconic work horses ! Very Happy

Good luck ! The forum is chock full of folks who can assist !
_________________
Emmons L-II , Fender Telecasters, B-Benders , Eastman Mandolin ,
Pro Tools 12 on WIN 7 !
jobless- but not homeless- now retired 9 years

CURRENT MUSIC TRACKS AT > https://tprior2241.wixsite.com/website

Charlie Powell · From: Kingsport, Tennessee, USA

The knobs are put on with a set-screw. They match the color of the knob well enough that they are not that noticeable. Loosen the setscrews and the knobs will usually just about fall off.

While I agree with the comments re: an amp this age needs servicing, (probably including a cap job) my original suggestion was geared toward a player that is not an amp tech. Loose pots can cause the problems indicated. I personally prefer to tighten them with the chassis removed and the caps discharged so that I can hold the back of the pot while tightening to prevent the pot body from turning and putting stress on the wiring and solder joints. Socket cleaning is appropriate, and they may need retensioning also following cleaning.

It is not my intention to insult Joe's capabilities and hope he does not take it that way. Nor do I disagree with other suggestions, and I hope other posters do not take offense. I would just hate to see damage done to such a fine amp by accident, or worse an injury from electrical shock, so I usually recommend taking the amp to someone with experience.
_________________
Charlie Powell

Tony Prior · From: Charlotte NC

Not arguing, just clarifying- Not all Fender amp knobs use a set screw , many Fender amps have PUSH ON knobs . These can be be either " knurled Shaft" or "D SHAFT" . Knurled Shaft pots were very common on Fender amps up thru the 80's maybe even longer.

_________________
Emmons L-II , Fender Telecasters, B-Benders , Eastman Mandolin ,
Pro Tools 12 on WIN 7 !
jobless- but not homeless- now retired 9 years

CURRENT MUSIC TRACKS AT > https://tprior2241.wixsite.com/website

Joe Shelby · From: Walnut Creek, California, USA

Charlie, no offense taken. I realize my electronics skills are basic and I've probably done all I can do with what I've got.

I checked the pot nuts for tightness and they're all fine. I used contact cleaner on the tubes by spraying the pins and then inserting and re inserting them a few times into the sockets.

I've cleaned the pots with contact cleaner and replaced the RCA cable in the reverb tank because that wasn't working.

So, I can live with and enjoy the tone settings I'm using now with thoughts of taking it to a tech in the near future.

Thanks to everyone for your help.

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