Treble Pot Attenuating Signal on new Fender circuit Build

[wittyjeff]

I just finished building an AB 864 Bassman clone of sorts.  The Bass channel works great, but the Normal Channel (to which I added a Mid pot mod) is acting strangely. The Treble pot will abruptly allow signal when turning from zero to past 50% and then begins to attenuate the entire signal like a backward volume control with the signal cut off to zero abruptly just before 100%.  Doesn't seem to really modify treble (although there's really only 0 to 50% of the control to determine that).  ...so basically no treble.  I've have this tone stack in and out like 3 times now trying different pot values, testing the pots (everything is new), and two or three completely different circuits.  I've attached my original circuit below and the last one I've got wired in there now.  Absolutely no effect on the symptoms/problem with any of the changes I've made.  I believe the Bass and Mid seemed to work better in the first circuit, but still... this is jacked up. While I'm sure I'll be embarrassed when we discover the source of the problem... I have run out of ideas or things to try.
Thanks in advance for your help/suggestions.

[PRR]

Tack 250pFd to ground on both plates. Does that change the sudden cut-out action?

[wittyjeff]

I connected a 220pF cap (that's what I had in stock at the moment) to each plate and connected to ground.  No effect on symptoms.

[Williamblake]

Then maybe cut this short and show sluckey a picture?

[shooter]

here's how to eliminate the TS .  that way you know which direction to focus on

[wittyjeff]

I've attached the complete schematic (as currently built), a photo of the complete guts, and some closer ones of the pre-amp and pots, plus the front for reference.

[wittyjeff]

Thanks Shooter!  I'll do that later today/tonight and report back.

[sluckey]

I've built quite a few Fender preamps with that tone stack shown in the first pic in the first post. No problems at all. There almost has to be a wiring error or faulty pot for your treble pot to behave as described. Because of your layout, it will be impossible for me to spot the wiring error(s).

Just as a side note, the Fender TMB tonestack has a particular operation that most people are not aware of. Turn all three tone controls to zero and all sound is killed. Then turn up only the treble pot and you will only hear treble sound, or turn up only the mid pot and you will hear only mid sounds, or turn up only the bass pot and you will only hear bass sounds.

This does not apply to your tone stack because your mid pot is not wired like a Fender mid. It's wired like a Marshall or Sunn.

[wittyjeff]

Thanks Sluckey, I agree, there must be a whacked wiring error in my hodge-podge of direct wiring.  I've taken out the pots and tested them individually and even replaced them just for spite.  I may need to just pull everything out (again) and redo it with a completely Fender TMB (but different than the one I tried previously).  Is there any way some upstream or downstream errors could have this effect on my Normal Channel pre-amp / TMB?  If so, what are the possibilities to check out before I completely redo this again?

[wittyjeff]

Also... could a "fried" 250pF cap (or one of the others) cause these symptoms?  Maybe I over-cooked one or something weird happened (or faulty from the factory)?

[sluckey]

Also... could a "fried" 250pF cap (or one of the others) cause these symptoms?  Maybe I over-cooked one or something weird happened (or faulty from the factory)?

Not likely but measure for any dc voltage on any of the pots to be sure. There should be no dc voltage on the vol, tre, bass, or mid pots.

I suggest you rip out all the tone stack wiring. Then wire up a simple AB763 treble/bass stack. Get that working. To add in the mid pot simply replace the 6.8K "tail" resistor with the mid pot wired as a 2 terminal variable resistor.

[2deaf]

Is the high side of your treble pot grounded? 

[shooter]

not according to the 3 different schematics, actual wiring 

[2deaf]

not according to the 3 different schematics, actual wiring 

Yeah, the actual wiring.  It acts as though the 250pF end of the treble pot is grounded.  Easy to check with a DMM if visual inspection doesn't do it.

[wittyjeff]

Sluckey, ...my thoughts exactly.  I'll redo with the AB763 as you suggested.  That's a tried and true Fender TMB and will keep it familiar etc.

2deaf:  that's a great thing to check.  The Treble pot shouldn't be grounded, but it sounds like youre saying that if it is grounded for some reason, that would elicit the symptoms I'm experiencing, correct?  That's easy enough to check, I'll check it out.

[wittyjeff]

As I suspected... I made a really really stupid mistake.  The treble pot WAS grounded (by an idiot).  For some unknown reason I just used the grounded post on that terminal strip to make that particular attachment (see photo attached).  It was right there in front of me all along (but impossible to see on the photos I sent you guys).  At least I did one smart thing and asked for your help. 
I really appreciate your help and I've learned a lot in this new hobby by reading your posts and websites.  I still plan to rewire that TMB stack to the AB763 like it should be anyway.  At least now I know that it will work once I do that.
At the end of the day, this amp sounds fantastic and the effects loop and Master Volume work as hoped, so I can count this build as another small step forward in my learning process.
Again, my heartfelt thanks to all of your for your time and suggestions.

[vampwizzard]

most of us have done really stupid stuff while learning.. thats the point of learning. Now youve got tools to troubleshoot the next problem!

[wittyjeff]

Thanks.  I'm sure I'll find lots of stupid mistakes to make along the way!