This is super strange.. what is going on here?

[Indyryder02]

Clone of Blackface Princeton reverb. Treble wasn’t working.. checked all connections. Ok. Treble cap ok.
continuity to cap from pot good. Read a thread of someone else having this issue.. they jumper wired the cap to pot. It works.. albeit backwards.. I tried this with the same results. I swapped lug wire connections on pot, works correct way now.. although backwards from Fender layout. Strange..
here it gets even weirder.. still puzzled how the jumper make it work, when I have no notable resistance through wire to cap.
i then remove the jumper from the cap.. it still works.. it works with the jumper wire connected to just the lug on the pot that would lead to the 250pf treble cap..
what would cause this? I’m sure there’s a logical explanation.
obviously I can’t leave it with a jumper on the lug to work either!

seeking help from the hive!
thanks In advance!

[Helmholtz]

Can you show on the schematic what exactly you did?

[Indyryder02]

Might be easier to explain from the layout.

basically, I have what tests as good continuity between the white wire leading from the treble pot to the 250pf cap.

For some reason.. the treble only works with a jumper clipped on to that same lug on the treble pot. The other end connected to nothing.. so the lead to the cap wasn’t even the issue. It’s connected fine and has good continuity.
it works fine with the jumper attached just to that lug.. and the other end of it to nothing…
what is the jumper out into space doing to cause the circuit to work?
This just seems to defy a lot of basics to me.. I know I’m missing something.. just curious why this even works like this.

it’s not a path to anything really? Or is it causing some resistance or .. I don’t know. I don’t get it! Haha

[Helmholtz]

Resolder the treble pot lug in question.
If that doesn't help, the treble pot might have a contact problem where it attaches to the wafer.

[Indyryder02]

I also had to swap the left and right lugs of the treble pot opposite of the layout for it to work in reverse.. less treble at ten.

[Indyryder02]

I had already resoldered when I swapped it to the left lug for the control to operate in the correct direction..
pot has an even sweep when viewing op to around 250k when viewing with my VTVM. Seems to work fine from all lug combinations.
yet no treble change with the control. Unless a jumper is attached to that single lug going to the cap. Other end of jumper doesn’t need attached to anything for this to allow the control to work. (Albeit it’s a subtle control to begin with)
I know I’m missing some basic fundamental here!

[Indyryder02]

Just like an antennae? Really strange.
just tried only the end of a jumper cable. Just the alligator clip by itself.. it does not work.. but placing a full jumper wire again, does work. Again. Other end attached to nothing… how this jumper attached to nothing makes the circuit work is very strange.

[Indyryder02]

Maybe it’s so subtle I can only tell it’s working with the noise the jumper is added.. maybe changing the cap will make it a more prominent effect.

[Indyryder02]

Ok.. viewing with a scope. It is working.. I was just losing it there for a minute apparently.. it only really has effect in the last 30 percent of control. And very subtle.. but this is the common spec used on practically all the blackface amps..

[Enzo]

SO far I have not seen anything about the most obvious suspect, the control itself. Helms suggested soldering it, but I don't see you tested it .

And try looking at the schematic. That might help you understand what your connections are doing electrically.

[Helmholtz]

Ok.. viewing with a scope. It is working.. I was just losing it there for a minute apparently.. it only really has effect in the last 30 percent of control. And very subtle.. but this is the common spec used on practically all the blackface amps..

That wire might add something like 10pF. Shouldn't make much difference.
Treble control in my BF amps works just perfect(-ly?).
I typically set it on 6 to 7.
You might check with the Tone Stack Calculator.

Consider a bad (or badly connected) 250pF treble cap.
Also a different taper treble pot might help.

[Indyryder02]

Yes, pot definitely seems suspect. Odd that it worked in reverse with the fender layout, and had to be reverse wired on the end lugs to work in the correct direction.. and the 70-30 operation. Interesting the other thread had this same issue as well with it being backwards.
prob losing my high end hearing! So that’s not helping! Had to look at the wave on a scope to believe it was doing anything!

[g1]

I think what you are hearing may be some kind of ghost effect due to stray capacitance, and not the real 'treble' control. When you do the reverse direction thing, is it still weak, or is it much more noticeable?
Post some pics of the amp guts if you can.
Is this something that just started happening, or was it there since built? Perhaps there is an incorrect value part somewhere or something.

[Dave H]

I'd replace the pot. I think it could have an intermittent conection between its CW lug and the track. Hanging a jumper lead on it could be just enough for it to make contact. If you set the pot fully CW and measure the resistance between the wiper and CW lug you may be able to see it go high resistance when the CW lug is waggled. Sometimes it can be made to work by squeezing the rivet/crimp connecting the lug to the track with long nosed pliers but it's only a tempory fix.

[Indyryder02]

Yes. I thing it was just me hearing the noise it added.. and I couldn’t really tell the difference tone it was making.
(too many years as a guitarist with loud amps! )

it’s a subtle control.. seems to do most in the last 30%