Treble Control Dead...!!

[wolf1419]

Fender Princeton Chorus, black knob, 1995, everything works except the treble control has no tone change from 0-10. Is this a cap, or a pot...? It has no affect except a slight scratchiness, even though it's been cleaned. Also the bass is very boomy from 4-10. Are these related...?

[Enzo]

Probably. Get out your ohm meter and check the pot.

[wolf1419]

Probably. Get out your ohm meter and check the pot.

Well, I did open the amp, got to the board(quite a small feat getting under that board w/o removing a bunch of stuff), checked the pot on the board and it appears to function, R value changes distinctly pin 1-2, and 2-3(from 0-infinity) similar to the bass and mid pots. C20 appears to be "open", since I can't get an ohm reading on any range, but I'm not certain with the value of that cap if it would allow a reading, or not. I tried all my ranges and it appears "open". I put it all back together for the practice and to be sure I didn't ruin or affect anything.
The treble pot still does not appear to affect volume in any way, at least not noticeable, but I haven't played it loud either. Something much more noticeable and a possible clue is that the midrange and bass controls are greatly affecting volume. They actually shut the volume completely off when counter-clockwise to 0. I don't think any amps do that.......is that a clear clue, and does that still point to C20 as the culprit??

[Enzo]

Pretty normal for no sound when ALL EQ controls are at zero.

Pots should never go to infinity. Legs 1 and 3 are the ends. They are the largest resistance that should be measured. The center leg - leg 2 - should measure from zero up to that max from either end.

Th R value should vary smoothly from zero all the way up to the max. It is not sufficient that it just changes.

[wolf1419]

Pretty normal for no sound when ALL EQ controls are at zero.

****That's no sound if either mid or bass are counterclockwise to 0 - don't think that's normal.

Pots should never go to infinity. Legs 1 and 3 are the ends. They are the largest resistance that should be measured. The center leg - leg 2 - should measure from zero up to that max from either end.

*****I probably don't have the proper range on my digital meter, so it looked like infinity, but is probably just very high - I'm understanding the pins o.k.

Th R value should vary smoothly from zero all the way up to the max. It is not sufficient that it just changes.

Now this could be a little telling. While measuring on the board the bass & mid pots are smoothly varying, whereas the treble pot is a little erratic with the digital meter, varying, but with weird jumps as you turn the pot(still going from 0 up, but some intermittant jumps on the way up, even though it is not apparent in the sound(no treble change). It would appear to be a dirty wiper, but does not explain no tone change at all, and the total loss of volume in either mid or bass control. It still appears to be affected by C20, the treble tone cap, being possibly open and at ground potential when either of those 2 pots are counterclockwise to 0, as this is a circuit that has mid & bass passing through the treble pot in the schematic to the next stage. Yeah, it sounds a little weird, but that's the design, which might also explain the boominess that people mention in this amp if bass control is anywhere above 5.

[Enzo]

REmove the control from the board and test it.

[antiunderscores]

does the signal have treble or no treble at all? i seem to have no treble in my signal at the moment because i intentionally bypassed a cap leading to the treble control (it doesnt appear to be working) this left me with very little treble, and the control acts as a volume, which obviously isnt happening happening to you due to the design. I know jack about this stuff but first thing i would try is replacing the cap. Just an idea.

[wolf1419]

does the signal have treble or no treble at all? i seem to have no treble in my signal at the moment because i intentionally bypassed a cap leading to the treble control (it doesnt appear to be working) this left me with very little treble, and the control acts as a volume, which obviously isnt happening happening to you due to the design. I know jack about this stuff but first thing i would try is replacing the cap. Just an idea.

I appear to have limited high end, but not as if the tone was all the way to 0, probably = to if it were set on about 3-4 and fixed right there. No changes using the treble pot(tested o.k.), so I ordered some caps and an inexpensive, simple, wide range Mastech 390-375 capacitance meter (0.1pf-20kuf) for this job and future repairs. I'll let you know when I achieve success.

[wolf1419]

I appear to have limited high end, but not as if the tone was all the way to 0, probably = to if it were set on about 3-4 and fixed right there. No changes using the treble pot(tested o.k.), so I ordered some caps and an inexpensive, simple, wide range Mastech 390-375 capacitance meter (0.1pf-20kuf) for this job and future repairs. I'll let you know when I achieve success.

Treble cap tested fine with the meter. It turned out to be a bad trace on the board connecting the treble cap to the treble pot, so I hardwired from point to point and it sounds great!!!! This amp can really get some great sounds if you do some experimenting with the gains and limiter.......many, many tones available, touch, sustain.....it's all there! Very underrated amp due to users unable to tweak it properly......luckily I have studio ears to help get the desired sounds. As is true of all amps, they just all sound different and that's what makes the world go round.