Fender Performer 650 Tone Stack distortion

[Stratfordade]

I’ve almost completed repair on a Fender Performer 650 that’s now running full power, but I’ve discovered there’s distortion in the clean channel when I increase the treble control. Lower trace shows input from sig gen which is fine all the way from amp input to treble pot output when it distorts as shown in upper trace. The distortion is even more jagged at output of following op amp U1B. Could this be caused by a problem in the tone stack caps particularly the 270pF treble cap C7, or is it more likely caused by the op amp following the tone stack? Could the bass or mid caps be culprits given that the treble pot acts as a mixer of the high frequencies at top of pot track and the low/mids at lower end of track? I’m not seeing any waveform distortion throughout the range of the mid and bass controls.

[g1]

What is your input signal voltage level? What is the AC voltage at TP2 when this happens?

[Stratfordade]

On the photo posted the level is slightly higher than for the test levels chart. But the test signals are all correct level - 8mV input and 32mV at TP2, albeit TP2 is only one half of the op amp. Note the test signals are all given for drive channel enabled. I only discovered the distortion when I went through the clean chain having checked levels in the drive chain through to the power amp. On clean the amp is nice and loud and someone may think it’s fine, but I couldn’t get as sweet a tone out of it as I expected hence delving further. I haven’t checked what you suggest which is TP2 when the treble is turned up (the test chart AC levels are for clean treble set to 5 and the distortion appears above this).

I’m away from home now but will check your suggestion. Maybe it will show up some problem with the op-amp U1. Thanks very much for your advice.

[x-pro]

Any tone block that moves the position of its controls away from the zero point introduces distortion to the sine signal.
The entire amplifier path is designed for pure sine.

[Helmholtz]

Any tone block that moves the position of its controls away from the zero point introduces distortion to the sine signal.
The entire amplifier path is designed for pure sine.

A passive tone control cannot introduce distortion as it's a linear crcuit.
(Distortion is only generated by non-linear components like diodes, transistors, tubes or saturating inductors.)
So if you feed it a pure sine the output must be a sine as well, indepedent of setting.
Whenever a tone control changes the waveshape, the input signal must contain some distortion, i.e. harmonics.
Looking at the scope pic of the input signal, I don't see a pure sine but rather something between a sine and a triangle.
So it already contains distortion, i.e. higher frequency harmonics-

[x-pro]

Helmholtz, I agree.

[Stratfordade]

I also noted the ‘peakiness’ of the sine but it’s a function generator (Thandar) so won’t be completely accurate.

[Helmholtz]

Helmholtz, I agree.

Good decision .

[Stratfordade]

What is your input signal voltage level? What is the AC voltage at TP2 when this happens?

TP2 shows a normal output of amplitude matching schematic test levels (8mV AC input, 32mV at TP2) when treble control is maxed. The signal from the treble pot after TP2 that feeds the non-inverting input of op-amp U1B is distorted.

Tomorrow I’ll replace the op-amp and see what difference that makes.

upper trace TP2, lower trace treble pot slider/op-amp U1B non-inverting input.

[Stratfordade]

Replacing op-amp U1 made no difference. I’ve cut one end of the treble cap C7. Signal fine at input end but it is distorted at output end so the 270pF cap is introducing some distortion.

[Helmholtz]

A small value cap can significantly alter the waveform of a somewhat distorted signal.
It does so by changing the ratio and the phase relations between harmonics.
If the input signal to the cap was a perfect sine, the signal after the cap would be a perfect sine again.
So the cap is not introducing non-linear distortion.

I'm not sure your amp really has a problem - unless it really sounds very distorted in the clean channel.

[Stratfordade]

Yes I can see how the phasing/harmonics will result in a changed waveform after the cap. I’m probably letting the great be enemy of the good so will put it back together as it is. The clean channel doesn’t sound at all distorted.

[g1]

I’m probably letting the great be enemy of the good so will put it back together as it is. The clean channel doesn’t sound at all distorted.

Agree there's no real reason to think it's abnormal if it's only at higher control settings. Lots of times when measuring power output I have to play with the tone controls to get a proper looking sine wave. Especially with guitar amps rather than more hi-fi type equipment.

[Helmholtz]

Lots of times when measuring power output I have to play with the tone controls to get a proper looking sine wave. Especially with guitar amps rather than more hi-fi type equipment.

That's why for measuring power output I like to feed my test signal to the power amp and not to the instrument input.
If that's too cumbersome, I start with a 400Hz or 1kHz squarewave and adjust controls for best square reproduction.
This ensures the flattest possible frequency response before I switch to the sine signal.

[g1]

That's why for measuring power output I like to feed my test signal to the power amp and not to the instrument input.

Yes, I always try to use FX Return or Pwr Amp IN jacks if available. Usually it's the classic tube amps that I have to use the inst. input and play with the tone controls.