Sympathetic Fuzz from Fender Princeton Stereo Chorus (Solid-state, non-DSP, black knobbed version)

[Mr. T]

A downloadable schematic is available here: https://elektrotanya.com/princeton_s.../download.html

Issue: Amplifier has a sympathetic distorted tone accompanying the primary clean or drive tone. It sounds identical to a fuzz channel bleeding into the clean channel. However, I don't believe it is channel bleed through in the traditional sense, as it presents on the so-called "drive" channel, as well, and is not affected by manipulating the knobs on the secondary, "off" channel. It also is not related to the speakers, as the problem persists using any of my external cabs. The problem DOES NOT present through the headphone jack, so my assumption is that it is in the output stage of the amp and not from the preamp or anything upstream. No obvious problem components on the board, though I replaced the power filtering capacitors for good measure. Golden-brown board coloration around the Zenner diodes (CR19/20 on the schematic) suggest they get pretty warm, but they tested okay. I've desoldered a leg on most of the electrolytic capacitors so I could check them with my meter - all were within spec.

Any advice for where I go next? Start checking resistors? Swapping IC chips? Could it be the IC power amplifier chips, themselves? I will note that this is a stereo amp and the issue is present in both L/R channels, though maybe to a lesser degree in one than the other. Thank you in advance for any guidance.

Background: I dabble in amplifier repair (mixed results, but more wins than losses), but typically tube amplifiers, and usually the obvious culprit components. Solid state has me stumped. I've completed a handful of mods, a successful "Champ" build, and saved a few lowish-value amps from the scrap heap. I've also inadvertently hastened a few amp's journey to the scrap heap. I have an iron, a multimeter, and a capacitance-dissipating safety screwdriver.

[The Dude]

It's not impossible, but highly unlikely that both amps would fail at the same time. So, I would start by measuring power supply voltages and make sure that voltages are up to snuff.

[J M Fahey]

A downloadable schematic is available here: https://elektrotanya.com/princeton_s.../download.html

Issue: Amplifier has a sympathetic distorted tone accompanying the primary clean or drive tone. It sounds identical to a fuzz channel bleeding into the clean channel. However, I don't believe it is channel bleed through in the traditional sense, as it presents on the so-called "drive" channel, as well, and is not affected by manipulating the knobs on the secondary, "off" channel. It also is not related to the speakers, as the problem persists using any of my external cabs. The problem DOES NOT present through the headphone jack, so my assumption is that it is in the output stage of the amp and not from the preamp or anything upstream. No obvious problem components on the board, though I replaced the power filtering capacitors for good measure. Golden-brown board coloration around the Zenner diodes (CR19/20 on the schematic) suggest they get pretty warm, but they tested okay. I've desoldered a leg on most of the electrolytic capacitors so I could check them with my meter - all were within spec.

Any advice for where I go next? Start checking resistors? Swapping IC chips? Could it be the IC power amplifier chips, themselves? I will note that this is a stereo amp and the issue is present in both L/R channels, though maybe to a lesser degree in one than the other. Thank you in advance for any guidance.

Background: I dabble in amplifier repair (mixed results, but more wins than losses), but typically tube amplifiers, and usually the obvious culprit components. Solid state has me stumped. I've completed a handful of mods, a successful "Champ" build, and saved a few lowish-value amps from the scrap heap. I've also inadvertently hastened a few amp's journey to the scrap heap. I have an iron, a multimeter, and a capacitance-dissipating safety screwdriver.

Start by scoping the output.

*Anything* we hear matches an electrical waveform applied to the speaker.

Inject 100mV 1kHz tone at the input, set all tone controls to 5, Master if present to 10, reverb to 0, and start raising volume slowly, watching the output.https://www.mediacollege.com/audio/t...6bit_30sec.mp3
It lasts 30 seconds so set player to Repeat or Repeat 1 so it restarts at the end.

If 1kHz too annoying, try:
https://www.mediacollege.com/audio/t...6bit_30sec.mp3

We expect a pure tone sinewave but you will find some weird stuff appears when you hear the Fuzz sound.

Take a screen picture/capture and post it here.

EDIT:

Start checking resistors? Swapping IC chips? Could it be the IC power amplifier chips, themselves?

No, that is a good way to destroy an amp and solve nothing, it´s called shotgunning for a reason, I´d call it "shotgunnimg blindfolded" which is worse. you must first find the real problem and where it comes from and then try to solve it.

[Jazz P Bass]

Try a different speaker.

[Mr. T]

Thank you all for the quick responses. I do agree it is unlikely both IC amps would fail similarly at the same time. I did attempt to scope it once, and the noise definately shows up at the output, but I had a hard time tracing it further back. I’ve recently successfully repaired a better scope, so I’ll give it a try and report back with shots of the waveform as soon as I can get a little time at the bench. As to the third post, it is definitely NOT the speakers. As mentioned in the post, I have tried it with assorted known-good external cabinets and bench speakers with no change.

[g1]

The problem DOES NOT present through the headphone jack, so my assumption is that it is in the output stage of the amp and not from the preamp or anything upstream.

This is very odd and may require closer listening. The phones out does come after the output IC's. All there is is a couple resistors and caps making an attenuator network.
Scoping at the phones out and comparing to the spkr out may give you more insight. You will need to have your particular headphones connected when scoping the phones out, in case their impedance is somehow affecting things.
And when you are scoping the speaker out, you must not connect the speaker (-) output connector to ground. So scope probe ground to chassis rather than CP2 or CP4.
Connecting ground to CP2 or CP4 will short out resistors R88 or R98 and create issues.
Any load box or speaker used for testing must be fully floating.

[J M Fahey]

This is very odd and may require closer listening. The phones out does come after the output IC's. All there is is a couple resistors and caps making an attenuator network.
Scoping at the phones out and comparing to the spkr out may give you more insight. You will need to have your particular headphones connected when scoping the phones out, in case their impedance is somehow affecting things.
And when you are scoping the speaker out, you must not connect the speaker (-) output connector to ground. So scope probe ground to chassis rather than CP2 or CP4.
Connecting ground to CP2 or CP4 will short out resistors R88 or R98 and create issues.
Any load box or speaker used for testing must be fully floating.

I suspect amp is unstable , complex impedance speaker loads make it oscillate (at some points of waveform), headphones are "easy" and to boot at the far end of a 100 ohm or so resistor, easy peasy to drive.

[J M Fahey]

In susoect sonething like this:



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