Fender Twin preamp fuzzing/crossover distortion

[harry]

Another amp problem! 3 weeks into a 10 week tour and this is the second amp to fail. I don't have much equipment: just a multimeter, a selection of components and soldering tools.

The amp is a Fender Pro Tube Twin and it produces a strange kind of distortion; it is most audible at low volumes - you brush the strings and it kind of fuzzes and cuts in and out - it almost sounds like a ring modulator as there's low ghost notes. When you strum hard it distorts: sounds alright but the overall volume of the amp seems reduced.

Using the Power Amp In jack, the amp works fine and free of distortion. Using the FX loop return jack still produces the distortion; so I'm assuming the problem is between the FX loop and the power amp in jack.

The schematic is here: http://support.fender.com/schematics...Amp_SchE45.pdf

I am going to check for leaky coupling caps and that the cathodes have the correct bias; other than that I'm stumped - any ideas? Is that transistor arrangement a power on mute for the preamp? Could that be at fault?

Any help or suggestions are much appreciated as always!

[Albert Kreuzer]

Using the Power Amp In jack, the amp works fine and free of distortion. Using the FX loop return jack still produces the distortion; so I'm assuming the problem is between the FX loop and the power amp in jack.

You have narrowed it down already. Just to be sure: Does the FX send produce a clean sound when connected to another amp?

I am going to check for leaky coupling caps and that the cathodes have the correct bias; other than that I'm stumped - any ideas? Is that transistor arrangement a power on mute for the preamp? Could that be at fault?

Yes, measure all the DC test points (the rectangular ones) and look for anything out of range.
And yes, the whole diode/transistor shebang is a power on mute and one of the main suspects. If you disconnect it, the worst thing to happen would be a pop on startup and/or shutdown.

[harry]

You have narrowed it down already. Just to be sure: Does the FX send produce a clean sound when connected to another amp?

That's a good idea, I'll try that before I start unsoldering things.

I have to admit I've not heard a distortion quite like this when working with straightforward triode gain stages, so I'm suspecting the transistor/diode arrangement. Thanks for the input, Albert; much appreciated!

[Enzo]

So the problem is between the PA in and the FX return. SOunds reasonable to me. Try this: Turn the reverb up some and bump the amp to shake the springs. Is the resulting noise clear like normal or does it sound like the problem? The reverb comes in after that first triode after the FX return. For that matter you could also feed a signal into the reverb return jack for a clearer thing to listen to.

In fact you can test that furst triode this way: Connect teh reverb drive to a small speaker or even to an amplifier. It wil sound tinny since C32 lops off the bottom, but ought to be enough to see if the problem is there.

And you want my stab in the dark? Disconnect Q5 and D8 and see if the problem goes away.

[harry]

Disconnected Q5 and D8. Works fine now - thanks a million, Enzo!