Fender Performer 1000 buzzzzzz

[drewl]

Hummmm....
Real nasty 60hz buzz on this performer 1000, much more so on the clean channel and only when a jack is plugged into the input.
We're talking the straight up and down spikes of noise, not hum.

Filter caps are good, power supplies are good.

I'm guessing something to do with the Inhibit circuit, which is like the mute on some of their other solid state amps.

It's coming from the preamp, plugging into FX return and cranking FX level yields no noise.

The buzz spikes are also on the reverb drive opamp, U6 pin 1.
No noise on any of the other preamp opamps.

Maybe you guys can see something I'm missing, or maybe an easy way to disable the mute circuit like the Frontman 212.

Thanks.

[Jon Snell]

Check the input jacks.

[drewl]

Cleaned, tightened and re-flowed pots, jacks and any suspect bad joints.

I'll check if all the switching functions of input jack are correct.
Thanks.

[g1]

Any chance somebody botched that ground mod in the service bulletin on pg.9 of that service manual? Or is this a fault that just started one day when the amp had been working normally til then?

[glebert]

What knobs change the buzz, if any?

[drewl]

Controls make no difference to the buzz, doesn't the ground mod only effect the distortion channel?

I didn't check to see if it was done, it didn't look like it, everything looks stock.

[drewl]

WTF???

So, while probing around and testing I stumbled across the fact that if I touched CR6 - CR9 anodes with one hand and touch the chassis with the other, the spikes be gone!

So they go to the switching select lines, let's see what's going on with those aaaaaannnndddd.......
C68 was missing and C70 had one leg clipped, replaced them and spikes be gone for realz.

They connect to ground from the select lines, why someone removed them, or if it was an accident we may never know.

[glebert]

WTF???

So, while probing around and testing I stumbled across the fact that if I touched CR6 - CR9 anodes with one hand and touch the chassis with the other, the spikes be gone!
.

Ah, the old "human body as capacitor" test. Good job finding the problem, that is a weird one!

[J M Fahey]

Somebody applied one of those "no Schematics involved" "Mods" so popular in some Forums.

Typically something like: "remove C64 - connect top of R48 to bottom leg of Q1 - cut track joining C42 to R148 , ground left, connect right to V4 cathode ...." and so on.
The potential for gross error is immense, doubly so because parts numbers change even in "same amp same model" but different versions.