120hz buzz from Fender reverb footswitch - only when on floor?!

[waspclothes]

Hey everyone, I got a really strange problem with my BF Deluxe Reverb.

I get some nasty 120hz buzz (spikes at 120hz) coming through the reverb - but only when I have the footswitch laying on the ground.

I know the footswitch is a terrible design, sending the low-voltage signal through the shielded cable to the switch itself. I tested the shield, checked the wiring in the switch (outside metal is tied to shield), checked the amp inside - no issues. I even tried it in different parts of the house.

If I hold the switch in the air the buzz is reduced massively (even with rubber gloves, so not grounding the footswitch through my body) and gets worse the more cable is laid on the ground.

This is with the original fender footswitch, but I also tried a few aftermarket switches with TRS jacks and cables - those were even worse (probably because the TRS cables were way longer than the original fender footswitch).

This is with no guitar attached, dead silent amp otherwise - 3 prong connector, new house with proper wiring. Tried multiple tubes in the reverb recovery circuit, tube already has shield.

Any guesses?

[Chuck H]

Is it possible you have house wiring under the floor where this was observed? Maybe take the amp where there's no possibility of AC wiring under the floor surface and see if the problem persists.

EDIT: I do know you said it was 120Hz but there are things about modern home wiring that can create the harmonic. So I'm just covering bases.

[waspclothes]

Is it possible you have house wiring under the floor where this was observed? Maybe take the amp where there's no possibility of AC wiring under the floor surface and see if the problem persists.

EDIT: I do know you said it was 120Hz but there are things about modern home wiring that can create the harmonic. So I'm just covering bases.

The room has underfloor electric heating (!) but I made sure the circuits for all the heating were off at the breaker. I wonder if it could still be related to this? EDIT: Tested this by bringing the amp outside, buzz was pretty much the same.

[Chuck H]

Please do. It's surprising how often the "nuetral" AC lead is still connected and conducting when things are otherwise disabled. As a painter that's had to pull and reassemble fixtures for the work I've been hit a few times because of creative wiring by electricians. Meaning that some circuits can be "technically off" but still have continuity with the AC home wiring. So yes. Please test in a known benign space to be sure. Just thinking out loud, but what could floor level elevation have to do with it other than something coming from the floor?

[waspclothes]

Please do. It's surprising how often the "nuetral" AC lead is still connected and conducting when things are otherwise disabled. As a painter that's had to pull and reassemble fixtures for the work I've been hit a few times because of creative wiring by electricians. Meaning that some circuits can be "technically off" but still have continuity with the AC home wiring. So yes. Please test in a known benign space to be sure. Just thinking out loud, but what could floor level elevation have to do with it other than something coming from the floor?

I tested it in the driveway, definitely no under floor heating there - just rocks. Same thing, 120hz buzz increases when the footswitch is put on the ground.

[g1]

Have you verified 3rd prong of AC plug is zero ohms to chassis?

[waspclothes]

Have you verified 3rd prong of AC plug is zero ohms to chassis?

Hey g1, plug ground to chassis is .5 ohm, my probes are .3 ohm.

Really strange, I took the amp upstairs and there's no buzz. Even with the electric floor heating on and I even ran an extension cord down to the original power outlet I was using downstairs to test if it was the outlet. The buzz remains on any place on the main floor I've tried though..

..and it's legitimately acting like the floor is the cause of the 120hz buzz, because the buzz is reduced if I flip the footswitch over (remember: the original fender footswitch just has a plastic bottom with no shielding).

[loudthud]

Have you tried reversing the Reverb/Tremolo RCA plugs where they plug into the amp ?

[waspclothes]

Have you tried reversing the Reverb/Tremolo RCA plugs where they plug into the amp ?

Yup, shielded one is definitely going to reverb.

[Helmholtz]

Seems your house has quite a bit of electrosmog.

I guess the hum stops when the reverb is switched off?

You might try covering the bottom of the switch with self-adhesive copper foil connected to the cable shield.

[Chuck H]

You might try covering the bottom of the switch with self-adhesive copper foil connected to the cable shield.

from post 5

I tested it in the driveway, definitely no under floor heating there - just rocks. Same thing, 120hz buzz increases when the footswitch is put on the ground.

[Helmholtz]

from post 5

What do you mean?

[Chuck H]

It was demonstrated that the footswitch does this even when sitting on a surface that isn't likely to have any potential to introduce noise. This makes it seem less likely that the noise is getting through the unshielded bottom of the pedal. I do think shielding the bottom of the pedal is a good thing to try. I'm just noting that right now the noise seems more dependent on the aspect of the pedal itself, independent of it's environment. Which is weird and I can't come up with any reason for it.

[Helmholtz]

It was demonstrated that the footswitch does this even when sitting on a surface that isn't likely to have any potential to introduce noise.

That we can't know, as it's not clear what the actual source of the noise is and how the noise field spreads.
It has been demonstrated that the noise field is strongest at the floor or ground.
The footswitch is open to fields at its bottom, so it make sense to add shielding.

[Tom Phillips]

The next thing I would check is the mechanical ground connections at the footswitch and reverb pan RCA phono jacks on the rear of the chassis. Make sure that all the contact surfaces are clean and that the connecting hardware is tight. This includes the ground contact fingers on the plugs and the associated outside surface of the jacks.