Foot switch ground loop buzz- Supro Saturn Reverb MKII 1648RT

[TASBrendan]

Hi all,

I have a Supro Saturn Rev. MKII that has a ground loop in the reverb when using the foot switch. The noise is in the reverb circuit and changes volume with the reverb volume, and only happens with the foot switch plugged in. It's absolutely quiet without it. When I move the pedal around it changes the noise, kinda like a single coil pickup or ungrounded pickup. There's a slight radio signal in there depending on which direction the pedal is rotated.

Some things I've already tried with no change are -
Using other switches and cables
The trem and verb both send to the pedal jack through individual coax cables. I removed the ground wire on the PCB at the reverb send side, trem send still grounded at PCB.
Removed both trem and verb ground connections from PCB, ground still connected at chassis through jack (same noise but also crackly)
Removed the TRS jack from the chassis, ground at PCB through trem send coax.
Removed trem and verb PCB grounds and grounded jack at closer ground point on PCB.
Clipping a 100pF cap in parallel with the reverb recovery triode plate resistor.
Clipping a 100pF cap from grid to ground.

*Have NOT tried a cap between plate and cathode

Thanks for the help!

Brendan

[Jon Snell]

Sounds like the tip and screen are reversed on the socket or plug meaning no screening or the foot switch is not screened.
A lot of these amps use a switch to short the input from the reverb to ground, it will pick up hum if not screened.

Note; You only get a ground loop if both units are connected to ground. A foot switch is not connected to anything except the amplifier it controls.
What you are hearing is mains pickup noise, from a badly connected lead.

[TASBrendan]

Thanks Jon! I'll check out some of the other grounding points more thoroughly. I can see mains hum on the scope at the grid of the recovery amplifier, so I know that it's getting in the signal between there and the reverb return. Touching the button on the pedal quiets the noise as well as touching the chassis.

[Jon Snell]

I have no schematic, but if it is not a widow maker, try earthing the chassis with a three core mains lead.

[TASBrendan]

Thanks Jon- it's a modern Supro from 2016(?). New design, not a reissue or anything but I can't find a schematic. The PCB is not complex, but is double sided. Also gets my vote for most inconvenient tube placement- three of the preamp tubes are horizontally mounted into the upper depths of the cab. Sorry, just venting there...

[Chuck H]

The noise changing with cable location and the radio signal are indicative of no shield. I don't think you're looking for a ground fault or loop, per se. You're looking for a shield fault. That might be a ground fault. Check that the cable shield lug on the amp jack is actually connected to ground. If it's not then check to see if the conductor lug of that jack is connected to ground. If it is then it's probable the jack is wired backwards as Jon indicated.

If you DO find that the jack is wired correctly, but the shield lug at the amp jack is not connected to ground then you need to determine the fault. Either right there at the jack or elsewhere. Just follow the lead and board trace testing with your meter until you find ground. The fault will be between there and the previous test point. If these are board mounted jacks you might find that the pad to pin connection on the board has failed. In which case a test should reveal ground on the correct trace but not on the jack.

[TASBrendan]

At this point I've disconnected every screen wire from ground at the reverb and trem sends and reconnected them one at a time with a jumper. With no other grounds connected, the pedal still grounds through the cable to the jack mounted at the chassis. I can see on the scope that the signal is getting to the grid of the recovery tube where the pedal connects. The only thing so far that has worked is to swap the cord with a higher quality (Hosa...) TRS balanced studio cable. My only guess there is better RFI shielding. This might be the simplest solution, as long as no one ever tries another cable or pedal...

[g1]

Is the chassis of the pedal getting a good ground connection?

[TASBrendan]

Chassis of the pedal is grounded- both buttons and jack are making good chassis connection and all connect to amp chassis ground.
Pulling the reverb rca wires one channel at a time makes the buzz worse. Touching amp chassis or pedal chassis makes the noise go away.

[Helmholtz]

Certainly no ground loop as others said before.

The footswitch cable might have a number of broken shield strands lowering effective coverage.

Also measure resistance between chassis and the safety earth contact of the power plug.

Are your observations without an instrument connected to an input?

[TASBrendan]

I'll take those measurements- The noise is there without any instrument plugged in. One thing is that it happens with other pedals I have, even a new one. It's not impossible that they would all have the same weak point in the shielding. Thanks

[TASBrendan]

Just to follow up- the problem was resolved by getting a new, higher quality cable. I emailed Supro tech support to see what they had to say, and they got back to me right away suggesting a new cable. They had never seen this problem before as part of the circuit design or failure.