Fishman Loudbox mini PRO-LBX-500 - buzz in mic channel

[Mars Amp Repair]

Hello folks,
So it's well known that the Fishman pots get some kind of debris in them that causes resistance to show up between the pot and the case of the pot. The Loudbox Mini I worked on had what sounded like a ground loop issue only when I turned up the mic channel volume. I inadvertently discovered it was a ground loop issue by loosening the ground screw on the power supply board & the hum reduced substantially.
I then discovered if I removed the preamp board to isolate the pots from grounding on the chassis, the hum went away.
Having experienced the shorted pot syndrome on other Fishman products, I measured the resistance between the individual pots in the Mic channel to ground. Turns out the mic volume pot had the lowest resistance to the (around 30ohms). I blew it out with compressed air while monitoring the meter & the resistance went to infinite.

It's interesting that this issue reminds me of the 'whiskers' issue that occurs in the old Germanium transistors. You can Google that one.
So with ANY weird issues that come up for these Fishman amps, you should see if blowing out the pots resolves the issue.
glen

[The Dude]

I've seen this myself. It seems a simple problem, but can be hard to track down since it's not a "normal" failure.

[Mars Amp Repair]

I've seen this myself. It seems a simple problem, but can be hard to track down since it's not a "normal" failure.

Yep. I began troubleshooting it as if it were a ground loop issue and just got to thinking as to why a solid state newer amp would have developed a ground loop issue.
Then i removed the preamp board and realized the buzz was originating from the ground path thru that ground,
To isolate the mic channel ground path, I cut the trace that grounded only the cases of the mic channel controls. That was a stroke of luck that I only needed to dremmel one trace to isolate them.
Then of course I found resistance between the case of the mic volume control and the leads.
The rest is history.
Thanx for responding.

[Jazz P Bass]

Thanks for the 'resistance to case' info.
I had contacted Fishman a few years back and they supplied me with the 'blow the controls out with compressed air' tidbit, which did resolve my issues.
I recently had a Behringer MX2642 come through where the Frequency and Mid control on two channels did nothing.
I found a resistance reading of 30 ohms from one leg to the grounded case.
I tore the freq control apart and cleaned it out and the resistance reading disappeared.
Weird stuff.