Behringer SF300 weirdness

[jhob]

OK, so not a DIY pedal, but I do have a DIY pedal on the board and a number of kits:

I've got fulltone 69 fuzz clone that I made from a kit ages ago, works fine. It's +ve ground and has a separate PSU to the rest of the pedals on the board (powered by Fame DCT-200 isolated supply) which are all -ve ground

Recently got a behringer sf300 fuzz and if I have the 69 on at the same time and then turn the 69 off it makes the sf300 go weird - gain goes really high, gain pot sort of reverses so it gets quieter as you turn it up and it makes a kinda yelp noise if you turn it on and off. Not all these happen all at the same time, or every time. And it happens after the 69 has been turned off.

I've also had the pedal go weird when the 69 fuzz hasn't been used, so that may be a red herring.

I actually sent the first one back as I thought the pedal was faulty, but second one is exactly the same.

All back to normal if I switch all the pedals off and on at the mains.

No other pedal is noticeably affected in any way, which includes buffered & true bypass as well as 3 other behringer pedals of the same design.

Any idea what's going on here? Can't work it out and it's bugging me!

I recorded a video to show what's happening: https://photos.app.goo.gl/YQmuniG5zjrDVWis9

I guess the nuclear option would be to rebox the sf300 and add true bypass.  I am going to be modifying it a bit anyway....

[ElectricDruid]

Here's the schematic of the 69 fuzz that seems to be involved in the weirdness:

https://www.musikding.de/docs/musikding/face69/69erschalt.pdf

[jhob]

Wondering if anyone has any input on this one? Still an issue and still one that I'm scratching my head over.

[ElectricDruid]

Is it related to having the 69 fuzz directly in front? Does it do it then, but not if you insert something else (a simple buffer, ideally) between them?

[jhob]

Is it related to having the 69 fuzz directly in front? Does it do it then, but not if you insert something else (a simple buffer, ideally) between them?

I think that the 69 fuzz is a red herring.  It still does this when that's not on the board.

[ElectricDruid]

I think that the 69 fuzz is a red herring.  It still does this when that's not on the board.

Stranger and stranger. And a pity because some kind of interaction with the positive ground was about my only theory.

Ok, so can you make the Behringer pedal go funny just on its own? Or does it only do it on the pedalboard with other stuff around it?

Also, does it do it when run off a battery? That would be an interesting result if it turned out that it doesn't.