Tascam 388 - WEIRD buzzing issue

[akpasta]

I have a Tascam 388 and I'm experiencing some very odd buzzing coming from Track 5 under very specific circumstances. Let me explain why it's so bizarre.

The buzzing happens only when the track is switched to "Mic" Input.

If you connect a mic straight to the input, there is no buzzing what so ever. It's totally clean.

If you connect an instrument via 1/4" to xlr cable straight to the input (useful for sending instruments through the on-board preamps), there is also no buzzing at all.

If you connect an instrument running through guitar effects pedals via 1/4" cable to xlr straight to the input, there is also no buzzing at all.

--- THERE IS BUZZING when you run an instrument through an effect pedals that is connected to a one-spot power adapter. But ONLY ON TRACK 5. IF you run the same chain to another track the same way there is no buzzing. The buzzing goes away the instant you disconnect the one-spot. It happens with multiple one-spots that otherwise function normally, but only on track 5.

I'm totally stumped.

[A.David.MacKinnon]

Its a ground loop. There may be a faulty or cold solder joint on the ground pin on track 5.

Is the one shot powering off of the same outlet as the 388?
What happens if you run the pedal into a di and then into track 5? If there’s still a buzz does it go away when you flip the ground lift switch?

[kslight]

My advice, only slightly snarky...take all one spots and throw them away. Nothing but noise problems like this, in my experience. Get an isolated power supply and warts like this usually go away.

Don’t know why it would only appear on track 5, perhaps something is not grounded anymore on one input or output, but if it goes away with a battery in place of a one spot, then it’s time to get an isolated power supply.

The 388 has a 2 prong plug as well, which seems to make it more susceptible to noise issues in my experience.