I have run into a strange issue with the effects loop on my Crate V50 combo. It seems to be getting Microphonic feedback with any pedal or effects unit I run through it. The feedback stays until the effects are turned off. Once the effects are on, the feedback happens even if the volume on the guitar is all the way down.

I have not taken the amp apart yet to diagnose the issue, but I wanted to know if anyone might have any idea what might be causing this?

What I have checked so far:

Preamp and Power tubes have all been checked to ensure I don't have a bad tube that is getting excessively microphonic. Tubes were changed two months ago, and have been bias checked.

Reverb tank was removed and bypassed to ensure it was not causing vibration. No change either way.

Effects loop jacks have been resistance and capacitance checked, and seem to be operating normally.

Five sets of cables have been tested to ensure I didn't have a shielding or grounding problem with the actual cables.

Three different guitars have been tested on the set-up with the same results.

It doesn't matter what pedals or other effects I am using, nor does changing the signal path order of pedals. I can use 1 pedal or 16 pedals and it still gets the feedback problem.

It does not matter which channel I have the amp in, clean or gain channel, same result.

Adjustment on the EQ does nothing to combat the feedback, nor does turning presence on or off, so I am not sure that the tone stack is the problem..

Boost switching has no effect as well.

The amp does not have this problem if I just run the signal path through the main instrument input. Only when using the effects loop.

As I stated before, I would like to hear some input from other techie types that might have some idea what is goofing up the system, before I actually have to tear the amp down and start metering everything.

I am thinking that it has to do with the loop circuit itself, but I won't know for sure until I can hit it with the diagnostic equipment.

Any advice would be appreciated! Thanks!
Interresting, you checked the difficult stuff before the easy stuff.

Check the levels on your FX in the loop.

Is the loop series or parallel? If it's parallel, your wet/dry mix might be a little thick if your pedals are also max'd out.

What FX are you using in the loop? Some come with noise. Perhaps try gating them?

??? check the easy stuff and let us know what you find. if that doesn't pan out, you might have a tube buffered loop with a tube going faulty... atleast that's what I'd guess without looking at it.
Sorry, it's been a few days since I posted last.

GrisKy, it didn't matter if it were series or parallel. Same effect both ways.

After poking around I found the issue. It was a short in one of the preamp tube sockets. It wasn't a full short, and was allowing noise into the signal. Really just enough of a short to cause a headache.

Thanks for the input though.