Weird whistle/mute issue

[stoneattic]

This just started with no change to the pedal board arrangement, and to make it more fun, it doesn't do it every time.

I have a pretty cluttered board of mostly modded Boss pedals. They are all true bypassed, with a T1M buffer at the beginning of the chain.

T1M buffer -> Turbo Tuner -> CS-2 -> AC-2 -> MT-2 -> DS-1 -> BD-2 -> TS9 -> amp
+ a bunch of stuff in the loop -> GE7 -> CE-2 -> BF-2 -> TR-2 -> DD7 -> BBE sonic stomp -> loop return

Here's the weirdness. If I use any pedal before the MT-2 the guitar sound goes almost completely away, replaced by a whistle, even after switching that pedal off. If I pull the power from the MT-2 the whistle goes away. If I click the MT-2 from off-on-off with the power unplugged, the audio comes back as long as the MT-2 is unpowered.

If I use the MT-2 without activating the pedals before it it's fine. The Turbo tuner doesn't appear to cause the problem and it seems most consistent with the AC-2. I replaced the switch in the AC-2 with no change and the AC-2 true bypass grounds the circuit when bypassed.

Some days it doesn't do it at all and everything works as it should.

I've tried different cables, so I'm pretty sure it's not a bad cable.

I have a gig tonight and I can live without the MT-2 if I have to, but this ahs been driving me nuts!

Any ideas?

[R.G.]

Either the MT2 or the pedal before it is oscillating. This can start as an electrolytic ages and doesn't do what it's supposed to as well any more.

True bypassing pedals can help this, especially if the input to a high gain pedal, like I think the MT2 is, is left open when bypassed. So it's possible that true bypassing the pedals helped set it up.

Can't tell any more from your description, but I think this fits the symptoms.

Every repair man hates intermittents.