MP1 - Master Clipping LED always on

[Fdesalvo]

LONNNNGGGGGG TIME!  Hope this note finds you all well and healthy.

I picked up a used non functional MP1 that's passing sound, though it sounds like there are some oscillations and the Master clipping LED is constantly on regardless of preset or edited level.  Has anyone seen this prior?

[MarshallJMP]

If you set the master too 0, is it still on? And it's only the master led? Do you have any sound comming out of it? Any more info you can share?

[Fdesalvo]

Hey there.  No changes with master and all gains set to zero.  There's sound, but it's almost like it's oscillating.  Same sound from headphone jack.

[MarshallJMP]

Does the master vol work ? Is the sound gone when you put it at zero? Is there a dsifference when you switch voicings (SS vs tube)?

Do you have a multimeter and some measuring experience?

[Fdesalvo]

Omg I didn't see notification for this. Master works as intended. I have oscilloscopes and dmm on hand.

Looks like the master IC U13, is a TC9176 and the data sheet shows inputs as pins 2, 5, 12, and 15. The MP1 preamp schematic shows U12 at each input. U12 has outputs to signals the clipping indicator it would appear.