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Topics - pa911

Hello, I am new to this forum. I have posted this query on one other forum without resolution.
The hiss measures about 20 db in front of the speaker with FX and Volume at minimum. None of the control knobs except FX type affect it (mainly changing the character of the hiss).
I have revision A and B schematics but the board in the amp is marked revision C.

Removing U10 eliminates the hiss but the amp is inoperative.
Removing U8, U9 and U11 does not affect the hiss.

Oscilloscope shows clean signal at TP31 and TP23 but noise at R53 and TP34.
This should localize noise to the DSP board. Swapping an identical board from my Fender Stage 100 (which is quiet), with a switch in the amp specific U4 chip, results in persistent noise. I believe this localizes the noise source to the U4 chip on the DSP board.

Does this sound correct? Does anyone know how to get a replacement U4 chip?

I have tried to decrease the noise by increasing R53 and R54 to 200kohms,reducing the gain at U10-A. Hiss is reduced by about 3db. FX also reduced. Will changing C49 help in filtering the noise?