Acoustasonic 30 DSP amp has loud hiss

[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?

[Tassieviking]

Here is the link to the other site pa911 mentioned :https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/986153-fender-acoustasonic-30-dsp-with-very-audible-hiss

I can't help but someone might be able to.

And the schematic: You cannot view this attachment.

[Tassieviking]

I am not sure if this is the same or if you can get it but its available in Italy.
https://www.folliemusicali.com/prodotto/fender-outlet-0057799000-pcb-udsp-micro-dsp-for-deluxe-90/

USA:
https://reverb.com/item/67584695-fender-dsp-board-with-processors-2002

If you think the fault is in U4 then see if you can get a new chip and have someone copy the program from the 100 amp into the new chip, it is only an Eprom.

[pa911]

Thanks for the replies.

1. I believe U4 is amp specific. On the Stage 100, only a single channel is used, while the Acoustasonic uses separate channels for instrument and microphone.

2. Copying a defective chip might end up with defective code on the copy.

The ideal would be to disassemble the code, but that is way above my skills.

[pa911]

Hello again,
Thought I would give this amp one more go.

Knowing that the noise seemed to begin at the U10 stage, I decided to test each component of this stage. This eventually showed a near short at the multiplexer stage with R66 reading 47ohms. This would cause an abnormally high gain for the DSP signal. After replacing with the correct 18kohm resistor, the amp is now quiet and DSP is much more usable. It appears that what looked like noise coming from the DSP board was actually the DSP signal.

[saturated]

Congratulations on the fix

Very impressive

  

[Tassieviking]

That was a good find, I once read a post when someone preheated an oven to 400F and placed the DSP PCB in there for 15 minutes as a last hope to reflow the solder, it restored it back to working condition.
I think it was a Fender where the DSP PCB's are no longer available anymore and they had a reputation of bad solder joints, you have to love the lead free solder that always gives people trouble.