Fender Acoustasonic 30 DSP with very audible Hiss

[pa911]

I have been trying to track down the source of a very audible hiss in the amp for quite some while now and have reached the limit of my understanding of the electronics. The hiss measures 20dB above room noise as measured in front of the speaker. Neither volume control, nor tone control affect it. Fx setting and volume will change the character and level of the hiss. The output with acoustic guitar and dynamic mic are distorted with a brassy metallic sound, made worse with any level of fx.
Things that I have done:
All filter caps have been replaced and upgraded, c104, c106, c79, c83,c99, c74, c105
Entire board has been resoldered
DC voltages are in normal range TP3 26.1,TP5 -26.2, TP4 15.5, TP6 -15.7, TP7 20.3, TP8 15.6, TP9 5.0, TP29 1.7
All IC's have been socketed.
The problem seems to localize to the DSP circuitry.
With the DSP board unplugged the output from instrument and mic sounds clean with only 3dB of hiss.
With U10 (mixer unplugged) there is no detectable hiss but also no instrument or mic output.
No change in the hiss when U3, U7, U8 and U9 are unplugged.
I am attaching the schematics for the amp. These are rev A and B. There are errors on these schematics. My board is rev C.
I am also attaching the service manual for Fender stage 100 DSP. It has the schematic for the DSP board. (I have the Stage 100 amp and have swapped the DSP boards with no change. The Stage 100 is quiet.)
I also attach oscilloscope traces at various test points on the complete amp with a 20mv sine wave input to the instrument channel with volume set to 61mv. (Have not tried on mic circuit because was unsure what phantom power would do to signal generator.)
With the DSP disconnected TP32 and TP33 show normal sine wave.

This is my first attempt at any posting here.

[Jon Snell]

What have you got on TP31 & TP34 with the DSP connected?
Your oscillogrammes look very much like crossover distortion.

[pa911]

I've been puzzling over the notched sine waves for the past month and have been blaming them on the DSP circuitry. But ever since I posted my question on this forum, the notched sine waves have disappeared. TP32 and TP33 now show normal sine waves about 600mv p-p superimposed on the 1.7vdc. The instrument channel has no signal input and only shows a flat 1.7vdc at TP31.
The hissing seems to start at the input of R53, where about 100mv of noise is seen. This goes down to about 20 to 30mv at TP34. The scope tracing is hard to describe so I am uploading a video of this as I scroll through the FX selections with FX level set to 4. The noise produces a thicken of the scope tracing.
By the time it reaches the amp output it looks like the last scope trace I posted earlier.
Can't seem to upload the video. Is this because of size or are videos not allowed?

[DrGonz78]

Can you trace the signal through the DSP board? As far as I can tell the the dsp has an AD conversion. Not sure if it’s codec part of the circuit.the aoutL and AoutR that does the digital to analog conversion. Sometimes digital converters create a hiss noise. I had a line 6 pod do something where the left channel was hissing but right channel was fine. Could not figure out how to fix it. Tried reflowing solder on the DA converter chips pins but the noise was from a bad chip possibly. I ended up cutting off the left channel output and essentially used the working right channel rerouted to left side output jack.

edit: just to add… that digital noise on the line 6 started occurring on and off again. Until one day it started happening each time it powered up taking 30 seconds to warm up to a hiss.

[pa911]

To Jon
I was not aware of an upload limit of 1mb, so cannot post any more images.

To DrG
By swapping the two DSP boards between my Stage 100 and the Acoustasonic I thought any problem with the DSP board would be eliminated, as both boards functional normally on the 100 and both boards hiss on the Acoustasonic.

[g1]

Did you say that your Stage 100 DSP uses the exact same DSP board, and that this Acoustasonic DSP board works fine in the Stage 100 with no noise issues and full FX available?
And do you have the same noise issues with the Acoustasonic if you put the Stage DSP board in it?

edit: I didn't see your last response. I think the DSP board can be eliminated as a cause of the problem

[The Dude]

Since you've eliminated the DSP board itself, but the noise appears to be coming from the DSP board in one of the amps, I would scope the 5V supply. It needs to be very clean to run a DSP.

[DrGonz78]

Yeah I was going to mention the 5 volt regulator and associated capacitors. I did see the 5v is present but the Dude has a good point about scoping it. Heck you could scope both amps and see if there is a difference.

[pa911]

C99 on the input of the 7805 has been replace with a 63v unit. C74 on the output of 7805 has been replaced with 63v unit.
The 5 volt test point TP9 shows a clipped 120hz ripple of 4 mv p-p. There is about a 0.5mv broadening of the trace which probably is due to my old equipment. Is it worth replacing C74 with a 100uf unit to cut down the ripple?

[Jon Snell]

A small amount of ripple is not an issue and the maximum capacitance should not exceed 10uF but 22uF is near enough.
Hiss is not usually power supply related.
Did you have a noise free signal at TP31 & TP34?

[pa911]

TP31 is noise free but definite noise at TP34. 60-70% lower than at input of R53.

[x-pro]

remove U11 and check the noise in TP34

[pa911]

With U11 out, 1khz signal at TP34 almost completely attenuated. Noise level looks about the same

[g1]

You have low noise at TP31, the DSP board is known good, and you have noise at TP34. I would try replacing U10.

Beyond that, the Stage 100DSP looks very similar aside from component names. You should be able to use it for comparison as far as noise levels at various points.

[pa911]

U10 and U11 have been replaced with no improvement.
I do note that the Stage 100 only uses one of the two DSP channels.
Will try to get some scope readings off the 100 later. Will need it to work next week.