Are you using two different guitars when you try the high and the low input ?
Does one of the guitars have active pickups and the other normal pickups ?
Some guitars have active pickups that have a battery and electronics installed in the guitars, they put out a larger signal then standard guitars with normal pickups.
That's the reason why some amps have a High and Low input (as far as I know).
One input is for the smaller signal of a normal guitar and the other input is for a stronger signal from an amplified active pickup guitar.
 This is not to be confused with amps that have more then one channel, usually one clean channel and one or more channels with distortion.

I think you will find that the amplifier is working like it should.
Does one input sound louder then the other with the controls set the same ?

 

First check the voltage on C102 in both AC and DC.
Should be approx 98V DC.
Check the voltage on the speaker out with no signal both AC and DC.
If you have a large DC on speaker out then don't leave a speaker plugged in.

Like Phatt said, send and return is the signal going into and then coming out of the DSP effects board, just like amps have send and return jacks that we can connect stomp-boxes to.
By placing the link in there you will no longer be affected by problems in the DSP board.

If you make the modification and still have problems, then its not likely to be the DSP board.

Next time you open your amp up could you please get some better photos of the board ?
even better if you can remove the sticker on the large IC so it can be identified.
It might not help to fix it but it would help understand what is happening in there.

A picture of the DSP board would be good, even better would be a part number if there is one on the DSP board.
There are some DSP boards on Ebay, also some other places on the net, but a part number or a picture would be great to identify which one is needed.

https://www.loudtechnologies.eu/shop/en/crate/dsp-effect-boards/g-10000030

Have you looked to see if you have 10v going into the DSP board ?
There might be a 5V regulator and maybe a 3V3 regulator on the board as well.

G'day Victor Van Dort
I am very confused with your post, why do you think it is impossible to repair ?
The schematic looks like a fairly easy one to read and follow, and parts are no problem to obtain that I can see.
The DSP board would be more difficult for someone with my limited experience with digital audio boards but it might be nothing wrong with that part.

It might be so simple that it is a solder joint that was bad to start with and has corroded a bit with age, simply re-melting the solder would fix that problem.
It could be dirt in a connection or plug somewhere or any simple problem that any technician would find in no time at all.

I feel that most amps can be fixed as long as the parts are available, and many parts that are not made anymore can be substituted with better modern parts.

There could be a young electronic engineer in the making among your friends or family, they might love to have a go at fixing it for you when they get a few years older and more experienced.

Do you have any electrical / electronic experience ?

Lots of pictures would be good, with enough pictures of the PCB's we can make a schematic for the amp.
with 575 watts max input and 2 x 15" speakers it must be a decent amp.

Not sure what is wanted either, possibly something like these:You cannot view this attachment.

I'm not sure how much this one cost.
10 off 12 watt PCB's  US$ 11.40
10 off 12 W front facia PCB's US$ 18.60
5 off Lab Series L4 PCB's US$ 8.00
Shipping cost US$ 23.10

I have most of the components, but I had to order some pots and transistors for them.
I also bought a toroidal transformer for the Marshall 12 watt, 30V CT 50VA.

To me they look like clipping diodes, stick some sockets in the holes and then you can experiment with led's diodes etc.
If you get a row of SIP sockets you can break them off individually and solder them in.

They might also work as a signal limiter so the signal after this stage stays below a certain level, the led's would start conducting at a certain voltage and thus limit the voltage going to C9 and C10.

Slowly making progress, I got some PCB's made so I can test make some.
I ordered the PCB's from JLCPCB and picked the cheapest shipping, I have too many other things going on so I was in no hurry to start.
They got to me faster then I thought so now I have to get some components I don't have on hand.

I also found a circuit for the 12 watt Bass amp so I modded the PCB so that can be build as well as all the other 12 watt Marshall circuits I found.You cannot view this attachment.

I made a PCB for a front panel as well, I always wanted to try that out.

I also got some Sound Lab L4 Bass pre-amp PCB's made for an experiment, I don't know if they will work or not but I added one  LM13700 chip instead of a CA3080 and a CA3094 as original. I am to lazy to breadboard the whole pre-amp to try it out because it is so bloody big.
 

I agree with Joe, it sounds like it might be the solder joints on the input jack.
It's a common problem when the socket is soldered to the pcb, every time the guitar is plugged in the socket is stressed mechanically and eventually the solder cracks.

The input jack is the shorting type from the schematic I have so it could be dirt in the socket, a quick squirt with Deoxit or similar and the stick the plug in /out several times might fix it.
 
 If you have a good magnifier, look over all the solder joints when the board is out and re-flow any that look suspect.

Here is another person with input problems, but different noise, good pictures is why I am adding it.
https://www.diyaudio.com/community/threads/fender-pr-241-repair-frontman-amp-made-in-mexico-high-pitch-sound.343735/

Good luck, and keep us informed of the results.

I was under the impression that was how it worked, one switch is for ch1 or ch2 and the other switch changed over to channel 3

Like this

Great explanation Phil, helped me understand it better.
Thanks.
Michael

If you want something that slips into that chassis, start looking around for another Peavey,
You like the sound of the amp so just get another one,

If you get the same amp you can start looking up some electronic lessons on the web, and then start taking measurements on both amps and slowly trace the fault yourself.
You might slowly learn more and more, get the amp fixed eventually.

I bet you would start building your own stomp boxes in no time.
I saw a Peavey Special 130 go for $65 at an auction where I live 2 weeks ago.
Keep looking till you find what you want.
Just don't throw it out, someone else would love to have a go at repairing it.