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.

A quick drawing of how most pre-out signals go into poweramp-in jacks, but not always on every amp.You cannot view this attachment.  You cannot view this attachment.
This is the switch that plays up when you lose signal to the power-amp with no jacks inserted, I like to get a small strip of normal printer paper and put a drop of WD40 or contact cleaner on the paper, put between the contacts and run paper up/down to clean the switch contacts.
There is no need to spray lots of anything in there, the paper will rub the dirt away, simple.

The coupling caps are C41 and C42, they are in parallel to form one capacitor of 0.2 uF
Measure the DC output of the power amp with black probe on gray wire and red probe on yellow wire going to the speaker jacks on the back panel.

IF YOU HAVE DC ON THE SPEAKER JACK, MAKE SURE YOU DO NOT PLUG THE SPEAKER IN.
DC on a speaker can cook the speaker coil, not a good thing at all.
Maybe isolate the power amp from the pre-amp, put a plug into the FXreturn jack, even plug the speaker into the FXreturn as this isolates the power amp from the pre-amp and it makes sure there is no input on the power amp.

Also, running a lead from the FXsend to the FXreturn does nothing, there is no buffer or anything else that magically appears.
If you trace the wires you will see that the fxsend is wired directly to the fxreturn socket, when you plug into the fxreturn this link is broken by the switch in the fxreturn jack.

Have you got +16V and -16V on the board ? Check all fuses if not.
Measure voltage across C61, C62 for 16V dc   (+C61 to -C62 = 32V dc)
Measure voltage across C55, C56 for 42V dc   (+C55 to -C56 = 84V dc)
What is the DC voltage on the speaker output ? is it + or - voltage ?
A few mV on the speaker output is ok, 40V is very bad.

Here is the schematic for you to check with.
Output transistors are Q7, Q8, Q11, Q12
You cannot view this attachment. 

I should also include the original schematics I started with I guess.
Also the OTA's datasheets.

I would love to make up a PCB for a full preamp and use a class-D power amp with it.
And also a Pre-amp pedal.

Has anyone else ever noticed that the TPA3116D2 Bridged 100W amplifier module with XH-M542 written on the bottom fits easily into a 125B pedal enclosure with plenty of room for 2 jack sockets and a power socket at the top end. (2-4 ohm in bridged mode)

Run it on a 4.5V wall wart for approx 5W output to 24V for 70Watt output at 1% THD
Advertised as 100W output or 120W on most E-bay stores, but distortion is 10% THD.
Google TPA3116D2 or XH-M542 amp and you will see the one I am talking about.

Rock up to practice with 2 pedals and a speaker cab and blow them away ?
 Most of us can grab an old laptop power supply from somewhere to power it,

Anyhow, here are the rest of the schematics :-

G'day guys and gals
I have been wanting to make a Lab Series Pedal for a while, recently I was talking to a young dude who plays Bass in "June And The Desert Flames".
While I was talking to him I realized he does not have a lot of gear and I can easily build some Pre-Amp pedals and some effect pedals for him fairly easy.
I have some PCB's for Sunn Beta pedals, Chunk System Brown Dog/ Agent 00Funk, etc at home.
I also thought it might be a good time to get into a Lab Systems PCB, the L4 Bass is simpler then the L5-L11 pedals so its a good starting point for me.

I have tried to change the CA3094 and CA3080 IC's to one LM13700 IC, but I am not that good on theory, I gave up working as an electrician 15 years ago because of all the narcotic pain killers I have to take every day.

Could some of the really great people who post here check my changes please and see if it is feasible to replace the 2 IC's with one LM13700 please.

I have read through lots of posts on this type of modification on the net and did the best I can, my brain is too forgetful to try working it out or trying to simulate it.
If someone knows how much the +- 15V rails use would also be great, I would love to power it from a LT1054 or similar on 9V supply.

Thank,
Michael

If you want to see the pictures larger, right click on the small picture and select open in new window, this makes the picture really large.