Have you checked the power amp to see what filters are built in ?
I'm sure there are LPF's and HPF's in there somewhere, possibly to protect the speaker they used specifically in that amp combo.

The return jack on the 5215 does not go straight to the power-amp as well, it goes back to the pre-amp PCB where it goes through the effects amp and then the output amp stage, and then onto the power-amp PCB.

There are a lot of areas where you can loose and alter the signal between the return jack and the speaker, I would concentrate on other areas before looking at the feedback circuit on the power-amp.

Perhaps you could lift the wire marked 'E' and use that to send/return to another amp just to hear the difference compared to the send/return jacks.

Its just more experimenting and more fun
Mick
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If you want to do as much testing as possible there are a few more tests you can do, the fact that you have 0.646v across C58 at the moment could indicate there is a shorted track or component somewhere in the circuit.
It might also be that the 7808 regulator is stuffed but CR24 is lifting the voltage to 0.646v.

I would short CR24 and remeasure the voltage across C58 to see if it changed.
 
I would remove the links I put an X on in the picture in post 21.
Lift one link at a time and then measure the voltage across C58 to see if it made any change, if you lift a link and the voltage goes up to 8-9v then there is a short somewhere that that links goes to.

There is a small chance that C58 has failed big time and is shorting out, but you can usually see if there is something wrong with a cap and they don't usually short out.

You can remove the 3 links and lift a leg on C58 and then measure the voltage on leg 3 of the 7808 regulator to make sure it is the regulator.
You would measure from leg 3 to ground.

I still think Phil is right and the 7808 needs replacing, but doing more tests can not hurt, all you can do is gain more experience.(and have more fun doing it).
Mick

Quote from: saturated on April 09, 2023, 02:38:06 PMLinear Voltage Regulators 3 Pin 1.5A Fixed 8V Pos Vltg Reg

That sounds like a L7808, if you give a us link to it or a picture we can be sure.
Any L7808, LM7808 or KA7808 in a TO220 package should work, make sure pin 1 is input, pin 2 is ground and pin 3 is output.
As far as I know the LM78## regulators are rated at 1 Amp and the L78## regulators are rated at 1.5 Amp, all the ones I have seen are In, Ground, Out (1,2,3).

Do not get the smaller 78L08 one in the TO92 package as it is only rated at 100mA and is wired differently.

I also think that the 7808 chip needs replacing.
I would check if the7808 gets hot first, just in case there is a short somewhere in the 9v circuit that brings the voltage down.

There are 3 jumpers that you can lift that feeds the 9v tracks on the PCB, if you lift them then all that is left in the circuit is C58, then lift one leg on C58 and check the voltage as well.
Its just a process of elimination in case you have to wait to get  the 7808 from a shop, you can eliminate all other problems while you wait.

Remember that the LM7808 is an 8v regulator and there should be 8v from leg 2 to leg 3 on that regulator, the output voltage is raised to somewhere between 8.4v to 9v because there is a voltage drop over CR24 that raises the voltage from 0v to near 0.4v to 1v on leg 2 of the 7808.
If you shorted CR24 then you should get 8v output from the 7808 on leg 3.

You could replace the 7808 with a 7809 if you can't get a 7808 easily, just short out CR24 and you should get 9v out.

Why do you think the problem lies with the power amp part of the 5215 combo ?
I would have thought it more likely that the problem lies in the pre-amp or the speaker itself.

Have you tried running the pre-amp out (Send) into another amps power amp (Return)
to see if the problem is in the pre-amp ?

And then another amps send into the 5215 return jack, just to see what changes happen to the tone.

If you happen to have another amp handy or another cab, disconnect the speaker in the 5215 and run the wires into another speaker.

Before you start modding the 5215 Amp PCB it might be worthwhile just trying a different amplifier module in there altogether, you have +- 50 v so find a pre-made amp module that you can try in there instead.
If you stuff up the mosfets it's nearly impossible to get replacements for them, there are very few NOS mosfets left to buy out there but there are thousands of fakes that will not work.
Good quality REAL 2SJ49 and 2SK134 Mosfets are really expensive.(If you can find them at all)

I also think that the 5275 amp schematic has some strong similarity to the 3210, I think it could be of some use to work out what the component values are that are unreadable.

Here is the 3210 issue 7 schematic if it helps.
It is a lot clearer then the issue 2 schematic but there are most likely differences somewhere.

I don't have any audio files so you can hear the 12w Marshall amp I made.
The PCB I made up is made so you can build all 5 versions I found schematics for, you just have to use the right components from the BOM.
I also changed the power amplifier so it can be made into a 30watt amp as well.
You can pick Lead, Reverb,Bass etc.
Link: https://www.ssguitar.com/index.php?topic=5164.0

I was never a good player and I have bad arthritis in my hands now so I can't be bothered to make any sound clips.
But I love to make up PCB's and build stuff when I can.

To get PCB's made you just have to upload the Gerber to the PCB manufacturer, I personally have only used JLCPCB as they are a good price.
The one problem with JLCPCB is that you have to order 5 PCB's minimum of the same type, but it still works out cheaper then buying 1 PCB from most online shops so I just end up with spares.
The shipping can get very expensive with JLCPCB as well, but if you pick the cheapest postage they are ok, and still fairly fast (2-3 weeks).

You can measure the output of the 7808 by measuring the voltage across
C58, it should be approximately 8.5 to 8.7 volts.
The diode going to the centre leg of the 7808 regulator increases the voltage at the output by the amount of voltage drop across the diode, about 0.5 to 0.7 volts.

You can measure the voltage going into the 7808 regulator by measuring the voltage across capacitor C57.
 C58 is just above the 7808 IC and C57 is next to the fuse on the right hand side.

You should have 17.5 volts across C57, if you don't then check the diodes CR20 and CR23. Also check the AC voltage from the transformer, red wire to red/yellow wire.

Unfortunately I haven't completed this one yet, my dad got really sick early last year so I flew up to where he lived and stayed with him for quiet some time.
And then it was the funeral, cleaning up his house and sorting out what I wanted to keep etc.
It kept me occupied for 6-7 months and then I didn't get back to the Sunn Beta project.
I did finish one of the Marshall 12 watt amps I made a PCB for though.

I remember I was tweaking the resistor values in the channel switching circuit so it would work better and only use 1/4 watt resistors, I don't think I had a lot left to do before the PCB was finished for the BASS version I was building.

I can not see any reason why it will not work, I left the Gerber files I sent to JLCPCB to get the PCB's made in the first post

Thanks for reminding me, I will drag it out and get into it again very soon.
Are you interested in the Lead version or the Bass version ?

I do not think it would matter, I think most signals going through the Op-Amps are in the Milli Volt range up to a few volts maximum.

There are some circuits that rely on the Op-Amp reaching the voltage supplied to start breaking upthe signal and making distortion however, but this is on purpose to give the tone some growl.

There are exceptions though and I hope someone who knows better will jump in and give their opinion.
Cheers
Mick

I recently bought a Signal Tracer/Injector Pen V6.3 from https://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=1&products_id=728.
I haven't tried it yet but I believe it is a signal injector and an audio amp all in one. It injects a signal on one probe and then picks it up and makes it audible on the other probe, should be easy to see where you loose the signal.
Depending on where you are you can also buy one from TH Customs who makes them.
 https://diy.thcustom.com/shop/signal-tracer-injector-pen-v6-2/
There is a Utube video of how to assemble and use it
Or just make something similar yourself.

Have you made sure that you have power going to the PCB in the amp ?
Do you have -9V on pin 4 and +9v on pin 8 of the 4558 OpAmps ?
Measure each one to ground, it should be 19v between pin 4 and pin 8.
Please check that you have the correct voltages on the PCB board.
Do any resistors look burnt ? , any capacitors swollen up ?
Remove the PCB and look at the copper tracks underneath, check all solder joints with a magnifying glass to see if they look bad, dull, etc.

I would start with a quick check of voltages followed by poking the components with a plastic pen or chopstick while strumming the strings, this might find a bad connection or solder joint even if you can't see it with the naked eye.
Remember to use something plastic to tap the components with and be very careful with any wires connected to the mains voltage.Fuse, main switch and transformer are deadly to touch, but like I was told when I was an electrical apprentice "It Only Hurts Until You Pass Out Or Die".
Cheers
Michael

Do you have another amp that does have send/return jacks?
If so, run a cable from the Peavey Preamp Out to the return jack on the other amp.
That will check if the preamps work in the peavey.

This might be the schematic for it, but I think it might be for a head unit and not a combo as it has speaker jacks and no speakers on the schematic.
Is it the small combo with 2 x 6-1/2 inch speakers you have ?
I think it has 2 x 10watt output so 20watt total ?

I cheated when I made a power amp in a 125B pedal for the bench.
I got a 3116D2 mono amp module ($8) from E-bay and stuck it in a 125B box.
I got a 5v fan from Tayda and a 7805 voltage regulator to drive it and 2x6.35mm sockets as well.
It can run at 4.5v to 26v and can put out 100 Watt of power at 24v.
I can run it on 9v on the bench, 12v from the car, it's very versatile with a pre-amp pedal in front of it.
I run mine on an old Dell laptop 19.5v power supply.
If I make another one I will add a 7809 regulator and a 2.1mm socket so it can power the pre-amp pedal as well.