I'd first be looking at the load you are driving.
I assume the use of 2 speakers,, maybe check the total load is not below 4 Ohm.
If 2 speakers are used try running one speaker and see if it distorts.
An 8 Ohm load (1 speaker the power devices will not run as hot and far less chance of distortion due to running hot.

One other possible cause is the bias is drifting when hot leading to Xover distortion.
i don't know the XR circuit but it may have a small trim pot inside for adjustment and as the unit ages that part might be effected by heat.
Talk to the teck guy again and ask him to check,, it may just need a clean of the trim pot.
Phil.

Well it might be Lead free solder which is really hard to work with at home.
For the humble home hobbyist far better to use lead solder.
Label will say 60/40, 60% Tin, 40% Lead.

Lead-free (Pb-free) solder really needs to be done in a factory that is setup for it. Yes it crumbles and does not flow like 60/40 solder.
Phil.

Those pots are more than likely standard 16mm pots with 5mm pin spacing. Alpha and Bourns make em, an likely others also.
The only thing you need to watch out for when ordering is the thread and shaft lengths and of course the taper of the pot.
If you can find them with Conductive Plastic wafers, even better as they will last longer than you. The carbon units do tend to wear out prematurely.
Phil.

As it's unlikely you will ever find original switching pots there is a simple way to resolve the switching for Fat and Bright.

If you note, although the switches have 6 pins they only use 2 pins so it's a simple On/Off switch.

I would simply mount 2 small toggle switches on the front panel and run short wires from the pcb pads.
You can then replace the Volume and Treble pots with what looks like a standard 16mm 25kA pots,, well that is what I would do.
Phil.

Quote from: Tassieviking on April 12, 2023, 09:57:45 PMAre you thinking Deoxit Phil ?
I used WD-40 in industrial situations, especially if there was acid in the air, but that is frowned upon in amplifiers by many people.

I think Deoxit does not wash away the special lube in pots or something like that while WD-40 and Electraclean type products might, there used to be a "Lectraclean" that used to melt most plastics on contact, nasty stuff if you are not careful.

One can of Deoxit will last most people a lifetime at home if you only use it for pots.

Thanks TassieV, yes Deoxit.
I did purchase a can of Cleaner from Jaycar years back (probably not Deoxit) but found that WD40 worked just as well. I've used it many times to resolve scratchy pots.
Of course if the wiper has worn through the track then replacement is the only option.
Getting back to this amp the level pot on the ParaEQ section might be the hardest to source as it's a tapped pot.  
cheers, Phil.

Pot replacement can be a nightmare depending on the layout.
If it was me I'd try a can of cleaner spray inside the pots just to see if they come good.

I can't recall the name of the cleaner but any electronic supplier will have several types.
I know some will frown on me  but I use WD40 or RP7 to clean scratchy pots with great results.

My Hifi amp had a really scratchy Volume pot and just a squirt of RP7,, been running fine now for the last ~5 years.
Phil.

That's ok just giving you other options, have fun with it.
Oh adding series resistance is not the same as CFB, you will just loose power.
Phil.

As TassieV mentioned replace with the more common LM7809 as the LM7808 might be harder to find. Just bridge CR24 with a wire or remove it and replace it with a wire link.
google LM7809 and check the pinout is the same as the 7808,, should be the same but wise to check.
Phil.

From the first post;
"Basically, I'm thinking of modding my 5215 to add current feedback, in the hope that it will **add some depth and presence** to the tone."

If the issue is lack of depth and presence,,, just get a tone pedal, GEQ or ParaEQ can work wonders.
It's a heck of a lot easier to do these things in the pre stages.
Why do things the hard way?

Messing with CFB might slightly change the tone but nothing like an extra bit of EQing in the pre stages.
I use a Cab sim type circuit on my pedal board and it works wonders.

I've been down these rabbit holes and found there ain't much difference.
I'm just sayin,,  up to you  

I said in second post;
"Paying close attention to the frequencies and the total bandwidth at the output."

The tone controls on most amps are just to tweak the freq response a bit but what you need to grasp is the Whole System Tone.
Every stage can have an impact on the final out come.
Using Simulations are a great way to help you understand just how and where the tone is won and lost.

For most guitar styles today most on board tone controls just can't cut the custurd,, you need more.

I worked out long ago that it's so much easier to tweak stuff on a pedal board than to do open heart surgery inside an Amplifier.
I still hate the idea of pedals but it's just so much simpler than trying to work inside amps,, some are a nightmare to tweak because of layout designs.
 

As already mentioned, not wise to mess with a working amp.
go get something cheap to experiment with and hear for yourself if it's really worth all the fuss.

Regards to that 231Watts you worked out,,, remember that your transformer voltages would drop by a fair amount long before you got close to 150Watts. Depends on the VA rating of the Tx
Phil.

Then Q8 (7808) is most likely stuffed.
Phil

Hi and welcome,
Very good info on Rod's site.
Mixed mode Fback explained here;
https://www.sound-au.com/articles/current-drive.htm

Front page here; https://www.sound-au.com/

You can also Dload Teemu's book on SS Guitar Amps here on this site;
https://www.ssguitar.com/index.php?board=2.0

Having messed around with this idea my observation is,, well it won't ever react like a well setup Valve power amp stage but it may help a little.

I found it much easier to do all the tricks within the preamp stages. Paying close attention to the frequencies and the total bandwidth at the output.
Then if needed to back off the Bass freq a little in the power amp stage.
(note speakers may need to be matched to the sound/tone you want)

If you wish to gain some power stage compression come soft clipping effect then you can simply add a couple of series 47R resistors to the power rails to power amp.

You could run the rig through a 60~100watt light bulb in the mains which limits the current the power amp can use and you will have the closest thing to Valve Sag.
I actually did this to a small 30watt amp years back,,I mounted the bulb under the chassis with a bypass switch. I sold it thinking I could do it to another amp i purchased and now regret it as the new one was a completely different setup.
 
You will loose full power but as the amp is big it will still be loud.
Of course you can't do this if the system has any digital crap on board or the PSU is a switch-mode unit.
If it's a straight simple analog system then it will sag.
you can just try different wattage light bulbs until you find the magic.
Oh and Note; the bulb has to be the old incandescent type.
Info on light bulb limiters here if you are not sure;
https://www.ssguitar.com/index.php?topic=2093.0
Re the Current protection,, it's there for dead shorts but could fail if you are running at full power and the output dead shorts for more than a few seconds. 
Phil.

Does the trem speed knob change the speed of the tick?
On most trem circuits the LFO still runs even if it is switched off.
Maybe check you have the right valve in the tremolo socket as that can cause issues.
Poke around the trem circuit with a wood chop stick and see if you can hear any change.
Phil.

Well you have no +9 volt rail.
Now go check you have +9Volts at the output of Q8 the 7808 reg chip.
The output leg is on the left (looking at the layout) it goes to C58.
If no +9V then likely the chip is fried.
Phil.
Speaker is not needed for this test.

Hi and welcome,
Q does the amp have an effects loop?
If so plug your guitar directly into the RETURN socket and if you can hear sound then

bridge the Send and Return with a short lead and if that resolves the issue then the switch in the return socket is faulty.
It's a very common problem.
Do that first then tell the outcome, if no go then we look deeper.
Phil.

Well this site is not really setup to give a start up course on audio electronics,,
but try this site; https://www.sound-au.com/articles.htm#begl
Scroll down to *Beginners' Luck*

Main page is here; https://www.sound-au.com/index.html

There is a so much info on this site I doubt I'll ever be able to read it all in one lifetime.

Sorry but it's all in English. Phil.