Great to hear it worked out well, thanks for the feedback as it will give others clues as to how to do such things.
Phil.

Welcome Hutch,
IIRC, there is an FX loop or Preout/Pwramp input.
Send a signal to return only which should disconnect the Preamp from power amp and see if that resolves the Buzz and over heating.
I know they run those LM chips at the voltage limit and they can fail if driven hard enough, especially if driving 4 Ohms.

Phil.

Quote from: Loudthud on May 11, 2023, 02:45:12 PMYou learn how to apply Ohm's Law to calculate Voltages, currents and equivalent resistances. If you skip this step or don't know how to do it, you can look pretty stupid on an internet forum.

Yep,  I'm laughing cause That describes me very well. 
Yes I do have times when I get stuck and have to go back and refer to books to recall even basic ohms laws.
I figured I was too old to go back to school so I cheated and just use sims to workout most of the maths.

I use sims and the breadboard to design build my own circuits.
Being able see a graphical plot on the screen at any point in the design makes it so much easier to work out just where you win or loose the mojo.
I can't afford scopes and other fancy teck bench gear and as most of the gear I build is fairly straight forward it's not worth spending on gear that will not get much use.
Phil.

Hey chum, unless you intend to build the next space shuttle leave that really hard stuff till later.
If all you want to do is understand Analog Audio Amps then you will learn faster by reading stuff on pages like this;
https://www.sound-au.com/

Open up Articals.
Learn some basics first mate, such as transformers and rectification then move on to basic transistor circuits.

Scroll down to *Beginners Luck*, a lot of good stuff to find there.
Stuff there will help you get your head around the basics.

Be warned you will need weeks to absorb it all as it's a massive site with heaps of good info.
Phil.

Good work, Glad to hear it's working again.
Re speaker, Either way is fine, it only matters when you have more than ONE speaker THEN you have to make sure the polarity is IN Phase.
When more than one speaker the sound still works and no damage is done while out of phase but you tend to notice a loss of Bass.

When I met my wife years ago she had a stereo unit and she said her friend had the exact same unit but it had much better bass so I checked and sure enough the speakers were wired out of phase,, swapped the wires on ONLY One and bingo she then heard the Bass that was missing.

Yes adding a slip on heat sink can't hurt.
Phil.

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