You said you heard a buzz before when powering up, so I assume you had the speaker connected.
It is best to first power-up with no speaker or load connected.
Then check that transistors are not overheating and there is no DC voltage at the output.
If all that checks out, then connect speaker and test.

Yes, I see now.
From the voltage drop (across the 2K2) you had with the 741's, assuming you are again at 22V with the 820R, you have increased from 8mA to 22mA.

So there is no defect, just an imperfect part substitution.  TL071 would have been a direct drop-in replacement with no mods.

The amp was working before with the 2K2 resistor, so changing it is just a 'band-aid' solution to some fault in the low voltage powered circuits.
You need to repair it rather than modify.
37V across 2K2 was 16mA of current, where was it going?

Does your friend have a multi-meter and know how to safely work inside of amps with power on?
If so, I would start by verifying that the preamp is getting it's proper DC voltages from the power supply.
This can be done by measuring the DC volts at both sides of the following resistors:
R20,R45,R29,R72,R10,R5

Yes, that should be correct.
It really shouldn't matter anyway, as it is a single speaker unit and you are not concerned with phasing.
The only other time it can be a problem is if the negative terminal on the speaker is connected to the speaker frame, and the frame is grounded.  (ex. some Fender Mustang amps)

As the sound came back and then went away again, it is likely an intermittent connection or bad solder joint.
Some chopsticking around the preamp area might help to isolate the problem area.

attached is a schematic that will hopefully be the correct version for your unit.
You cannot view this attachment.

First, put a patch cord between send and return, and try your guitar into the regular input again.

These amps will burn their output transistors if the heatsink is not completely mounted to the chassis, including the bar.  And the removable 'bar' has to be oriented properly or it will not mate up right with the heatsink.

I would not replace the 1C03 transistor unless it is proven to be defective.  Solid-state components do not wear out or degrade, like electrolytic capacitors eventually do, so they are not normally replaced if they are working.

One of the speaker connections will be routed through the 'phones' jack (so the amp is quiet when using headphones). 
If it is through the 'tip' connections, that would go to the speaker (+) terminal.  If it is the 'sleeve' connections that disconnect the speaker, then that would be for the speaker (-).
If you can post a picture of the trace side of the board in that area, it should be easy enough to figure out.

Here's a Carlsbro schem that uses the 1C03 in TR6 position.
Unless you are getting very strange HFE readings out of circuit, I would be hesitant to replace it just for that.
There is a 1C04 acting as it's complement in TR7 position, does yours have that?  If so, how does the HFE reading on that one compare?

Was the amp working, or did it have a fault you are trying to fix?

Nice work Phil.
Sorry to hear about your wife.  Hope you can have some peace for Christmas.

Do you have the proper footswitch?  Without it, these amps don't really work right, you get a mix of clean and dirty channels.
TR2 determines what the eq is doing, depending on which channel is selected by the footswitch.  It may be fine, you can look at the pinout for BC184 and see if the correct leads are going to the correct places (emitter to ground, collector to VR6).
Small value caps that are in the signal path are prone to degradation and cheap, so you might just want to replace them.  Or check them with a dedicated cap tester.

You can't just isolate single components in circuits that are otherwise not identical.
The input cap has to do with the other caps and resistors around it, do the other amps have the 100p to ground?
The gain pot bypass cap will affect the frequency response somewhat.
Do the other amps use the TIP100 and 105 output devices?  Those caps in the power amp are for stability.
Not sure if changing the IC would have any benefit. 
Sometimes putting modern parts in old circuits makes them worse, as they were designed around limitations of parts.  For example, chips with better bandwidth or faster slew rate can make amps sound more harsh.

Quote from: Tassieviking on December 03, 2023, 10:17:37 PMhttps://carvinaudio.com/pages/guitar-amp-schematics

Excellent.  From the SX package on that page, the SX50 is pg.14 and 15 of the pdf.

Quote from: smsuryan on December 03, 2023, 08:50:50 PMnot sure how to distinguish where the "blues button" is located and omit the clean channel. 

The blues button puts back the 500Hz that is notched out for metal and hard rock tones.  It is only on ch.1.
It is shown as S1A and S1B around IC's A3A and A8A.
Around Vol.3 pot (which should be labelled Vol.2?) you can see the 500HZ notch circuit with no defeat switch.

With unit cold and load connected, I would adjust so you have no more than 5mV DC across each of R222 and R223.  That would equate to approx. 10mA idle current through the output devices, which should be plenty.  Even 3mV should be enough.
Alternatively, if you had a scope, at low level signal output into load,  you would reduce the idle current till you see a crossover notch, then increase it till the notch just disappears.