Very odd indeed.

Is one of the plugs TRS?

Thanks for the resolution.

Click on the link: https://www.ssguitar.com/index.php?action=dlattach;topic=3606.0;attach=7940

Here is the schematic set. (TGM30_Page 7)

Are the controls broken or simply 'scratchy'?

Dexoit works wonders for cleaning up potentiometers.

https://www.ssguitar.com/index.php?action=dlattach;topic=3501.0;attach=4607

All the information that you need is on the schematic.

" Ambient temperature was about 77 F"
Please provide the next data set.
What is the actual temperature of the back panel?

Is this anomaly seen when playing the amp or at idle?
If at playing: what wattage output?

My point is that the amp consumes power.
O/k.
Power equals heat.
There is a known power dissipation at idle and at full power output.

Whether or not there is an issue with the unit depends on these specifications.

Why are you considering changing the op amps?

MC1458 (LM1458) was a low cost IC and it worked well in the day.

The modern equivalent would be RC4558.
A lower 'noise' IC is the NE5332.

If it is the TDA2050 chip amp that is bad, they are no longer manufactured.

A valid substitute is the LM1875.
Drop in replacement.

I have attached the schematic.

Ok.

I was looking more at component designators.

Here is the correct schematic.

That is a most common diode.

You must place a 1N in front of it.

1N4007.

I am of the opinion that you should not attempt to bridge the amps together.

See that triangle with the -1 in it?
That is an inverted signal as compared to the Left channel input.
If you want to try to cobble up a circuit for that, be my guest.

I am not too sure but it appears that there is a RIAA circuit at the inputs.
It's a compensation circuit for the phono pickup.
That will affect the input as far as a guitar goes.

Could you elaborate on "only want to use one speaker'.

I would not try to bridge the two output circuits.

My son has one of these for his phone:

I would imagine it would not be that difficult to wire up a send jack from the amplifier preamp to a 3.5mm jack.