Fender M-80 Loud Pop on Switch on/off

[phatt]

99% chance You just blew your output transistors.
Buy new ones, and this time make damn sure with a meter continuity test that there is no short between the transistor and the heat sink **Before you power on**. you might have also stressed other components by doing this, So be warned you will need to use a **limiter** when you replace the power transistors as the other driver transistors might have failed as well.
How to make a Lightbulb limiter; https://www.ssguitar.com/index.php?topic=2093.0

As has been explained already,,,
You will not without major alteration be able to eliminate the pop.
Sorry if it sounds harsh but you just created a much bigger problem for no good reason.
A wise man once said;
"If it ain't broke don't try and fix it" 

[g1]

So here's an update. On replacing all 4 power transistors, I eliminated the loud switch on pop.

No you did not eliminate the turn on pop. 
As soon as you turned on the amp it blew.  There is probably DC on the output, which is why you hear the hum.  If you keep running it with DC at the output, it will burn out the speaker as well.

[J M Fahey]

I believe the plain M-80 is the HM series amp.  Schematic attached.

I'm not looking at them side by side or anything but the power section looks identical to me based on the details I remember from studying the chorus model's power section yesterday. Of course the problem might not be the power amp; you never know.

Yes, the power amps are identical.
Only difference is that the mono one has 4 power transistors for 100W into 4 ohms, while the stereo one has 2 50W amps with a single transistor pair each.

But operation and troubleshooting are the same.

[argenta]

Happy New Year all. I did indeed replace the power transistors and the driver ones as well. With the power transistors, I made sure they are insulated from the chassis, however, when testing for shorts between the transistors and the heat sink I observe the following:
The meter reads short circuit initially, then over a few seconds resistance increases to open. Why is that?

[joecool85]

Happy New Year all. I did indeed replace the power transistors and the driver ones as well. With the power transistors, I made sure they are insulated from the chassis, however, when testing for shorts between the transistors and the heat sink I observe the following:
The meter reads short circuit initially, then over a few seconds resistance increases to open. Why is that?

Have you built a lightbulb limiter yet like Phil mentioned?  https://www.ssguitar.com/index.php?topic=2093.0

If you need one, PM me.  I have one collecting dust that I'd be glad to send your way if you pay shipping.

[g1]

when testing for shorts between the transistors and the heat sink I observe the following:
The meter reads short circuit initially, then over a few seconds resistance increases to open. Why is that?

Those transistor collectors are connected to the supply rails.  The heatsink is grounded to the chassis.  When you measure, you are measuring from supply rail to ground. 
What you are seeing is the meter charging up the supply rail filter caps.