[morzh]

IT IS GOOD TO BE PARANOID!

Today I got the small 3-prong cable (10A, AWG 18, so it fits through the existing bushing in the back).
I thought that since I have an extra Ground that was not used before plus an external load - the phonograph, plus it is not original - it would make sense to install a fuse.
And because this chassis is a super pain to take out - the fuse should not be an inline type, it should be accessible.
So I installed a 2" holder, put in 3.15A fuse, routed the wires, and - voila! - we have a nicely fused radio.
Then I connected the phonograph, the hum did diminish with the Ground though it did not go away completely, and the pickup could be heard when rubbed with a finger against the needle.
And then I turned the motor on, and the lights went out on the radio.
I immediately knew it was the fuse. Took it out, all black.
So I replaced it and repeated the whole process with an isolating transformer with the GND still connected. All worked fine.
But when plugged in the outlet directly - it blows up.

Upon examination I found out the wire connection of the phonograph is questionable, but obviously it worked since the Earth was absent. Not it does not.

Once I find out exactly what it is, I will describe it.