The plug on the end of the power cord for that radio has some jumper wires inside. They serve to 'set up' a power cord either for mobile use (required an accessory box) or as a 120-Volt AC base radio. The mobile cord had jumpers on different pins.
Yes, they sold a mobile DC-power setup for it.
Takes more than just hooking the three wires from the power cord to the correct three pins. Needs those jumper wires on the RIGHT pins.
Sounds like a great subject for someone (else) to put in a YouTube vid.
So, to hook up a 3-wire AC power cord to the S312 21-pin "Jones" plug,
Shows hot side of the AC line to pin 3 This wire is usually black.
Neutral to pin 10. This wire is usually white
A jumper wire from pin 5 to pin 6.
A jumper wire from 7 to pin 8
Doesn't show where the third wire (green) ground should go, only shows a 2-wire cord.
Connect the third-prong ground wire to pin 7. Along with the jumper to pin 8.
Make sure the fuse is no larger than 4 Amps. ** FIRST ** !!
Then turn it on only with a variable-voltage supply, turning it up slowly from zero Volts to minimize damage if there are any faults in the radio or the hookup.
Simply plugging it into the wall and flipping the power switch is commonly called the "smoke test".
Naturally, if you let smoke out of it this way it will be damaged even more than it was before.
And putting the smoke back in always costs more than it did to let the smoke out.
Any tube-type radio this old has around a dozen or more electrolytic capacitors in it. They are meant to last ten or fifteen years.
NOT 40-plus years.
Unless the radio is NOS NIB with zero miles on it, you should find at least one of these capacitors gone bad. Sometimes they will short and pop a fuse. Sometimes they'll swell up and leak corrosive liquid onto the surroundings. Other times they will simply become "open circuit" as if not connected at all. The side effect of this is typically a low-pitched hum sound either on the receiver audio, transmit audio or both.