[SiriusHardware]

The power device (RF output transistor?) whose screw you accidentally drew a spark from will be insulated from the chassis by a plastic washer surrounding the screw, and a mica or rubber washer between the device or by an insulating sheet between the local heatsink and the chassis. If it was only momentary you are unlikely to have caused any damage. If it had been sustained, you would have drawn heavy current through the coil(s) going from the transistor's collector up to +V supply and could potentially have damaged those coils but hopefully the fuse - there is a fuse, right? would have blown first.

Can you do a quick meter check and see if there is a low resistance path between the main metal chassis and the metal can of any of the receiver coil 'cans'?

Historically, a lot of this generation of radios had their metal chassis DC insulated from the circuit board ground, but 'RF connected' to the circuit board ground by multiple ceramic disc capacitors which went from the circuit board ground to nearby tag points on the metal chassis.

The reason for this was, I guess, so that radios so built could be used in vehicles with either positive or negative earth systems.

You say that if you touch the PTT pin of the mic socket to chassis the radio goes into TX so it sounds like the radio chassis is connected to circuit board ground. I'm just wondering if it is supposed to be.

I have a very similar Midland 77-104 which is the 'original' straightforward mobile version of that radio, the Portapak being a special 'variant' of it. I know where it is but there is a lot of stuff between me and it. If I can get to it I will look and see if the PCB ground on that one is hard-connected to the metal chassis. Unfortunately mine happens to be the CEPT (Midband) version so there will be some differences anyway.

It would be good if Techman could check his and see if the chassis on his is DC-connected to the ground area on the PCB as well.