Motorola 77XM AM/FM Radio found today

[Captainclock]

Originally Posted by jr_tech

Inside the radio, what is the third wire connected to? My guess is that it goes to the FM antenna terminal (perhaps through a small capacitor) and *not* to chassis ground. The loop antenna is for AM, not FM.

jr

Actually surprisingly enough it attaches to a terminal that has a small black wire that from what I can figure traces to the power switch. so not directly to chassis ground but it does reference to ground. I'll need to see if I can find a service manual/schematic for this unit online somewhere and see for sure what exactly the purpose was of the 3rd wire on the power cord, but from what I can figure, it doesn't function as an antenna of any sort, and since the cord is no longer attached except for a couple of inches that was left attached to the chassis there's no way to see what the original plug end looked like.

Update: I found the schematic on Nostalgia Air in Rider's Volume 18, apparently I was looking at the cord wiring wrong, the extra wire was actually one of the outer wires of the cord and not the middle one like I thought and the extra wire IS a built-in antenna of sorts (kind of an early version of the built-in antennas you see on your modern clock radios where the wire is wrapped around the power cord and using the power cord/household wiring as an antenna.) Go figure... Interestingly enough you can "disable" the built-in antenna by removing the jumper off of the FM Antenna terminals and then attaching a dipole antenna to the terminals or an outdoor antenna for better FM reception in fringe areas.