Motorola 77XM AM/FM Radio found today
 

Hello everyone today I found at one of my favorite antique store haunts was a 1950s vintage Motorola AM/FM Radio Model 77XM that's going to need a lot of work, because the original cord is pretty much gone except for a couple of inches of it sticking out of the cabinet, and the cord was a 3 conductor plug where the middle (ground) conductor operated as a built-in antenna of sorts besides the external antenna connections for both AM and FM. There was someone else that had this same exact radio on here that they overhauled and I was wondering if whoever that was might be able to chime in and give me some tips of what I might do with getting mine going, especially as far as getting a proper replacement cord goes, which I was thinking of trying to find a 3 prong power cord from like an old light fixture or computer monitor or something and cutting the plug end off and wiring that into the radio like the original was and then wiring a plug end onto it and going from there.

Any tips or advice would be appreciated.

-Levi

Pictures posted below.

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.

Try this... schematic on page 1 & 2 will enlarge good enough if you select them... shows the 3rd wire going through a 50 pf cap to the FM antenna terminal.

https://www.tubesandmore.com/schematics/motorola/hs-102

hs-102 is the chassis number for the 77xm.

jr

The schematic is probably online (google "Nostalgia air" for one good source) consulting it would be advisable.

EDIT: JR posts faster than I can write.

I just did consult the schematic and confirmed what was suspected initially in my post. So could I use a standard flat-wire 3 conductor cordset from say an old LCD TV or an old Computer or some sort of old appliance cordset (not the ones from a stove or anything but from a refridgerator or something even) and use that for the replacement cord on this radio as long as its wired up like the original was?

Should be ok, be careful to not connect the third wire to anyrhing at the plug end.

Or just use a std 2 wire cord for power and another length of wire (like 2 to 3 feet) attached to the antenna terminal to function as an antenna. That way you can adjust the position and length of the antenna independently of the line cord position.

jr

Wow that radio sure has some sick looking caps !

I would just use a standard 2 wire cord. Add one of the cord clips
that almost every FM radio uses. The clip & cord make
a very low pf cap to couple the FM signal to the radio.
The cord antenna has nothing to do with AM. That is covered by
the loop ant built on the back. Loops are almost always
superior to long wires on AM. They are very directional for
starters.....

73 Zeno

OK so I guess I will probably just replace with a standard two conductor cord and just use a kink-on FM Antenna terminal on the power power cord for the external antenna. Although what will I do with the built-in circuitry for the original built-in antenna that was utilized through the original 3 conductor power cord? could i just remove that part of the radio circuit completely? Also I'm assuming that the big capacitor that's run across the power supply was a death cap that must of gotten destroyed from a lightning strike, or some other kind of power surge?

That's not a 50's model, it's from 1948.

That cap across the line probably shorted, got hot as hell/ugly, and burned open inside all on it's own from age.

If you test power up sets with original caps you will see a cap fail like that eventually.

OK well I kind of figured that out after I posted this thread because I looked at the riders and even after seeing that the pilot lamp was a 110v night light bulb like my 1946 philco was I realized it was probably 1940s and not 1950s. I'm guessing that since this has Modern FM on it besides AM that must mean this was a higher end unit since most radios from 1948 were either AM only or AM and Shortwave only and no FM.Either way besides the cord being gone on it the unit is in fairly good shape yet surprisingly enough, and is complete.

It must of happened a fairly long time ago as the wax on the cap is hardened and discolored like its been sitting a while (well and that and considering that the original power cord was cut off who knows how long ago) it seems that this radio has probably not seen a power up in at least 20 years I would guestimate.

I looked up the schematic on N/A and was surprised to see it still used the 4.3 mhz IF frequency, like the pre-war FM sets used. :scratch2:
Those early FM sets weren't that good of a receiver, plus they drifted like mad.
Well, at least it isn't a Fremodyne! :D

Interesting, I hope this isn't going to need a lot of realignment because I really don't know a whole lot of realignment stuff except for just what I can do by ear, I have an oscilloscope which could be used for that but I don't have a frequency generator (I had seen one once at one of the local flea markets for $20 but I didn't have the money on me at the time and when I did have the money later on I went back to go get it and it was already sold!) Anyways What is Fremodyne? And why would they use the old FM (42-50 Mhz) as the basis for this radio which uses the modern FM band? Also the FM Tuner mechanism is really interesting on this radio it uses a "piston" like mechanism that's controlled by a small gear that's mounted on the AM tuning capacitor for tuning the FM... :scratch2: