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:

The set uses the slug-type tuning principle. They were a bit more stable than the regular tuning condenser. Many of those early FM sets could be aligned with an AM signal generator, by following the proceedure in the service notes. I wouldn't bother with FM alignment, as it wouldn't help that much.
The Fremodyne was a form of super-regenerative circuit, developed for inexpensive FM receivers. Heathkit, even made a kit form in the late 40's. It received FM stations, but it wasn't a true FM receiver. Fidelity wasn't that great, as it used some form of slope detection. :sigh:

interesting, so the way this radio sits currently it may be fine without a realignment needed? Fascinating. Anyways I think I'll just replace the power cord and the capacitors and go from there. I noticed that the orginal bulb for lighting the dial was a colored C7 style Christmas light bulb that was yellow in color but most of the paint flaked off over the years. I was thinking of experimenting with different color bulbs in this radio like with red, or blue or even green to see what kinds of different effects I can get from the radio dial. What do you think? It would be kind of like how some people put different color bulbs into old stereo receivers to make the ordinarily white glow tuner into a more interesting blue or green glow dial.

The original lamp was probably white or clear. :boring:

I'm not hearing any objections so I guess I will then try out some different colored bulbs then. :thmbsp: :scratch2:

The candelabra-based 'Xmas tree' bulbs commonly used in 'nite light' applications are very short lived, unlike the originals. Even with a diode in series to run them at half power they burn out quickly.

SO basically any of the modern "nightlight" bulbs which is what this uses (except that the original was a 10 Watt bulb according to the Rider's service Information which they don't make 10 watt nightlight style bulbs anymore and the next closest would be either a 4 or 7 watt bulb.) what I would and could do is try and find a box of NOS C7 Christmas light bulbs (which they do show up quite often at the antique malls near me, especially around Christmas time) because it seems that's what you're saying by saying that the older ones lasted longer than the modern ones do.

In my experience, the 4 and 7 watters made in the last 30 years or so are very short lived.

The original 10 watters used in the 'Exit' signs in my building, installed in the 1960s, lasted a good 20 years before they started burning out. The only replacements available were the 4 and 7 watters, which required constant replacement until I finally got rid of them recently and put in LED retrofits.

And BTW, before somebody hollers, I did not run the 'Exit' signs at half power. I ran a single bulb with a diode to test how long it would last, and it burned out almost as fast as without a diode.

My job, as an electrician on second shift was to do exit lite maintenance. Lamp life was highly dependant on how well the filament was supported, which is true with just about any incandescent lamp. They used to buy 130 volt lamps. Some signs used low wattage fluorescent lamps.
At the end, we installed LED signs with ni-cad battery backup. That's the kind, I have in my two buildings. :thmbsp:

I Use these LED bulbs on my vintage night-lights and radios that require a line volrage bulb. They work fine for me and run very cool (only .36 watts!).

http://www.amazon.com/G7-Power-Beatt...ight+led+bulbs

not affiliated,
jr

Nice find :) You might be thinking of me. Mine was partial recapped and partially working when I received it. I haven't had a chance to finish restoring it yet.

http://www.videokarma.org/showthread.php?t=260088

OK, I thought someone on here had one as well but wasn't sure who which is why I asked. Anyways how was the cord repaired on yours or does yours still have its original cord still intact?

I might give that bulb a shot once I get the radio up and running. :thmbsp:

First time I've seen a radio make use of the AM pentagrid converter 12BE6 tube as an FM IF stage. I figured that that would be possible, and sensible, to put that tube to work when the radio is in FM mode. Don't know why this wasn't done more often. You'd need a switch contact that disables the AM local oscillator. Just short the cathode portion of the osc coil.

Luckily, it still has the original. The third wire ends just short of the plug. I'd use a modern three wire electrical cord with a two prong plug. Use the third "ground" wire for the antenna.

OK Thanks. That's what I was thinking of doing myself but I thought I would get a second opinion from someone on here first. I'll probably take a trip to the hardware store this afternoon and see if I can find the kind of cord I need.

And then they skipped over V-4, which *could* have been used as an AM IF.

Do you know why they designate V-1 as first *and* second FM converter? It doesn't look like a dual conversion circuit. :scratch2:

jr

I don't know but I do know that I have my power cord for the radio now which I had found an old computer monitor power cord (the flat kind) at Goodwill today in a basket of miscellaneous cables so I got it home lopped off the ends and got a plug end put onto the one end (leaving the ground cable cut off just shy of where it enters the plug end) and now all I need to do is get a small rubber grommet to hold the wire into the chassis (they had a couple of pieces of cardboard with holes drilled into them in different spots with two rivets holding it in place originally) and wire in the new cord and I'll be all set as far as the cord goes.

Now as far as the capacitors go, will I necessarily need to replace the old blown out death cap with a new one? Couldn't I just cut out the old death cap and just leave it out of the circuit?

You could try LED bulbs in your radio, but they may use miniature switching power supplies and may RFI the radio.

Someone may have used a bulb stolen out of a Xmas light set to replace a burned out original bulb. Yellow being the best match for the dial.

Scope probe held near the bulb just picks up 60 hz and a tiny bit of modulated carrier from a nearby AM radio station... no switching hash.

jr

Don't use anything except the bulb it was designed to use (or something close to it). On sets with series strung tube filaments, the bulb is a vital part of the circuit that, when blown or removed, will reduce the life of the tubes (especially the rectifier).

When you first turn on the radio the bulb glows brightly, quickly dims, and gets back brighter once the tubes are all warmed up. If you remove the bulb, the tube filaments get all that power at turn-on. A 50 cent bulb is a lot cheaper than a rectifier tube.

Usually very good advice for a series strung set. but in this case, the bulb is a 120 volt "night-light" sized bulb connected directly to the AC line after the line switch. Almost any candelabra base bulb (including a C-7 xmas bulb) may be substituted. The set does not use a rectifier tube, it has a Selenium rectifier.

jr

That's exactly what I was going to say, but you beat me to it! :sigh: Anyways, I think I'll definitely see about getting some C-7 style Christmas lightbulbs of varous colors to try out in the radio. I'll also see about that LED C-7 Style bulb you linked to earlier in the thread.