1955 - Motorola - 5MFS8 - Radio Repair

[doug7740]

This winter one of my projects is to try to repair the original AM radio from my 6 volt, 1955 Thunderbird. When I purchased the vehicle the radio never played, except for an occasional loud static sound from the speaker. Since the push button that controls the on/off switch would only stay in the on position, I had no way of turning the radio off. I also noticed that when I would turn the tuning knob, sometimes the pointer would stop mid-way and you would have to use the station selector buttons to get it to operate again.

After removing the covers from the radio, I performed a complete cleaning and inspection. During my inspection, I discovered that the lever that engages into the slot of the on/off push button had too much clearance. This was the reason that the push button would not stay in the off position. By slightly bending the lever and with some light lubrication, I was able to get the on/off push button to function again.

The tuning control is an interesting design. It utilizes two clutch plates that connect the tuning knob to the tuner carriage. When you turn the tuning knob, the gear on the end of the tuning shaft rotates the first clutch plate. Since the clutch is engaged, the second clutch plate turns the gear that is on the end of the spline shaft and moves the tuner carriage. When you depress one of the station selector buttons, the clutches disengage the tuning knob shaft, allowing the tuner carriage to move without rotating the tuning knob. These clutches are also designed to slip, to prevent damage if you continue to rotate the tuning knob after the tuner carriage reaches the end of its travel.

The problem with tuning knob on this radio was that the clutch material had deteriorated from age and was no longer adhered to the clutch plates. After removing the clutch assembly from the radio it was disassembled; cleaned, lubricated, and new friction material was made from gasket paper. After cementing the new friction material to the clutch plates, the clutch assembly was reassembled and installed back in the radio. Now the tuning control was functioning as designed.

After testing the vibrator and replacing 3 defective tubes, it was time to see if the radio would play. To my dismay I was only able to received one station, and it was a very weak one at that. Upon further inspection I discovered that the top of the movable core (A8) of Osc. Coil (L4) had broken away from the adjustment rod, and the movable core was not moving with the tuner carriage.

The broken core and adjustment rod was removed from the radio, and a very intelligent friend of mine squared the top of the core on a lathe, and drilled and tapped the core’s center to match the thread pitch on the adjustment rod. To keep the movable core from turning on the adjustment rod, a lock nut was used. After reinstalling the repaired movable core (A8) back into the Osc. Coil (L4), we were able to receive a much stronger local station.

Before I can cement the adjustment core rod to the tuner carriage, I have to properly adjust the Osc. Coil (L4). The alignment instructions mention an alignment tool that is made with a wood and rubber strip.

Can someone in the group explain this alignment procedure?

So everyone can get a better understanding of the repairs that I made here is a full post with photos- https://forums.fordthunderbirdforum...-replacement-recommendations.13189/post-80205

Doug
1955 Thunderbird Blue

 

[biddle]

On a related note, anyone looking to modernize the radio should check out this Ford Thunderbird radio on Amazon

 

[zeroroadkill]

Doug,
I figured out why (maybe) the light doesn’t come on. I checked the bulb and it was fine. Then I noticed the bulb wire goes to some sort of rheostat looking device but without an adjustment but a couple of other wires going to the yellow wire next to the power wire. I cannot find a female connector anywhere to plug the yellow wire into. I assume this goes to the light switch rheostat?
And what is this adjustment for, the antenna/tuner?
Thanks

 

[Gary Tayman]

Okay, tine for me to chime in. First off, that yellow wire is not original. The original black wire may have had a similar bullet connector, but this yellow wire tells me that somebody has played with this set already, and who the heck knows what's inside.

Here's what you SHOULD have: the power wire for the radio (with fuse holder) and the power wire to the lamp (with bullet connector) both go to the radio's on-off switch. This is a double-pole switch that's circular, and mounted to the side of the tuning assembly. These wires may not go directly to the switch -- the wires themselves may be soldered to a spark plate, which is a 1" square copper wafer that's soldered to the case of the radio. Then another wire goes from there to the switch, and then to the lamp.

The spark plate is an old-fashioned way of protecting the radio's components from various spikes and surges that might be present in the car's electrical system. Why? Good question, as protecting the light bulb might be overkill, and radio tubes are impervious to much of this (unlike transistors where one zap can blow it). Om top of that, that buzzy vibrator produces more spikes and noise than the rest of the car combined with the exception of the ignition. But we're looking at state-of-the-art, 1955 style. Anyway, that on-off switch is notorious for contacts getting dirty over time and acting up. It's usually the radio side, and the "fix" is to move the radio wires to the lamp side, and wire the lamp directly, bypassing the switch. Sometimes one might simply attach the lamp wire to the power so it comes on with the radio, regardless of whether the headlights are on. Rarely is this done professionally -- look inside and you can expect to see wire splices and electrical tape, or whatever. At least on the 55 you can reach the switch -- the 56-57 sets nearly always have the bottom case bent up from someone trying to reach that switch. You CAN get to it -- there's a trick -- but hardly anyone knows how. When I get these sets for conversion -- let's just say practice makes perfect -- I can hammer out that bottom cover so it's hard to tell it was ever bent.

If the switch appears bad, allowing the radio to work but not the lamp, try this: carefully squirt some WD-40 into the switch from the opening underneath. Click the switch about 100 times. Hold another button in while pushing the first one -- this lets you click it on and off quickly. Then see if the lamp starts working (assuming the bulb is good and the wiring is intact). Once in awhile it's too far gone, but in most cases it comes right back.

Of course the real fix would be to replace the switch with a new one, but try to find one. Most of them are Centralab, but a few 57's have CTS switches. Neither one is available.

That screwdriver adjustment near the antenna connector is the antenna trimmer. Hopefully you haven't messed with it. If you have you may need to readjust. Raise the antenna all the way, tune to a weak station around 1200-1400, and slowly adjust back and forth -- you'll find a hot spot where reception is best. Leave it there.

Those repro sets -- oh boy, don't get me started. They look almost original, I'll stop right there. I can convert an original radio to modern stereo, and customers send me older conversions for upgrade, but I will not touch one of those sets with a 200-foot pole.

 

[zeroroadkill]

Since someone has been under the dash replacing lots of wires it appears that they didn’t use the bullet connector on the blue/red lamp wire for the radio. I just spliced in a connector to the lamp wire for the yellow radio wire. The yellow wire did have an old bullet male connector. Now it works
Sure does buzz
Got the clock installed too. Tried several times to get it running with the motor and brushes but it kept stopping so I had a quartz movement installed.
Thanks Doug & Gary for all the great advice!!!

 

[Gary Tayman]

Those quartz movements from ISI are WONDERFUL! I offered conversions of these for awhile, but stopped for a few various reasons. But I've got one of these in my 1964 Thunderbird and it works really well.

Interesting you have a blue/red wire for the lamp. For many years you could always tell the lamp wire on the radio, as it's always blue with a red stripe. But that started around 1958; the 55-57 Birds just had a black wire on the radio.

 

[zeroroadkill]

The radio bulb wire is yellow
The car bulb wire is blue/red