Anybody feel like discussing Old Mopar/Motorola hybrid AM radios?

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Bill Crowell

Bill Crowell

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I find these quite interesting because they represent both the last dying gasp of the vacuum tube and its highest form of evolution before it was totally replaced by the transistor.

These radios used so-called "space charge" tubes, which used only 12 volts on their plates, in the front end and IF stages of the radio. This allowed Ma Mopar to get rid of the vibrator power supply that was necessary to produce the high B+ voltage for regular tubes.

The space-charge tubes performed only low-level signal amplification. The audio power amplification was done with transistors because, due to their low plate voltage, a space-charge tube can really only amplify signal voltage, not current; they can't handle more than maybe 50 mw. of power. The secret to being able to use such a low plate voltage is that the first grid after the cathode isn't used as a gate, like in a normal vacuum tube; it is instead biased so as to accelerate electrons toward the weakly-charged plate, rather than cutting them off as a regular grid would do. Physically, these tubes are called "miniature 7-pin button" tubes, which I think are smaller than regular 7-pin miniature tubes.

I've got a Motorola hybrid radio in my '62 Valiant V8 which doesn't work. I know that it is either a model 119 (the cheap model with no RF amp stage), or a model 203 (with an RF stage), but I won't know which until I remove it from the dash.

Would anybody be interested in a thread devoted to documenting, troubleshooting and repairing this radio?
 
Sorry, but you lost me at "space charge". I never took a course in electronics, though I wish I had.
 
I know, nobody cares about this stuff anymore, and I can't really say that I blame them. I am probably only interested in it because it reminds me of my childhood.
 
I have limited experience with early Mopar radios. My dad had a 1955 Chrysler, in the early 60's the radio failed. My uncle was visiting us and had electronics from the Navy in WW2. He took a small multimeter, and tested the transistors, found a bad one, and replaced it with a transistor from a pocket radio, fixing it. The germanium transistors at that time were point contact, and were prone to failure.
 
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OK, fratzhog lover, you've got it!

I will take detailed pictures of my radio, and I've also got the Motorola service manual for it. (I'm sure Motorola, if it still exists, won't mind if we scan a few pages for strictly educational purposes.)

Once I determine what model I've got, I'll post pictures of the radio and its schematic.
 
Thank you sir! Technology is great and radios today are fantastic for what they can do, but I love the golden era. I have a j39 in my front room!
 
An oldie

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They are great.
I want to see the schematic and how the tubes were biased.
I used it for a few years and was amazed at its fidelity, without my needing to remove any capacitors.
Still have it, but it's in storage.
 
Bumping just cuz it's a topic close to my heart.
I have 2 US utility patents on a low voltage vacuum tube device I invented in 2002, perhaps the last patents to incorporate such technology.
To me, transistors seem like crude devices.
Particularly bipolar types...they want to be On or Off. They are not happy being operated in half-conduction as an analog device.MOSFETS are better but still have relative electron immobility compared to a vacuum tube, which is very happy anywhere along a load line. Looking forward to the schematics.
 
Sorry for the delay, Darthomas. Here is the cover page from the service manual, the schematic of the deluxe model that has an RF amplifier stage, and the top and bottom views of the chassis.

I'll pull the radio out of my car and start troubleshooting it soon. If any of the space-charge tubes are bad, I wonder if I will have any problem finding them? For that matter, I wonder if I have the proper cards for my cardmatic tube tester in order to test these tubes? (Haven't checked yet.) It might be pretty hard to find a tube tester that will test these if I can't use my cardmatic.

Darthomas, what do you make of this "placir chassis" stuff; i.e., is this any different from the usual practice with circuit boards that devote one side of the board to a ground plane?

"Plated Chassis Board Description - Motorola's placir chassis has plating on both sides of the chassis board. The exposed side contains the actual circuit connections while the plating on the component side of the chassis board provides a convenient chassis B- (ground) return for components and circuit wiring. Plating on the component side also provides shielding between the top and bottom of the unit similar to a metal chassis which incorporates hand wiring. All B- connections are carried through the placir chassis in multiple paths to the B- side. This insures a good low resistance return path. Four separate and independent paths are provided to ground. Refer to "Plated Circuit Chassis Servicing Techniques" manual (Motorola Part No. 68P636536 for information pertaining to servicing the plated chassis board."

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Sure, Yote, go right ahead as far as I am concerned. I'll try to help you if I can. Also, I can tell that Darthomas and KitCarlson, among others, are really smart about this stuff, and I am sure they would be willing to help you, too.
 
Thank you Bill,
I have '72 origional am radio in car with two '4-8 ohm compatible' rear speakers in parallel. Also 8 ohm speaker in connection with origional fader switch wireing. It sounds good. However on this forum it is said I will ruin my radio using 8 ohm speakers. My research indicates two 8 ohm speakers wired parallel draws 4 ohm but in series is 16 ohm . Correct me please if this is wrong. Our local radio guy (retired) thought it wouldn't be a problem as these older radios could ' take it'. I would like more opinions on this , especially from those of you that REALLY know this stuff , as I don't.
Yote
 
Yote said:
Thank you Bill,
I have '72 origional am radio in car with two '4-8 ohm compatible' rear speakers in parallel. Also 8 ohm speaker in connection with origional fader switch wireing. It sounds good. However on this forum it is said I will ruin my radio using 8 ohm speakers. My research indicates two 8 ohm speakers wired parallel draws 4 ohm but in series is 16 ohm . Correct me please if this is wrong. Our local radio guy (retired) thought it wouldn't be a problem as these older radios could ' take it'. I would like more opinions on this , especially from those of you that REALLY know this stuff , as I don't.
Yote
Click to expand...
You are absolutely correct. Two 8 ohms in parallel is 4 ohms. Two 8 ohm speakers in series is 16 ohms, so you can connect two 4 ohms in series and get the correct 8 ohm impedance.
 
It is my understanding that these radios require a 4 ohm speaker. I have also read that speakers labeled '4-8' ohm compatible are in actuality 8 ohm. Do you think my system will be okay???
Yote
 
If it's labeled 4/8 ohms it should be fine. There would have to be different terminals on the speaker for 4 or 8 ohm connection. Do you have a pict5 of the speaker?
 
I have never had any problems in 40 years from using 8 ohm speakers.
The output transistor's DC "source" is through the speaker. So if a higher impedance speaker is used, less current will flow and the transistor will be maybe a little less powerful, but sort of protected from harm by being operated more conservatively. The thing to avoid is a speaker of less than 3.2 ohms, which would cause excess current in the output transistor.

The "placir board" just seems like clever circuit board usage for the day, but many service men hadn't seen them yet in 1961.
So they gave that explanation.
 
The only thing I see notable about the way the tubes are used is that they have more or less full voltage to the plate because there must be some voltage drop to couple the signal to the next stage, but there are no resistances in the cathode circuit to create bias in the tube. It just runs as much current as you get with 10 volts on the plate, which is very little.

I think I have Model 204.
Mine has an occaisional "pop" as capacitor C11, a .01 mfd cap has developed leakage.
Now I know what I'll need to do.

Bill, what are the symptoms in yours?
Do you hear a low "thud" in the speaker when turning the radio on/off?
You should.
 
Yote said:
View attachment 1714979384 View attachment 1714979385 My speakers in question. Front speaker on top , rear speaker x2 on bottom.
Click to expand...
While the spec 4-8 Ohms is impedance, which is not just DC resistance, it includes the dynamic resistance to the audio.
However, what is important to the transistor is the DC resistance regarding the operating point.
Measure the speaker resistance with the wires disconnected.
If they are closer to 8 ohms wire them parallel.
If they are closer to 4 ohms, wire them in series.
If they are 6 ohms, try both ways and use whichever way is truly louder.

Make sure they are "in phase".
If thats parallel, both plus together, both minus together.
If it's in series, its into the positive speaker terminal, out the negative to the other speaker's positive, then the negative out to the return wire.
If it's hard to tell whether the speakers are in phase, you can connect a 9 volt battery to the wires (disconnect radio) and note which way the cone travels when applying voltage..feel the cone from behind and it will be clear which way it goes. Make sure the two speakers both go the same way, even if the terminal designations would have you thinking it's wrong with plus on one, minus on the other to get it so the speakers are in phase.
 
I've got a bad feeling about trying to troubleshoot this radio until I better understand the circuit's theory of operation. There's too much risk of going off on a wild goose chase in troubleshooting unless you really understand how the radio works first.

So did we ever figure out how the bias is being handled in this receiver, or are the tubes being run "full tilt", without bias? In the case of the amplifier tube elements, in what class are they being run?

I'm going to ask these questions to some of my friends who know more about electronics than I do. Also, there are some websites concerning space-charge tube circuits that I need to look at again.

However, most of the academic literature concerning space-charge tubes is written in Russian, because U.S. manufacturers only made space charge tubes for a few years before switching over completely to solid state. The Ruskies continued to use them, though, and eventually developed, to a very high level, fingernail-sized subminiature space charge tubes for their satellites before they switched over to solid state radios. Their space-charge tube satellite radios actually outperformed the solid state ones that they later used in their satellites.
 
Nothing special, just design optimized for low voltage effectiveness.
They're full tilt but it's a small hill.
Think of it as any standard radio.

Did it thump quietly when on/offing?
Did you check the speaker first?
What have you checked?
Maybe a tube just fell out.
Motorola was not just some fly-by-night manufacturer so there can be some faults with anything but I was amazed when mine came to life with hay still in it and sounding like FM.
Read USPTO No 6982549 regarding the many advantages of low voltage on the plate.
I want to put this radio in my 2013 "@hem, cough" "Charger".
I swear it's just a Dart on steroids.
 
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