Mars Thunderbird Mobile Transmitter


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Picked up the Mars Thunderbird Mobile AM transmitter below a while back and I am going through and setting it up using an old Heathkit HP-23B power supply. These mobiles were imported from Japan back around 1959 to 1960 and later to a shipping port here in Northern California. The guys used them in their cars during the commuter runs like to and from San Francisco every day.


Then here is the inside top view of the transmitter.


Then here is the underneath side view of the transmitter.



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Then the transmitter was also featured in the March 1960 issue of QST magazine below back then as well.




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Here is a screenshot of the transmitters modulation envelope someone took at some point. It's not the greatest stock, but that can all be corrected.

If you also notice it wants to FM a little too so the B+ line going to first two 12AU7 stages needs to be regulated with a couple of small series zeners and a resistor.


Then here is some input/output data the person also documented then as well and at different plate voltage levels.



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The other issue is the microphone. I originally chose the cheap Astatic mic shown in the first pic because the input needs drive. The 6AQ5 tube has no mic amp in front of it. The transmitter was designed for a carbon mic because they were feeding the mic input transformer primary with a small DC voltage from the bottom 6AQ5 cathode resistor. No one uses carbon mics anymore though and for good reason. A good description of the way a carbon mic sounds is to take your best mic and wrap a thick dirty sock around it. Then slip a 1-foot cardboard tube over that. Then talk into the other end of the tube. That’s pretty much how carbon mics sound so using one is never really an option.

I did finally manage to come across the used D104-M below. I took it apart and the crystal element is still intact with no damage. I just need to go through and clean the switch and wiring up and replace a few components. I might use the cord from the new cheap one if it would fit and stay in the same way. The amplified Astatic hand mics have an output impedance of a few thousand ohms and that's what most carbon mics were so I’m trying to make it work with the input transformer still in place just like it is by leaving the DC on it, but cap coupling the mic to block it. The transformer is a step-up transformer so say like 5K in and 500K out to the 6AQ5's grid. Astatic actually started including the amplifiers in their microphones back in the 1960's because of this very type of radio and the early tube transceivers which followed. They all needed drive from the microphone or from an external amplifier placed in the middle.


I couldn't come across the schematic for the 4-wire M model above, but the M6 model came out the same time and was just the 6-wire version of it (aka Minute Man II). They both had the same transistor circuit below. It's just one FET with the output tapped from the drain resistor pot to act as a buffer for the high-Z element and to also get the voltage up. That is why it's the perfect mic to replace the carbon mic on the little AM Mars mobile. They are just very hard to find today, at least all intact and in working condition.



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Then here is the full schematic for the transmitter.


I was looking at how to regulate the 12AU7 tube, but not the 6AQ5 tubes in the mobile. Running the transmitter at +300V with the 12AU7 regulated at +250V would probably be ideal. The trick though is the voltage coming out of the HP-23B power supply. It's not exactly +300V so I'm trying to decide what voltages to go with before ordering the parts. The below idea will keep the oscillator stable so it doesn't FM, but it wont affect the AF or the final RF stage during modulation. This is a very simple modification worth doing and it shouldn't take a very big zener because the 12AU7 B+ doesn't draw much current.