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-23 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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Here is a screenshot of the 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.


Then here is the full schematic for the transmitter.



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Was also looking into building a possible vibrator pack to run the transmitter from 12V as well and I did find the old pic below where someone used an old Sun/Air pack to do it. This is something I can maybe come up with later, but for now the HP-23 supply works fine. That Sun/Air pack may have been for 24V though and not 12V. It's an old aircraft vibrator pack.


The other issue is the microphone connection. I 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 as shown from the schematic because you can see how they were feeding the mic input transformer primary, T1 with the 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. What I wanted was one of the old original NOS Astatic Minute Man mics below (D104-M), but you just don’t see them around anymore. They were the closest thing to the actual D104 mics and sounded almost the same. 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. T1 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.

Original D104-M (aka Minute Man I)


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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 in and stay the same way.


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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I had completely forgotten these old mics used the darn 7.5V dog collar battery below. They were always a bit of a pain. When I opened it up I went "Yikes!" Flashback... I had to order a couple.

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