But the tubes found in the RT-159A are special. Miniaturization was critical for this bit of mobile electronics, and accordingly most valves used are about the diameter of a pencil, quite a bit smaller than even the low-gain tubes found in mainstream vintage products.
So how did it work? We get a great start from the schematic diagram helpfully placed as a sticker on the inside of the rear case cover. With a little more poking around on the Web, more details emerge on the RT-159A circuit.
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In the diagram, tube V1 provides the crystal-controlled initial oscillator used to set the reference frequency for both VHF and UHF transmissions/receptions. Tubes V2 and V3 are both part of doubler circuits and the output of V3 is the 121.5-MHz signal, so a 30.375-MHz crystal frequency is implied. The output of V3 is followed by V4, also a frequency doubler, and source of the 243-MHz UHF output signal. When run as a receiver, valves V5 and V6 serve as detectors for VHF and UHF, respectively, to demodulate incoming signals when the antennas are in Receive mode. The diagram implies a simplex circuit--no simultaneous transmission and reception seems possible.
For the audio circuits, tubes V7 and V8 are used. V7 is the first-stage audio amp with V8 serving final-stage audio output and modulatation. Output from the audio section is routed back into the appropriate part of the V3 and V4 doubler stages to modulate the audio signal on to the 121.-MHz or 24-MHz carrier frequencies. Base audio input and output--mic and speaker, essentially--are by way of a shared transducer labeled LS1.
The electronics construction is decidedly labor-intensive, and was surely state of the art for its day. Passive components are connected with a mix of carrier terminal boards on which the parts are mounted, and directly wired parts connecting to the sockets are used for the tubes. Even the carrier boards use hand-soldered discrete wires to get to the right places, with additional wiring to route the tunable inductor bank (the yellow-sleeved devices pictured).
Cast metal enclosures along with relatively weighty electronics and antennas lead to a 1.25-kg (~2.75-lb) radio, and the tube-based design required a high-voltage rail for operation, so it's likely the external battery had an inverter of some sort, further increasing both weight and bulk. Still, the mission-critical nature of the RT-159A probably made it a burden well-tolerated. One can only wonder what action this particular radio may have seen.
David Carey is president of Portelligent, a TechInsights company that produces teardown reports and related industry research on wireless, mobile and personal electronics.
