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Taking the wrap off a 50-year old radio

Posted: 23 Jan 2009 ?? ?Print Version ?Bookmark and Share

Keywords:survival radio? frequency? transmitter? RT-159A teardown?

The RT-159A provided a simple but critical function, transmitting a fixed tone or a pilot's voice in a rescue request.(Click to view full teardown.)

Picture a downed military pilot or injured boat captain adrift during the Korean Conflict or Vietnam War. Likely, he'd take out the Hoffman RT-159A survival radio to call for assistance.

The RT-159A we analyzed here was found on eBay, home to almost all objects mainstream and obscure. How things like a 50-year-old survival radio show up for sale on the world's largest auction site continues to amaze me, but as a confirmed collector and pack-rat I'm delighted eBay is around as a vibrant outlet for vintage electronic rarities.

The RT-159A, a crude tool by today's standards, provided a simple but critical function, transmitting a fixed tone or a pilot's voice in a rescue request. Likewise the radio could receive signals on the same two frequencies used for transmitting!121.5MHz and 243MHz, designated as VFH and UHF, respectively. The 121.5-/243MHz slots remain as emergency frequencies to this day. While scheduled for monitoring phase-out in 2009, transmission on these frequencies will still likely set search-and-rescue wheels in motion.

A sliding band-select switch on the outside of the radio allows for the choice of desired frequency. Other controls and externals in the RT-159A include a trio of buttons for selecting Transmit, Receive and Tone (presumably overriding the default of voice input), with all three using a shared locking slidebar to avoid inadvertent calls for response. A four-pin connector on the bottom of the case provides for hook-up to the external battery.

Two extendable mast antennas reside in another locking assembly at the top of the radio. By sliding a switch, an antenna platform that can be raised above the radio is released. From that platform, the two masts can be extended to differing lengths for the two frequencies. Instructions printed on the radio show UHF mode served by laying the masts in their short, unextended form as a dipole pair (opposing directions) across the platform. Alternatively, the masts can be fully extended for use in the VHF frequency, where a longer wavelength requires a correspondingly longer antenna element for similar gain.

Given the radio's genesis in the mid-1950s!an era when electronics were still largely implemented pre-transistor !vacuum tubes supply all the active components in the RT-159A. Opening the case for the first time brought forth the aroma of heated filament-based electronics, phenolic circuit boards, aged solder flux and old component varnishes. Memories of my early!and often only marginally successful!attempts at TV repair using the old Radio Shack tube testers came flooding back.

Special tubes
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.

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 121.5MHz signal, so a 30.375MHz crystal frequency is implied. The output of V3 is followed by V4, also a frequency doubler, and source of the 243MHz 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 121MHz or 24MHz 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 1.25kg 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
President, Portelligent

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