The PRC-38...
PRC-38 VHF USB/FM radio set ... is a mystery. This little piece of Collins clockwork must have cost a fortune to design and develop, but seems to never have been produced in any real quantities.  In the Collins 'HF Communication Equipment 1964/65' catalog it is presented as a PRC-38, but is also referred to as a 618K-1.  Did they think they'd sell this thing commercially?  Could be Collins developed it to compete for a particular requirement, and then maybe got beat out by Cincinnati Electronics and their PRC-70.   The fact that the PRC-38 will do FM at 20 MCs may be a throwback to the days of separate Artillery, Armor, and Infantry FM bands of 20-27.9 MCs, 27-38.9, and 38-54.9 MCs.

  This radio is marked 'PRC-38/XN-2' and 'ORDER NO. 33013-PM-64-91' -  it carries serial number A19.  The manual covers radios from S/N A10 to A22 built under contract DA-36-039-03391(E).  The radio is shown on it's bottom face here, strapped into it's rucksack frame.  Connectors for both 'old family' and 'new family' audio are present, as well as a connector for a KY-8 voice crypto unit and also a connector labelled 'ANT CPLR'.  (This remote 'coupler' is nothing more than a set of coils switched in or out depending on what frequency you're on -  the automatic antenna tuner is inside the radio.)  Like the similarly sized PRC-41, it also has a vehicular mount which sits the radio on it's side.  The central portion of the PRC-38 front panel can be removed and remoted from the radio,  like bolted to the Jeep's instrument panel...

  Yup, you heard right, this thing can be remote-controlled -  it has a full-blown Autopositioner system controlled by the front panel frequency selector switches.  What with the Autopositioner, the automatic antenna tuner, the syllabic squelch, the exotic PA tube, and the secure-voice capability,  you can bet that Collins charged Uncle Sam cost-plus for the engineering on the PRC-38...

  Another interesting tidbit -  the Eimac 4CS100L PA tube.  The "S" in the number indicates that the tube is conduction-cooled, being bolted directly to the backside of the cast aluminum front panel.  A chunk of beryllium oxide is bonded to the tube's anode and acts as a thermal link to the front panel.  The 4CS100L is an instant-heating tube and uses a scary little trick to further shorten warmup time (and probably tube lifespan as well...).  The tube has a second anode which serves only to indicate that the tube is up to operating temperature.  By the datasheet, the 4CS100L filament wants 2.1V at 7.5A and will bring the tube up to operating temp in 2 seconds -  the datasheet also quotes a warmup time of 150 milliseconds at 6.2V!!!  When the PRC-38 is keyed, the PA tube filament gets 4.2V until the secondary anode begins to conduct, at which point a relay drops the filament voltage back to the rated 2.1V.  Does anyone have an extra 4CS100L (a.k.a. a X2013) lying around?  I'm beginning to think I should have a spare...

  The automatic antenna tuner in this critter is pretty much standard Collins issue -  a motorized roller inductor for loading and another for phasing, with separate 800 CPS servo amplifiers driving each motor.  The servo motors and amps get power only when the operator pushes the 'TUNE' switch.  To tune up,  just select a frequency, wait for the Autopositioner to stop, and then push the TUNE switch and hold it 'til the loud tone stops.  (The transmitter's sidetone amplifier is fed with some of the output of both the loading and the phasing servoamps, so you'll hear an 800 CPS buzz whenever either motor is moving -  when the squeal fades away, the antenna tuner is tuned-up & happy.)  If you're looking for some haunted house sound effects for Halloween, this amplified noise from those twin interdependent servomotors will do just fine -  otherworldly skreets, yowls, twitters, yawps and rasps such as you've never heard, like the four horsemen of the Apocalypse are having a friendly rubber of bridge just adjacent your tympanum...
 
 
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