[Milsurplus] ARC-38 trivia!

[Ray Fantini]

The ARC-38, a and 618S only use the dynamotor in the transmit mode or when in CW it runs the whole time you're in CW. Weird trivia about that family of radios. The filaments are all powered from the 28-volt DC bus on the airplane. But the radio will not receive or work unless you have 120 volts @ 400 cycles, you need the 400 cycles source for running the servo assemblies in the slug rack and SMO if it's a 38 but the 400-cycle source also feeds a half wave voltage doubler that provides the B+ for the receiver and SMO assemblies. The dynamotor only provides the high voltages for the modulator and PA assemblies. The same thing also happens in the 618T family of radios so if you have no source 400 cycles you don't have a radio.  More trivia! I have a habit of pulling the 120-volt 400 cycle source from the input of the servo systems and 180L tuner after the radio sets itself on channel and tunes itself. The radio works just fine without the AC on the servo systems and there are two separate AC feeds, one for the servo and one for the receiver power supply so it's easy to do.
Extra bonus points: the use of a half wave voltage doubler without any transformer directly from the 120-volt source was also common practice in television sets of the fifties and early sixties. On a TV set it results in a possibly hot chassis depending on how its plugged in but in a aircraft one side of the AC system is grounded and everything is hardwired so that won't happen.

Ray F/KA3EKH

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