I haven't read this group since last week so I am glad to see I
helped promote an interesting discussion.

        The A F B TV Radio would run on DC because it had no
transformer in the power supply.  It's cheaper to build equipment that
way, and the equipment is lighter, but it certainly makes it more
dangerous to work on if you don't have an isolation transformer.

        The A F B radio was simply a solid-state version of the way
many table-model radios were made a long time ago.

        As for why aircraft use 400 HZ, you are all right but there is
even one more reason.  While I don't know why the exact number of 400
Hertz, the biggest reason for using a higher frequency is the size of
inductors such as what you have in motors and transformers.  That's
the big reason.  Airplanes need to be as light as possible and iron
makes very good ballest.  The less iron you have in all the electronic
and electrical equipment on board, the less fewell it takes to simply
get off the ground.  400 HZ is still practical for mechanical
generators to produce so that was probably the best engineering
compromise.

        I don't think we'd want to live in a 400-cycle power
environment, however.  You can hear 400 HZ and its 800 HZ harmonic
extremely well.

        When I first started working in our computer center in 1989,
the IBM main frame ran on 400 Hertz.  There was a motor-generator room
near the computer that had two units which converted 60-HZ AC to
400-HZ ac at 115 volts, just like an airplane, and fed that in to the
computer room.

        When one walked around in that area, all you could hear was a
loud 800-HZ whine.  In the computer room, you could hear whines from
anything using that power.

        It would also be harder to keep it out of audio equipment
although there was no audio equipment in the area to bother.

        I think that we'd get pretty tired of hearing that mosquito
whine everywhere very quickly.

Martin McCormick