[Milsurplus] 115 VAC 400 Hz tolerance, and the B-36

Mike Morrow kk5f at earthlink.net
Fri Jan 14 10:47:39 EST 2011


>Trivia: the early versions of the B-36 used an ARC-8 (BC-348/ART-13) and later used
>AN/ARC-21 I am still looking for pictures, parts  or any information on the huge
>mysterious "drum" radio. Think the last time I brought this up the opinion was its extinct.

The AN/ARC-21 is an interesting set.  The RT-128A/ARC-21 would be the most common version,
and there were a couple of different power supplies, one being dynamotor-based, the other
being 400 Hz AC transformer-based.  There were also several different auto antenna tuners,
though none had a JAN "ARC-21" designation, AFAIK.

The pressurized drum of the RT-128A is 18 inches in diameter, and the weight of the
RT-128A alone is more than 140 pounds!

The full and proper installation should include the RT-128A/ARC-21, plus the AN/ARR-36
auxilary receiver (R-224/ARR-36), plus the AN/APN-70 LORAN A set.  

I've got most of an R-224/ARR-36 (minus external dynamotor and a few pieces of hardware).
It's the only one I've ever seen, but there should be greater probability of finding one
of those than finding an RT-128A, since the R-224 didn't undergo a USB conversion process.
The RCA R-224/ARR-36 filled an HF auxilary receiver role with the RCA AN/ARC-21 for the
USAF, similar to what the Collins R-648/ARR-41 did with the Collins AN/ARC-38 for the USN.
It uses modules and construction similar to those in the AN/ARC-21.

I suspect most of the AM RT-128A/ARC-21 units (which were made by RCA) were converted by
the USAF and RCA to USB RT-400/ARC-65 units by the early 1960s.  I have heard of both
AN/ARC-21 and AN/ARC-65 sets being released to AF MARS members in the 1960/70s.  There
are doubtless surviving units still out there.  I also suspect that the RT-128A/ARC-21
to RT-400/ARC-65 conversion process for the USAF got RCA interested in developing their
RT-311/ARC-38 to RT-594/ARC-38A conversion process for the USN. 

Control boxes for the AN/ARC-21 (also used by the AN/ARR-36) and the AN/ARC-65 show up
once and a while on ebay.  In the past ten years, there was an RT-400/ARC-65 on ebay
(I won it for about $40, but then found that it couldn't be shipped.  Later another list
member purchased it in a subsequent auction and picked it up locally.  He sent me
pictures, and it was in excellent condition.  I was envious of his 140 pound treasure!)
I've seen one RT-128A/ARC-21 on ebay, which I also believe another list member bought,
but IIRC it was missing the drum that it fit in.  The only AN/ARR-36 that I've ever
seen is the one I got on ebay a decade ago.  The tech manuals are not that rare, and
sometimes show up on ebay.  Somewhere in storage, I've got the manuals for the AN/ARC-21
and the -65.

So the RCA drum sets are not too mysterious.  But they are unusual.  Honestly, I'd love
to have a complete AN/ARC-21, ARR-36, and APN-70.  And if I had it to do over, today
I'd be willing to drive cross country for that AN/ARC-65 that I won.

Mike / KK5F


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