Guard Channels (was [Milsurplus] RE: Nutso (RE: WW2 B-29 VHF radio?)

[Mike]

ROBERT W. DOWNS wrote:

> nowhere in either manual can I find a mention of the actual frequency
> or frequencies of the guard channels.

I've got the following VHF sets with in-service crystals:

RT-18/ARC-1 with the guard channel at 121.5.
RT-19/ARC-4 with the guard channel at 140.58.

Also, the following with no guard channel, but set up for:

R-28 and T-23/ARC-5 with channel C at 140.58.
R-77/ARC-3 with channel C at 121.5.
SCR-522 with channel C at 121.5.

I think that the standard "emergency" guard channel frequency of WWII
was 140.58 (I erroneously said "142.58" in an earlier post.)  It appears
that this channel was usually assigned to channel C or 3, no matter what
set.  Sometime after the war 121.5 became standard for that purpose (and
in the military, 40.5 VHF-FM and 243 UHF-AM.  40.5 x 3 = 121.5).

BTW, I've seen several T-23 and R-28/ARC-5s in my day, and every one
that still had the crystals was set up as follows:
Channel      Frequency
  A           116.10
  B           126.18
  C           140.58
  D           142.74
For some reason this surprises me.  I wonder if the ARC-5 VHF units came
from the Western Electric with these crystal frequencies standard.

> The AN/ARC-3/36/49 manual mentions a modification to the R-77/ARC-3 and
> R-428/ARC-36 adding a connector for the purpose of connecting the 12MC
> output of the guard channel converter (no nomenclature given that I can
> find).  The receivers became R-77A and R-428A.

I also have been unable to find any info on the ARC-3 external guard
channel front end.  I'll bet the design never made it to service before
most military command comms went to UHF-AM in the late 1940s.

BTW, I could use the right-side external and internal covers from an
R-28/ARC-5 if anyone has a parts unit available.  Or I could uses the
whole receiver if it is complete.

Mike / KK5F