[ARC5] Mods; ARA, SCR-274-N, and AN/ARC-5 AF transformer T-1; and More

[Mike Morrow]

Robert wrote:

> ...there were only three known official modifications made to any of
> the receivers after they entered service.  The first was the conversion
> by 8th AF of the frequency coverage of some BC-455's.  The second was
> the installation of the motorized tuning on certain R-26, R-27 and ARA
> equivalents.  That was done during the War.  The third was the addition
> of ground straps to the receiver dynamotor shock mounts.  That's most
> commonly seen on BC-453-B's and I think was done post-War.

Here's a FWIW on the first modification that you mention:

USAAF - UK “Standard Beam Approach” (SBA) Receiver (1942).
BC-455-B receivers were modified in the UK to cover 28 to 41 MHz for the
“Lorenz” type of directional VHF (dot-dash) beams used in the UK for
airfield instrument approaches using the “Standard Beam Approach” (SBA)
system.  Externally, the modified receiver had a dial calibrated from
28.0 to 41.0 MHz, plus a UK-style antenna connector on the left side.
It was stamped “MOD CBA”.  Gordon White discussed these in "CQ Magazine",
June 1968.  Unit nomenclature:  BC-455-B MOD CBA  (28.0 to 41.0 MHz)

> If anyone can think of a fourth...

I'll list two more that I know about:

(1) There was the USAAF AN/CRW-3 Guided Glide Bomb Control (1944).
Some SCR-274-N BC-454-B and BC-455-B receivers were incorporated into
the glide bomb radio control system AN/CRW-3.  This control system was
part of the late-WWII GB-4 glide bomb.  The standard receivers were
modified slightly to provide a connection to the RF and IF stages for
an external electronic gain control signal from AM-41/CRW-3.  The dial
was removed except during frequency adjustment.  The original nomenclature
was altered with a paper tag to end with a “-BM”.  The receivers were
mounted in special mountings for the AN/CRW-3.
Unit nomenclature:  BC-454-BM (3.0 to 6.0 MHz), BC-455-BM (6.0 to 9.1 MHz)

(2)  There was also the USAAF AN/ARA-1(XA-1) Radio Compass Adapter (1943)

This develpmental AN/ARA-1(XA-1) utilizes a loop-fed BC-453-B and/or BC-946-B
with a compass adapter (modulator) to control an aural output and visual
LEFT-RIGHT beam indicator meter.

I don't know the exact nature of the modifications made to the receivers,
but perhaps this is stretching the issue since this system never saw
deployment.

> The missing output transformer was part of the "customizing" AKA
> hacking.  None were ever made without one or the other of the four
> transformer variants.

The earliest version of T-1 is that used in the ARA with 600-ohm secondary.
The early SCR-274-N -A models have a 8000-ohm secondary, and the later
-B models have a 8000/600-ohm tapped secondary.  I'm drawing a blank about
a fourth version, not counting the A.R.C. type 12 models.

Below are some details about T-1 in the ARA, SCR-274-N, and AN/ARC-5
receivers from the various instruction manuals.

ARA and AN/ARC-5 models:  T-1 600-ohm secondary (A.R.C. part 5631).
Primary 4000 turns #40 enamelled wire, tapped at 2000 turns.
Secondary 500 turns #32 enamelled wire.
Primary DC resistance 1160-ohms, secondary DC resistance 21-ohms
(+/- 15%).

SCR-274-N -A models:  T-1 8000-ohm secondary (A.R.C./WECO part 6308).
Primary 4000 turns #40 enamelled wire, tapped at 2000 turns.
Secondary 1800 turns #38 enamelled wire.
Primary DC resistance 1028 to 1300-ohms, secondary DC resistance
292 to 370-ohms.

SCR-274-N -B models:  T-1 8000/600-ohm secondary (WECO part ES-691027).
Primary 4000 turns #40 enamelled wire, tapped at 2000 turns.
Secondary 1800 turns #38 enamelled wire, tapped at 1325 turns.
Primary DC resistance 1028 to 1300-ohms, secondary DC resistance
292 to 370-ohms, tapped secondary DC resistance 86 to 110-ohm.

Mike / KK5F