[Milsurplus] The Great SCR-AF-183 Mystery

[David Stinson]

As you all know, during WWII Aircraft Radio Corp
sub-contracted to Western Electric for construction 
of the SCR-274N.
But it was not the first time that happened...
(Disclosure:  I own BC-AE, -AF and -AH-230 and GF-1).

A.R.C.'s original 1932-1934 designs for the GF / SCR-183
type sets had some serious performance and reliablity issues.
The early sets like -AB, -AC and GF-1 had an unreliable 
antenna tuning linkage, low power out ( 1.5 watts ), 
a poor modulation scheme, poor neutralization circuit, 
drifty components etc.

Re: The Signal Corps transmitters:
By 1935, most of the problems with the A.R.C. SCR-183 
had been "ironed-out" and culminated in the SCR-AE-183,
order SC-132368 June 5, 1935 with some further 
"tweaking" refinement in the 1936 -AG and -AH variants. 
 While they looked the same externally as earlier sets,
 the -AE/AG/AH replaced the old (1)VT52 (3)VT25 circuits, 
which originated in the BC-AB-200 and GF-1,
with a completely redesigned (2)VT52 (2)VT25 scheme 
that delivered more power (4 watts), was stable and reliable.
With minor variations, these 1935-36 sets were the "template"
for all the -183 and -283 sets that followed.  

Now comes the mystery:  The BC-230 design was fairly 
set with the -AE order of June 5 1935.   Then A.R.C.
sub-contracted for Western Electric to build SCR-AF-183,
order 10999-NY-36, dated July 27 1935.  One would think
this would be a step in the progression from -AE to -AH, 
but it's not.  Not at all.  

With the exceptions of nomen plates, the connector
and some mechanical changes to prevent using any coil set
other than an -AF variant, the BC-AF-230 transmitter 
is a straight and faithful knock-off of the 1933 GF-1. 

I can state that it is a GF-1, rather than a BC-AB 
or -AC-230 copy, because the GF-1 came with 
slots on both the transmitter front and plug-in coil for 
paper frequency tuning charts- one chart for each coil type.
The BC-AF-230 has the same charts in the same places, 
while the -AB, -AC and -AE do not.  Both the -AF and GF-1
have white-faced meters.  The -AE (at least mine) and other,
later transmitters have black-faced meters.
The -AF transmitter is speced for a single coil of 6200-7700 KC
(C-200), the Navy frequency range speced for the GF-1.  
The -AF did offer one "alternative" coil set (C-201) 
for 3050-3800 KC.   The -AF also specs a "crystal control"
coil for (C-182) for Army Airways on 4495 KC.  
Note this is not the same coil as the common C-184, 
which fits all the -AE and later -183 sets.

The "bible" for early SCR-183 / -283 fans is TM 11-200.
It covers 183 variants -AF, -AG, -AH, -AJ, -AK, -AL, -AN
and the first 24 volt -283 variants built as -AL-283 and -AN-283.
While the chapters covering the other sets usually refer back 
to the -AH as a "template" and note minor differences,
the -AF is so different from the others as to rate its own,
extensive chapter with cautions against mixing -AF parts
with later sets.

So here we have the question:
Why would A.R.C. sub-contract to W.E. to produce 
a set that had been obsoleted years before?
We may never know, but it would be interesting to speculate.

Spec notes for the SCR-AF-183:
Built by Western Electric on order 10999-NY-36 July 5 1935.
Transmitter ranges and coils noted above.
The manual says that the receiver, rx control box, dynamotor,
and junction box are also parts of SCR-AB-185 and all those
except the receiver control box are part of compass SCR-AB-186,
but that doesn't jibe with the circuit diagrams.
The receiver, BC-AF-229, is nearly identical to the later -AH 
variant; the -AF uses a resistive divider to develop screen voltage
which is located in the dynamotor, while the divider in the -AH 
is in the receiver itself.  The audio output low-pass filter in the
-AF is in the output transformer's primary circuit, while in the
-AH it is in the secondary.  The dial mechanics on the -AF 
are also different, the tuning dial being flat rather than conical.
Receiver coil ranges included 
(primary issue) dual 201-398 and 4150-7700 KC (C-185)
("alternate" issue) dual 201-398 and 2500-4700 KC (C-183)
                     and single 4150-7850 KC (C-192)

GL OM ES 73 DE Dave AB5S