[ARC5] SCR-274N VHF Resurrection (long)

[David Stinson]

What is probably the only SCR-274N VHF in the galaxy 
is running in my shop.   The receiver has been subjected
by some doofuss to nit-wittery and is awaiting autotune parts-
which are being supplied by the kindness 
and generosity of some of our terrific members-
but it is on the air.  I owe you big, fellas :).

http://home.netcom.com/~arc5/274NVHF/SCR274NVHF.JPG

This may look like the AN/ARC-5 T-23 / R-28 set 
with which we are all familiar, but it is not. 
It is the early Army Air Corps set 
from which the AN/ARC-5 VHF was born.  

This is a very interesting radio.
As you know, there was a big push mid-war
to add VHF capability to tactical aircraft.
Western Electric was producing their sub-contract
version of the HF SCR-274N and many thousands
were already installed. How could W.E. best "get in" 
on the new contracts? The result was an ingenious
innovation that became SCR-274N VHF,
which provided a simple means of adding VHF to any 
274N or Navy ATA installation. To add VHF to 
an existing HF set, one need only install the radios in
the existing racks, replace the control boxes and
add one simple cable. Takes about 5 minutes.
It out-performs that unstable black lump of a radio, 
SCR-522 by a mile.   I don't know why it wasn't
adopted as a standard right away.  
Maybe it was the epidemic of  "Love-All-Things U.K."
or perhaps Bendix was able to buy higher-quaility 
Senators and Congressmen than Western Electric; 
probably a little of both.

Major units:

Transmitter BC-950
Receiver BC-942
TX Control Box BC-938
RX Control Box BC-944

Only about 3500 of the Army sets were completed
under orders 2808-WF-42 and 7461-WF-43.
Most of them were re-fitted and converted to the later, 
more familiar and common AN/ARC-5 VHF
under Navy contract NXsa-40001.  

The BC-950 transmitter is quite scarce.
The W.E. engineers had one "head scratcher" problem:
the 274N screen modulator is not approriate for
the 950's 832 PA stage. It needs a smaller plate modulator.
Some time ago, I peeked into the BC-950 and discovered
an 815 push-pull modulator stage:

http://home.netcom.com/~arc5/274NVHF/BC950MOD.JPG

I did not go further in tracing the modulation stage at that time, 
since I was working on the receiver and control boxes first. 
All resistors and capacitors tested good to excellent
and none have been changed.  The 832 PA had gone
gassy and I replaced it.  A tube and "lube-n-tune" 
was all it needed to get it going.
Most BC-950s were refitted to ARC-5 T-23 by removing 
the 815 modulator circuits and plugging the holes
in the chassis. Later production sets do not have the
holes at all. You can find transmitters that were in various
states of completion on the production line before
they were re-worked into T-23s.

No receiver with a BC-942 nomenclature plate
is known to have survived, although re-plated
Army Air Corps receivers can be told from later 
production by their Signal Corps inspection stampings,
internal markings consistant with Army radios and,
most telling, a set of tubes marked specifically 
for Army sets (usually marked "U.S.A.").
Later production had mixed or full JAN tube sets.
Also, early receivers may still have the older O-101
channel selector mechanism, while late production
will always have the updated O-102.
If the SQUELCH control is behind the small slide
on the front of the receiver, it's an O-101.
If the control is behind the large door, it's an O-102.
Other than the selector mechanism and markings,
BC-942 and R-28/ARC-5 are identical.
The receiver needed the 3-.22 uFd cap restuffed,
one 5 uFd filter, one .003 uFd Micamold and one 474 kOhm
bypassed with a good parts, plus a full "lube-n-tune."
Sensitivity for comm level signals is 1.2 uVolts.
Not too shabby for a 1942 VHF front-end.

Many more 274N VHF control boxes were finished 
than the radios themselves.  While most were 
re-fitted to AN/ARC-5, the Army boxes 
do appear on the market now and then.
Documentation for the 274N VHF appears to 
have been destroyed, since none has surfaced.
Discarding the old manuals was common practice
when a set was retro-fitted to a new version.
I had to discover technical details of the set
by tracing circuits and noting physical differences.
Here are nomenclature plates:

http://home.netcom.com/~arc5/274NVHF/BC938PLT.JPG
http://home.netcom.com/~arc5/274NVHF/BC944PLT.JPG
http://home.netcom.com/~arc5/274NVHF/BC950PLT.JPG

I assumed that, since the transmitter control box (BC-938) 
included MIC and KEY jacks which do not appear 
on the later AN/ARC-5 C-30,
W.E. would switch the audio feed from the BC-456 for 
the HF rigs to the internal 815 modulator in the BC-950.
So I was surprised when I finished tracing the 
TX control box and discovered MIC audio is not switched,
at least not in the BC-938.
I began to wonder if there was an external junction box.
Here is the BC-938 Transmitter Control Box circuit,
created from an amended C-30 diagram.
Green wires are added.  A wire in the C-30 from pin 16
on J-402 to pin 11 on J-401 is deleted:

http://home.netcom.com/~arc5/274NVHF/bc938.JPG

This got me wondering where and how the mic audio 
switching took place, if not in the control head.
A day of tracing circuits in the BC-950 transmitter
brought the surprising answer:  there is no switching.
The mic audio goes to the BC-456 screen modulator 
just as always.  The audacious W.E. engineers solved
the "inappropriate modulator" problem with a neat trick.
The modulated "screen" voltage is fed to the TX 
rear connector, but does not connect to any
DC circuit in the transmitter.  It is coupled through
a DC-blocking capacitor and resistive attenuation
to the primary of  the 815 plate modulator's grid
transformer, where this audio drives the internal modulator:

http://home.netcom.com/~arc5/274NVHF/bc950aud.jpg
http://home.netcom.com/~arc5/274NVHF/BC950Tran.JPG

Thus, without any significant changes to an SCR-274N
HF installation, one could "drop in" VHF and go;
one of the original "Plug-n-Play" solutions.
And the audio is excellent.

I've incorporated this into my SCR-274N.
The only things needed to change between HF and 
HF/VHF configuration is swapping two radios
and two control boxes.  Two sets for one ;-).
It's too bad the SCR-274N VHF wasn't given its due;
it's an excellent example of engineering innovation.

GL ES 73 DE Dave AB5S