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

[Mike Morrow]

Dave wrote:

> 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-

This is absolutely stunning, Dave.  A real technical achievement!

What did you wind up using as the basis for your BC-942 receiver?
Were you able to get anything useful from that junker R-28 we traded
a few years ago that was missing some of the gears in its O-101
tuner, but appeared to have once been a BC-942?

> 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

The easy "conversion" (compared to the BC-950 transmitter) of the
BC-942 receiver to the R-28/ARC-5 by changing nomenclature tags
must have eliminated the chance of any surviving with the original
BC-942 plate. 

> 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. 

> 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.

Without implying any disagreement, I'm interested in the basis for
stating superior performance.  Superior adaptability to an existing
US (but not UK) set is obvious, yes...but superior performance? 

> Maybe it was the epidemic of "Love-All-Things U.K." or perhaps
> Bendix was able to buy higher-quality Senators and Congressmen

I suspect that the desire for a set that could serve both UK and US
needs AND serve 14-vdc and 28-vdc aircraft was favorable to the
SCR-522-A.  If an aircraft was to have no HF command set, as were
many UK and some late-war USAAF aircraft, the SCR-522-A would have been
easier to install than a VHF-only SCR-274-N, I believe.  But probably
most important was the SCR-522-A being ready well before any other
usable VHF set in the theater for which VHF was most important.  The
early WECo "tunable" SCR-274-N VHF units (BC-695 receiver and BC-699
transmitter) must wasted effort and delayed WECo's final output.
Also, while not *easy* to re-channel to new frequencies, IMHO the 522
is *easier* than the WECo VHF design.

> 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:

The WECo design team must have welcomed the plate-modulated AN/ARC-5 that
allowed use of the external MD-7 modulated plate HV supply for the T-23
version, with an AF power dissipation resistor (R-329) in the T-23 to drop
the excess AF.  But I still wonder if the BC-950 PA plate could not simply
have been fed with the modulated screen voltage output of the BC-456 with
equal success to the method chosen for the BC-950.

> Thus, without any significant changes to an SCR-274N HF installation, one
> could "drop in" VHF and go...

Which is what the USN did with the R-28 and T-23 version.  And the USN
quickly condemned that nasty UK-style channel push-button select control
box C-30 in favor of the smaller, lighter, more versatile, and doubtlessly
less-expensive C-30A, while the USAAF continued the push-button nonsense
with the troublesome C-118 for their new AN/ARC-3. 

The nomenclature plate on your transmitter is interesting:  BC-950-A-121 .
I've never seen such a taxonomy.

It looks like hams will be hearing your BC-950 on 146.52 (and 116.1) MHz AM.

> 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 ;-).

A problem...your BC-456 should be a later aluminum-finish -B/E model!  :-)

But seriously, the next thing you could add to your installation is a BCB
BC-946-B in place of the BC-453-B, and a R-1/ARR-1 feeding it for a USAAF
version of the ZB homing system.  I'm pretty sure that such installations
did NOT use the ZB antenna switch or control box, so only the R-1 and the
BC-946 (fed only by the R-1) would be needed, plus a hard-to-find MC-415
BCB dial on the BC-944 control box.

After that, if you wanted to really go overboard and didn't mind the 
anachronism, you could add the BC-608 and BC-616 "Pipsqueak" components.

> It's too bad the SCR-274N VHF wasn't given its due.

But it was...with only minimum improvements, for the USN.  I suspect that
by the time it was ready to roll, the 522 had already filled most USAAF
needs, with the excellent AN/ARC-3 coming down the pike.  The USN seems to
have been behind the VHF curve.  They were using a few make-do
WE 233/AMB/AN/ARC-4 sets, plus some SCR-522-A sets, and it appears that
their installed base of existing AN/ARC-5 command installations merited
adopting the WECo VHF design while awaiting the WECo/Westinghouse AN/ARC-1.

That is really outstanding work, Dave!  Your description of the research
is a real learning experience and joy to read.  Seeing the picture of the
SCR-274-N VHF set is like looking at history.  Thanks!

Mike / KK5F