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

[Mike Morrow]

> Actually sorta angels on pin heads here but the first would have been in 
> RAT.  Same one was of course used in RAT, RAV and ARA.

Yes, that's A.R.C. part 5631, although the RAT and RAV manuals give slightly
different values of DC resistances compared to later manuals.  I was restricting
my discussion to command set receivers.

> Second type, according to standard Signal Corps doctrine, would have 
> officially been considered, called or referred to as 4000 ohms, not 8000.
> And the tap on the Third would have been 250, not 500 or 600.  Reason is that 
> typical Signal Corps headset and loudspeaker impedances at the beginning of
> the War were 7000 to 8000 @ 1KC and equipment was expected to operate 
> satisfactorily with one to several jacked in.

That's exactly why I don't attach much premium to specifying 8000 vs. 4000-ohms
or 600 vs. 300-ohms.  The real AC impedance varies wildly for each audio frequency
passing though the AF circuit, which is further complicated by the various
parallel loads depending on the number and type of devices plugged in and
which loads are selected to AF output bus A or B, the number of paralleled
receiver AF outputs (up three from the command set alone), and yet another
difference develops when the modulator sidetone output is connected into the
mix during transmit (although the receiver outputs are disconnected then).

It's surprising how well these simple everything-in-parallel AF systems worked!

> Incidentally, the 1325 turn tap on the SCR-274-N B-model transformer 
> secondaries was from the top, not the bottom.

Yet the secondary DC resistance at the tap (terminal 6) is specified from
tap to bottom (case).  Very bad technical writing!  

> The fourth I was thinking of was in AN/ARC-5 but a more careful check shows 
> that although mention of the 2000 turn primary tap was omitted from the 
> Parts List description, the receiver schematics (and a quick look inside a 
> receiver) shows that it was there and that the A.R.C. part numbers are the
> same (5631).  So delete the fourth one.

The AN/ARC-5 manuals that I have show the A.R.C 5631 part number in the Table
of Replaceable Parts for T-1, plus Stromberg-Carlson part number STC-640268
for the units they made.

> Anyway, if you check most TM's and T.O.s or AN's for WW-II Signal Corps 
> equipment you will see 250 and 4000 ohms for Lo-Z and Hi-Z sources.  Korean War 
> vintage and later L0-Z magically becomes 500 or 600 for Lo-Z (no High-Z 
> except in the RT-77/GRC-9 which was a carry-over and it still said 250 and 4000 
> on the internal selector switch positions).

IIRC, the AN/GRR-5 also has high and low impedance outputs from the AF stage
of the R-174/URR.  I forget what the PP-308/URR does with them.

Mike / KK5F