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

[WA5CAB at cs.com]

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.

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.  Later headsets (HS-30, 33 and 38 
accounted for 98% of them) have listed impedances of 256 or 8000 (depends on cord 
and matching transformer chosen) for the HS-30 and 600 ohms for the others.  
I've no idea why the HS-30 in Lo-Z (direct) configuration was 256 unless 
that's just what it turned out to be and it worked so they used it through the 
end of the War.  Strangely, the two Lo-Z loudspeakers (LS-9 and LS-11) were 
also called 250 ohm, not 500.  I suspect that at least part of it was 
doctrine, not fact.  But of those sets later in the War that had Lo-Z taps on the 
audio output transformers, most had reversable tags on the front panel.  If 
the tags didn't say Low Impedance or Lo-Z and High Impedance or Hi-Z, they 
said 250 and 4000, not 500 and 8000.

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

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.

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).

In a message dated 01/20/2013 20:21:45 PM Central Standard Time, 
kk5f at earthlink.net writes: 
> >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.
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480