[Milsurplus] Tips on Running an Original SCR-274N

[David Stinson]

I have a complete SCR-274N 1 RX / 1 TX 
set in original configuration I've handed to a friend
to play with. It's in original configuration, 
save the mic element, a 4:1 UNUN hidden in the 
antenna switch, a series antenna cap,
a small box with a "NET" switch to net the RX/TX 
and a relay with contacts for keying an external amplifier.
Had to borrow some parts from another set,
but my friend is trustworthy ;-).
I wrote him a "users guide" with a few tips on 
running the rig. My understanding of how to use
these radios keeps evolving and improving over time,
so some of this advice (like the antenna cap values)
is different from what I might have written five years 
ago, and I'm sure what I write five from now
will be a little different (assuming The Almighty decides
to put-up with my nonsense that long).
I thought some of us might find it useful, so 
have copied it here for anyone thinking of building
an ATA/ARA or SCR-274N rig.
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All the caps have been tested and bad ones replaced
with tested originals that sat overnight at full voltage 
and didn't leak, but remember this gear is 70 years old. 
It's not a matter of if caps and other parts will fail- it's when. 
I've run the receivers on dynamotor for 10-12 hours at a time 
with no trouble, but that don't mean a thing.
Might run another 70 years and
might fail tomorrow- no way to tell.
Listen to the dynamotors in operation.
You'll be able to tell if one starts to "lug-down."
Hot smells, too. You know what to look for.
If anything like that happens,
switch off and we'll take a look at it.
Of course, the best way to "stretch" the old guys is to pull
the dynamotors and use a lower-voltage external supply.
Will leave that choice to you.
Mode: 
The rig was designed to be used on AM.
CW was used only as a "back-up." 
Unlike with ham hacks, 
the original- design is keyed with a big B+ relay 
and the antenna relay. It is not designed for fast keying.
High-speed CW will eventually damage the relays. 
The manual states keying speed on CW should be restricted 
to 12 wpm. 15 seems ok for me.

Set-up:
Each transmitter has a black antenna lead with a 
capacitor in it. They are marked "80" and "40."
"80" is 200 pFd at 2.5 KV. "40" is 50 pFd at 2.5 KV.
The capacitor goes at the transmitter end.
These must be used between the transmitter and
the antenna switch to match the rig to 50 ohms.

You'll see a copper braid lead running from the 
transmitter, the RX control box and up to 
the antenna switch, then over to the RX rack. 
It is essential this remain in place as it's 
part of the grounding. There is a ground plane
under the paint, but the connection at the antenna
switch is not good enough by itself and the rig 
will not properly load without it. You can tell if 
there's a grounding problem because the mike
will get "RF hot" and power out will fall off.

Let the transmitter warm-up for at least 15-20 minutes
and it will be as stable as any tube rig 
and better than a lot of them. If it drifts a lot,
there's a problem.

You need a speaker matched to 600 ohms 
for good audio output. You'll have all 
the audio you can stand.
A low-Z speaker will not work.
Speaker connects at the bottom of the 
receiver control box. You can use the jacks
on the front of the receiver rack but will need
to throw the switch there to the correct audio buss.

Station needs a good connection to ground.
Grounding is very important in WWII rigs 
of all kinds. Don't trust the old grounding posts
on the racks unless you test them with an ohm meter.
I hane not yet completely refurbish the racks.
Grounds and chassis connections are a major source
of trouble in these old rigs. If you ever work on 
restoring one, be sure to test all the grounding points.
You'll see a grounding post on the left side of the 
receiver rack, but the solder braid does not go to 
it. If you test with an ohm meter, you'll find that post
has gone hi-Z, probably from oxidation. I need to
remove, clean and replace it but didn't get to it.
So test from your ground connection to the antenna
relay ground post. Should be a few ohms or less.

Coax jumper connects to the center antenna switch post
and to the ground post on the side. The little box 
at the center has an RCA connector on the side.
This is a set of dry relay contacts for keying an amplifier.

Tune-up:
Load the rig into a 50-ohm dummy load first.
Don't pay attention to the "RF Amp" meter
as it won't move much for a 50-ohm load.
*The rig must be tuned in CW mode.*
It is screen-modulated, so improper tuning
will result in crummy-sounding or low modulation.
Set the transmitter to freq with an external
receiver or counter. 

Once the transmitter is tuned, set the receiver
to "CW" and turn the audio control almost
all the way down. Switch the little box to "NET"
and zero-beat. Don't forget to switch back
to "OPERATE" or your ears will protest.

I didn't get a "SIDETONE LEVEL"
control installed, so there's no side-tone.
If you look at one of the small connectors
on the modulator, you'll see a cut wire with 
liquid tape on the ends. If you want side-tone,
you'll need to experiment with a resistor value
inserted here. I'd start at a meg.
Straight connection level is way too high 
and will result in feedback.
I don't do CW with the rig very often so
just I leave it open. 

Place the mode switch in CW and tweak 
the link and roller coil for max output.
Then switch to AM. That's it.
You should get about 30-40 watts CW
and about 12-15 watts AM.
No need to re-tweak unless you move 
a big jump in freq.
Do not re-tune for max in AM position.
Use your antenna tuner to tune the antenna
to the rig, not the rig to the antenna.

If you use an external amp (I do),
the tuning will be a little different from 
going straight to the antenna.
tune the rig first into a dummy load,
connect to the amp and tune the amp up,
then *with the rig in CW mode,* re-tweak
to match the amp input. Switch back to 
AM and you're ready. 
To install an external amp keying relay:
use a 24 volt relay. On the side of the receiver rack,
there are two 6-pin connectors. On the vacant one you
aren't using for the normal sidetone/muting connection,
hook the relay coil between pins 5 and 6.
To install a "NET" switch, open the wire lead from
the 6-pin dynamotor connector to the receiver connector
that runs the muting relay in the receiver rack
and insert a SPST switch.
Opening this lead keeps the relay "off" and the 
receiver running so you can zero-beat.
(don't have my diagram here- you'll see it on the diagram)

Be gentle when swapping rigs. 
Those old pins are easy to break and a pain to fix.

Don't shout into the mike. 
Close-talk it at normal voice levels.
Loud talk will overmodulate.

One note: I had an occasional intermittant where the 
transmit dynamotor would stop running. I think I fix
it but if it happens again, just bump the motor and
it will restart. I'll clean-up another dyno if it gets
to be a problem.

Got to get to church-
Give me a shout with any questions.

GL ES 73 DE Dave AB5S