[Milsurplus] Re: [ARC5] SCR-274-N

[David Stinson]

----- Original Message ----- 
From: "Floyd Petri" <fpetri at eastex.net>
Subject: [ARC5] SCR-274-N


>I have obtained all the components for a SCR-274-N ...
>Then I will proceed to test all components for proper working 
>and repairing all that do not work....

Congratulations and best wishes for your new project, Floyd.
Here's a few tips I've found in getting mine running:
http://www.arrl.org/news/features/2006/06/17/1/21-lrg.jpg

I recommend testing and trouble-shooting each receiver, 
transmitter dynamotor etc. before assembly.  
If not, you will have multiple problems 
and the task will become overwhelming.

Long ago, I used to advocate replacing all the capacitors 
in the receivers.  I do NOT do so now.  Over the years,
I've learned better ways and now find many of the old caps stil
serviceable.  These days, I grab junkers I would have passed up 
just to get the caps, screws etc.  I find, on average, about a third 
(sometimes half) of the caps will pass an overnight leakage test 
at 250 volts.  In my personal SCR-274N, all the caps are
now tested originals.  However- that's doesn't mean I "trust" them.
70% failure after 60 years probably means 95% after 70 years if 
kept running at full voltage.  I run the dynamotors only on special 
occasions, like a public display.  Otherwise, the receivers are run
on +28 VDC B+ sans dynos and they work fine with either
headphones or an amplified speaker.  
Many caps that are leaky at 250 volts 
seem to work OK at 28 volts.  
I have an RAT receiver with caps that would fry resistors 
at 250 volts, but it plays fine as paint on lower B+ 
and the rig stays cool.  
If you keep the B+ low, you don't have to change a cap 
unless it's darn near shorted or flat open.

The transmitter bathtub cap is a different story. 
The transmitters don't put out much power at lowered B+
(though they will work), and even modest leakage 
in that cap will cause all kinds of grief.  Same story with the
BC-456 modulator caps- check them close as they will be
running at full voltage.  If you have a good checker like the
ZM-11, you'll find many of them still serviceable.

In all sets, look for a problem that's starting to show up in
the variable capacitors.  The bars at the ends of the stators 
which have the connection points for the wires are pressed
onto the end of the stators.  Hi-Z is developing between the
connection bar and the stator.  You'll find sudden, intermittent
loss of  TX power from mistuning and the same for receiver
sensitivity.  Get you a big soldering gun and solder between
these connection bars and the stator.  I have one BC-454
that had the problem between some of the plates of the 
*rotor*.  The connection where the plates were pressed into
the rod was getting intermittent.  I solved that one by using 
a 12-volt supply, a pilot light and some soft, solid copper wire
all in series, which I used like a miniature "spot welder."  
I "spotted" the cap plates at the junction and that
solved the problem.  That's the only case of rotor trouble
I've had, but the stator deal is getting common.
The "spot weld" might work with the transmitter cap 
connection bars as well, but I haven't tried it yet.

Jiggle/rotate all the IF cans, RF cans,  remote drawer,
antenna tuning caps, IF can caps and BFO cap 
around a bit to clean the contacts.  
You'll need to realign the BFO/IF
but that's no big deal with these.  Don't mess with the 
trimmers on the receiver tuning cap unless you have 
reason to believe they're intermittent.

When burnishing the relay contacts, use nothing more abrasive than
brown "sack" paper or a real relay burnishing tool. 
Don't forget the relays in the receiver rack.
The antenna change-over relay is particularly "hinky" and needs
a lot of attention.  If your BC-442 doesn't have the official
"M1" modification of a 1-meg resistor between 
the receiver antenna post and ground,
I recommend installing it.  
Keeps static charges off the antenna and out of your radios.

I've written several times about disassembling, cleaning and 
polishing the antenna tuning components.  Some have argued
with me.  I recommend it; you won't be sorry.

Flat tubes in the receivers are uncommon, but I have found them.
Check the transmitter tubes closely.  Often, they've been abused.
Look for grid emission in the 1625s and weak 1626s.
If you work at it, you can get all "Army" tubes in your SCR-274N.
I know- that's kinda "over the top," but I enjoy having all
my tubes marked "U.S.A." or "U.S. ARMY" in a Signal Corps set.

A good, solid ground between all the various pieces is essential.

> I am lacing all cables and wiring just as it was in WW II Bombers.
I got very lucky and acquired an original harness from a P-40.
Finding original wire is tough.  I may have some around.
I'll look this morning.  Perhaps a friend of mine can help.
I'll check.  Do you have the original twine?  Common brown
cotton "household twine" you find at WallyWorld is close.
I can send a photo of original lacing if you like.
Look over your connectors.  If you want to be strict in 
your set, all the connector should have "PL-xxx" numbers.
6000 or 9000 series connectors are either Navy or JAN.
I have a big plenty of the white, plastic insulated wire
they were using by the time of the B-29.
If you decide to go "late war," I can supply lots of it.

Spline shafts- get some for the sake of your collection,
but do yourself a favor: don't install them if you plan to 
operate the set.  Tune a receiver with one and you'll find
out why the engineers designed them broad.

>  Also I need to identify the white wire seen in photographs 
> of the radio equipment in WW II bombers that is shown 
> connecting the receiver antennas and transmitter antennas 
> together and to the antenna relay....
I also have an original antenna wire from a 3-receiver rack
that was removed intact.
It's just white cloth-covered wire, "daisy-chained."
>From left to right, the "correct" receiver stack is 
3-6 MC, LF, 6-9 MC.  Cut two pieces of wire
that fit between the receiver antenna posts.
Strip so you have half an inch of bare wire extending
below the antenna posts.  Twist together and solder.
add wire on the end up to the antenna switch.
Bend the half-inch below each antenna post clockwise
to wrap it around the post.
If you want to be super-authentic, you'll need some 
paper/cellulose tape wire numbering markers for
each of the wires.  Don't remember the numbers 
right now, but 

If you look at the photo linked above, you will see what
looks like a fat, white wire between the two transmitters.
This is an original, also taken from a set of transmitters 
removed intact.  It is a piece of the stiff, copperweld-like
wire used on the trailing antenna spool (Mike: Do you know
what this wire actually is?  Hard to bend; doesn't seem like 
copperweld but looks like it.).  The "white" is a chain of 
ceramic insulating beads.  On each end, there is a disc
the same diameter as the beads with a small hole in the center,
through which the wire goes.  The disc is soldered to the wire
with a big puddle on the outside, looking like a little dome,
and keeping the beads tight.
I've tried to replicate this, but the little disc is the hard part;
I ended up using a standard washer with some solder braid to 
hold the wire centered, but the result just doesn't look right.
If anyone knows where you can get what looks like a washer
but has a small center hole, please let me know.
The other white wires in the photo are replicated
"bead strings."  Note that later in the war, use of the 
beads during installation was deleted, at least for AN/ARC-5,
probably because wire insulation got better.
Perhaps someone on the list has some of the hard wire;
I've used all mine up.
Later, when the antenna relay was close to the transmitters,
they would just run the common cloth wire, one from each
transmitter, up to the antenna relay and join them their.
Either way is "correct," but the beads look cooler ;-).
If you decide to go with beads, let me know how long
your runs will be and I'll as many as I can spare.
I have a B-17 restoration effort coming up which is going 
to take quite a few, but I think I'll have enough left
for a set, or at least a good start on one.

Sidetone:  
This one is a pain in the neck I haven't solved yet.
Originally, this ran through the aircraft intercom,
which set a decent level.  If you hook sidetone directly
between the receiver rack and modulator as shown
in the manuals,  it'll blow your ears off.  
Worse- clicking in the earphones with 
CW keying will do the same. 
Don't try a telephone click suppressor
you can rob from the back of a telephone earpiece;
BTDT ("been there, done that").  Oh, it kills the howls 
and clicks just fine.  And the impedance reflects all
the way back down the chain and kills your modulation.
For now, I just clipped the wire and wear the headphones
forward on my head 
(now I understand those photos of ops doing that).  
I know I can solve it; just haven't gotten to it yet.

Mike Hanz has some excellent insights on a transmitter keying
issue he and another gentleman studied last year, and he and
others have great ideas on tuning and loading.
Link, Mike?

That's all I can think of right now.
Please let me know if I can be of any help.
I've had one -274N to -274N QSO and wouldn't
mind a few more.

GL OM ES 73 DE Dave AB5S

p.s. The SCR-274N was designed as a 'phone rig
and it really does work better in that application.
More on operating the rig when you get further along.