[Milsurplus] Refurb and Checkout Part 2- Modulator, Boxes & Racks

[David Stinson]

SCR-274N
Refurb and Checkout Part 2- Modulator, Boxes & Racks

You'll need screwdrivers, some good contact cleaner
like De-Ox-It or Cramolyn, some light gear grease,
that light machine oil from last time, 
burnishing tool or clean brown paper sack
and the schematic/wiring diagrams.
Use the correct-sized screwdrivers to avoid gunched-up screws.
If a screw sticks, see the proceedure under Part 1.

Leave the fuse covers in place for now
to protect the fuses and holders.
Remove the top and bottom covers of the modulator.
Remove the back covers of the racks and boxes.
Place all the screws some place where a dropped screwdriver
or errant elbow won't send them to the screw gremlins
(Those little irritants are going to either give or take some
 screws in any case; why make it easy on them?).
Remove all the modulator tubes.  Test them, if so inclined.
I've never found a bad tube in a modulator,
but testing is cheap insurance.

Inspect all the connector and dynamotor pins.
Use a soft brush to clean as much crud out
of the connectors as possible.  Be careful you don't
snag the leaf springs on the pins and pull them out.
Look for bent, broken leaf springs, solder (ack!)
or other nasties.  If you have solder on pins, you
have a big job ahead.  Get a good iron, a bunch of
Solder Wick and a solder sucker.  Go to the 
Well of Patience and dip-up a bucket full.
Better make that two bucket fulls; you'll need them.
Your goal is to get enough solder off the pin
that the leaf springs will "spring" correctly.
Good luck.  I'll say a prayer for  you.
Gentle use brass tubing to compress the
springs while cleaning them can help; see below.
Don't try to force a connector over a soldered pin;
you'll break the pin and/or ruin the connector.

If you have a bent pin, don't just "wily nilly" bend
it back; the pins have leaf springs that are inserted in
the ring around the bottom of the pin.  If you just
bend it back, the spring will come out of the ring
and break off, leaving you with a bad pin that's
bound to go intermittent.  I don't know the exact size
(had mine in the junk box), but you can go to the 
nearest hobby store and get brass tubing that will slide
over the pins and compress the leaf springs.
Gently, slowly bend the pins back straight with the 
bit of tubing.  Replacing a broken-off pin is a major
headache, so take it easy.

Locate all the relays.  
Burnish the contacts using a professional tool if you're lucky, 
clean brown sack paper if not.  
Use nothing more abrasive than the paper.
The contacts can be bent, so take care.  
The contacts will be a bit "hinky" 
when you first power them, but will smooth-out soon.
Burnish the telegraph key contacts in the TX control head.

The headphone and mike jacks rarely cause trouble.
When you first insert a headphone connector, 
rotate it a few times.  

Lubricate the mechanical parts of the complicated-looking
rotary switches in the control heads.  Work the lubricant in
until they work smoothly.  This action may be good enough
to clean the contacts, too.  If not, you'll need to put a little
contact cleaner on them.  Check the solder joints on these
switches closely.

Use a little gear grease and machine oil to lube the
RX control head crank mechanicals.  
If one is frozen, don't force it.  Use 
some penetrating oil and patience.  
Unless corroded,  it will eventually turn loose.  
If it's forever stuck, I think I have some spares around here.

The pots are sealed and are usually OK, but not always.
I've had a few go noisy.  I know only one way to fix it,
but you must be brave ;-).  I take a very small drill bit 
in a slow drill, stack washers and a grommet on it so it won't 
go very deep and drill a small hole in the back of the pot.
A shot of De-Ox-It and the problem is solved.

Prop the RX rack with the toggle switches facing upward
(BC-442 switch as well, if it has one).
Dribble a goodly amount of De-Ox-It into the switch 
at the base of the toggle.  Let it soak, then work the toggle.
You may have to do it twice or more.  
Some of the cleaner will eventually make it to the switch contacts.
Wipe excess cleaner away.

Lubricate the latching devices on the front of the racks
with the machine oil.  If they are frozen, wrap a piece of 
thick paper or leather around the knurl before attempting to
break the thing loose.  Don't use a lot of force, because these
things will twist in two.  If you mutilate one, let me know.
I might have a spare.

Now take the covers off the fuse holders.
Gently clean away any crud.
Do not pull the fuses unless they are obviously bad.
The fuse holder clips are made of some metal that
has not aged well.  They break off easily.  If one is holding
a good fuse securely against a gentle tug, leave it alone.
If you don't have a spare fuse in the "SPARE" place,
don't insert one; no need to flex the contacts.
If your fuse is missing or bad, go ahead and try replacing it.
The holders may be OK.  If they break off, let me know.
I think I have a spare or two.  
When done, put the covers back on to protect the contacts.

Gently slid your rigs into the racks and look for 
a correct fit.  If you have bends, use big blocks of 
wood and a clamp to gently squeeze them out.
We have members who are excellent at this kind 
of thing who can advise you.

Look for several large, black, wirewound resistors
in the modulator.  Test them for opens.
Test the big, weird-looking secondary loading resistor.
Test the modulation and microphone 
transformer windings for opens. 
Look for any obviously overheated resistors.
If one is fried, find out why and fix
(usually a shorted bypass cap).

OK, now for some real fun:
Inspect all the solder joints.  
All of them.  Everywhere.
If one looks even slightly suspect, re-heat it.
Especially at tube sockets and switches.
Trust me- you'll be saving yourself some real future headaches.

Even more fun:  
The capacitors in the modulator.
Disconnect and test every one at full voltage.
The modulator is probably going to run full-out, 
so you  need to be merciless.  
If a cap doesn't hold up to full voltage 
or seems even a little "twitchy," replace it. 
You can do it the easy way 
(disconnect the wire at the cap,
  solder in a replacement where the 
  other end of the wire connects),
or the honorable way ;-)
( replace with good original or re-stuffed).

Put everything back together.
When you insert the tubes, 
gently rotate them in the sockets to scrub the contacts.
In a few cases, I've needed to De-Ox-It
a tube socket, but wiggling the tubes usually does  it.
When finished, count how many screws 
the gremlins got or left.

Now go get a glass of wine. 
Rub some Rogain on your head 
to replace the hair you've pulled out.
Relax. Enjoy the moment, because things are 
about to get even more...ummm... "interesting."

73 DE Dave AB5S