[ARC5] 10-Meter 274N RX - The 20-27 RAT-1

[David Stinson]

Well we're all interested in Ken's ongoing adventure
with the modded 10-meter radio.  So I thought I'd 
present some "OEM" info from the RAT series.

I have the last 20-27 MC "Command Set" receiver 
produced:  CBY-46109 RAT-1 serial number 50.

http://home.netcom.com/~arc5/RAT-1/RAT120-27.jpg
http://home.netcom.com/~arc5/RAT-1/CBY46109plate.jpg

Normally, I would not consider disassembling an 
authentically "Rare" (vs. "Ebay-Rare," lol)  RAT receiver,
but this one had already been badly goobered by some doofuss.

I've got it working but man, was it a MESS.
Drill holes in the front and back- he even wallowed-out the 
index peg on the dial rather than just align the poor thing.
Worst of all, the nit-wit chainsawed-out the entire top
of the dynamotor deck.  I used a junker to "graft" a new 
deck on the back.  I got it "off center" by about a millimeter on 
one side.  Probably no one but me would notice- it bugs me
but I'm not going to hack it back apart again:

http://home.netcom.com/~arc5/RAT-1/RAT1rear.jpg

See all those grey spots on the back?
Those are filled drill holes all around the replaced connector.
As you can see, I still have lots of "backing and filling" to do.
And, as I have come to expect from this kind of VANDALISM,
the thing never played again and did not one lick of useful work
after all this needless, thoughtless abuse.
Anyways.... I digress.... Sorry  ( not really.... snicker ;-)

As someone previously mentioned, the variable cap 
is part number 4606,  44 pFd per section:

http://home.netcom.com/~arc5/RAT-1/4606Varicap.jpg

The little oscillator padding cap is one of those cute
A.R.C. "hand-thrown" jobs made of little sheets of 
mica and conductor assembled on a nut-and-bolt
armature which they tightened until it was "right."
Should have made a photo of that.

One very cool thing:  The tuning cap is date-stamped
and also stamped by the person who did the alignment.
God rest you, "RV," whomever you were:
http://home.netcom.com/~arc5/RAT-1/RAT1Date.jpg

In contrast, the varible cap for the 6-9.1 MC rig 
is 65 pFd per section.
The modded unit uses one plate, 
doubtless to give you some bandspread.   
Tuning is kinda "touchy" when the dial covers 7 MC.

The IF is 4200 KC.  The IF coils are still sealed and
I don't want to open them.  The seals on the RF coils
were already broken but, thank the merciful Almighty
and for a wonder, doofuss had not boogered with them.

All coil forms are .69 inch outside diameter ceramic.
(Thank you, Mike Hanz, for the "right tool" to measure this).
Slugs are in place.

Here are the coils:
http://home.netcom.com/~arc5/RAT-1/RATCoilset.jpg
Top to bottom:  Antenna, Mixer, Osc.

http://home.netcom.com/~arc5/RAT-1/RAT1Antcoil.jpg
      6 turns single winding.

http://home.netcom.com/~arc5/RAT-1/RAT1Mixcoil.jpg
I cannot count the turns on the top winding; it is "pie-wound"
and at least a couple of layers.
Bottom winding is 6 turns.  Winding starts where you see it
at the top right (yellow area of the screwdriver).

http://home.netcom.com/~arc5/RAT-1/RAT1Osccoil.jpg
  Top winding is 5 turns.  "Tickler" winding is 2 1/2 turns.

Operational notes for those interested:
I've got this set fully operational as-designed on an RAT-1 dynamotor,
but it's been many years since I last worked on it- like, maybe 1997!  
So I went back through the alignment, fired it up and checked for
gremlins.  All OK after IF, BFO and RF alignment, but very dead bands. 
Even fairly stable after a good warm-up. 
Using the 12SK7 RF amp (if you can call it that at 27 MC),
sensitivity is considerably less than that of the modern receiver
to which I compared it (sorry- my service monitor signal generator
calibration is busted).   I saw that someone mentioned the higher-
amplification 12SG7 and I just happened to have one.  Popped it
in with no circuit changes and the improvement was significant.  
You could use this on 15 meters if the band would ever really open.
For "giggles," I stuck-in a 717A from the VHF rigs (yes, I know the 
filament is wrong; this was just a short trial) and that really made
the few signals on the bands jump-up.  Is there some RTTY 
contest going-on today??  Strong CW signals from a K6 station,
but only one lonely "Chicken-Bander," comparable to 
what I could hear on the Racal  RA6790.

Hope this has been of interest.
GL ES 73 DE Dave AB5S