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

[Geoff]

----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>


> 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



Nice report Dave and I wouldnt be too hard on the designers about 
sensitivity. Take any single RF ham receiver of prewar and it wouldnt be any 
better.

Now you can see why us hams of the 50's were so much into mods, especially 
Novices or teenage Generales trying to use 10 and 15M with obsolete radios. 
I used to swap in a 717A in place of the 6SS7 in the HQ-129X on those bands 
just to hear stations and this was during Cycle 19. I didnt realize I was 
previously hearing only about 25% of what was available.

For G&G try a 6AB7 and 6AC7 if you can work out a quick way to solve the 
filament issue and compare to the mushroom.

Carl