[ARC5] Chasing Bugs Out of a RAT, And a Tube Tip

[David Stinson]

My 12-Volt, CBY-46084 RAT, 20-27 MC is a victim of "improvement;"

an aborted attempt to put the thing on 10 meters. 

The top is tagged as Serial #25, while the back skirt is 

identified as Serial #5.  Swapped tops are not uncommon.

 

Some goofuss buggered the RF coils, drilled the chassis and top, 

exterminated the dynamotor connector and performed other

grievous  blasphemies.   Another proof that God loves us

and is still on His throne:  the guy only damaged the Nomen plate

a little and didn't mess with the wiring under the chassis.

Of course, it didn't work.  So he tried to put the coils back and

made a mess of it.  When he couldn't get it to be an ICOM, 

he tossed it on the junk pile; a too-common story.

 

https://photos.app.goo.gl/wxgMD9Hn3sIxMY2f1

https://photos.app.goo.gl/StZjtESrInfg57rD2

https://photos.app.goo.gl/GJeDVTbcSPDdHACs2

 

The poor thing has been on the shelf for about two decades,

looking all hang-dog and lonesome, wondering if its turn will ever

come.  Well, I finally got to felling guilty and put it on the bench.  

You should have seen it wagging its dynamotor deck.

 

Disassemble and clean/lube/De-Ox-It, then the obligatory check for 

high-Z grounds. 

When I pulled the internal cover over the tuning cap, got a nice

surprise.  I love "personal" marks like this.  Whomever you were,

"RVP," I salute you:


https://photos.app.goo.gl/iFmLjvzgssHElrF73



100V current-limited on the B+ line (no fil volts)

and surprise surprise- none of the caps are leaking, even after leaving 

the juice to them for an hour.  That's nice because I'd hate to disturb that

pretty, ruby-lacquered wiring.   I had a NOS Dynamotor plug so removed

the dismembered corpse and installed the new one.

Going to find a clean ARA top cover and swap the plate.  The chassis 

holes are small and on the side so I'll probably just fill them with epoxy

and touch on some paint.

 

Luckily, I have a 20-27 RAT-1 so had a good coil set to use as a  pattern

for repairing the ham-burglared coils.  I found the correct wire on a 

scrapped 2.1-3 MC PA tank coil.  It went easier than I expected.

Reassembled, powered and wondered why I couldn't get 4.2 MC

through the IF stages.  Found out it works better if you actually 

connect the test lead to the signal generator.  Alignment by the 

manual and the set is playing well.  

 

A Tube Tip:
The 12SK7 RF amp isn't much at 20+ MC.  I've always wanted to try

the Western Electric 717A in that socket but of course, the 717A has

a 6-volt, 175mA filament.   The "fix" is easy.  Since the fil supply is 

12 volts, need to drop about 6 volts from the filament buss connected

to the tube.  6V x .175 mA, just a little over a Watt to dissipate. 

Grab a 5V, 5W Zener diode and a 1A or better power diode.

connect them in series, cathode to anode.  Put the diodes

between the tube and the filament buss, the *cathode* side

connected to the filament buss and the *anode* end to the

tube pin.  The Zener drops about 5V.  The power diode adds

about a 0.7 V drop.  The tube gets "close enough" filament

voltage.  I tried it and the 717A really revves-up the RAT

after touching-up the RF stage alignment to match the tube.

I put it back of course- Not looking for performance.  

It was just a fun experiment.
Need a 12SK7 and only got a 6SK7?  This will fix'ya.

 

73  DE Dave AB5S

 

 

 

 

 

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