RCA CTC5 Special advice
 

I picked up a RCA 21-CT-7837 yesterday with a dud tube. Filament is open, but getters looked good. Tried rewelding the filament this afternoon. Got some flashes but wouldn't hold. It's got the original 21AXP22.

It's missing 3 of the legs, and the RCA/Special badging on the front. I'm debating whether it's better as a parts unit, or to try and find another CRT. Chassis is clean, but transformer and flyback haven't been tested. Cabinet has some scratches/dents, but it's a solid 7/10. Advice is appreciated

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If you got a spark or two trying the filament weld, then a weld is still possible. I would
look further into trying to do a weld. I would consult welding books, and look at the
metal used in the filament, and check welding experts to see what current & voltages
they use, and stay under that and try it again. Forget what anyone here says, using
a charged capacitor to weld a filament is the way to go, but you need a good
understanding of the joul energy in the charge, and the knowledge if the
energy in an arc weld to pick a good cap, & voltage to make it work...
You know there are weld on dent pullers for auto body work that
make a nice spot weld on the body for dent pulls that can
easily be broken without deeply penetrating the body skin.
Somewhere you can find some good info on how to
get a better filament weld than just guessing on
cap size, & voltage.... This would be a good
project... No reason this can't be done.

Good Luck...


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Any reccomendation on where to look? The problem as I understand it is that the majority of welders use low voltage and high current, but that might not work with a larger gap in the filament

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Your not looking for a welder. You need to understand the energy requirement to make
a weld in the filament wire. Know also that you are welding that in a non-oxygen
environment. You need to look at joul energy in a charge held by a cap of
specific size, and with a specific voltage charge, Is this joul energy
of sufficient size to make a tungsten & whatever it may have
been mixed with of a specific diameter to make a good weld.
I don't know where to look, not welding books.

You might want to experiment - get some filament wire and
figure out what size cap & charge makes a good weld.

I know there are methods to weld filament wire inside a
tube, I remember reading about it years ago in tv repair
magazines....

..



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Did you reflow the solder at the base pins ?

jr

+1

I have a Philco 21AXP22 that needed the pins resoldered....Then about a year later it died again and solder wasn't working...So I soaked the base in ammonia to release the glue desoldered it and found about half the leads including heater leads had rotted off close to the glass I soldered 2-3 Strands from some lamp cord to the stubs and filed down a few of the glass mounds to reveal a solderable stub. I meticulously cleaned the stubs so they would take solder fast, gently prewarmed the neck with a plumbing torch, then quickly soldered the new wire with an iron (dwell too long and the glass will crack and bye bye vacuum), then I threaded the new wires through the base, exhaust siliconed it to the neck and soldered. Tube still lives today 5 years later...

:D:D
Then waited for your heart to slow back down to below 200 BPM, and your blood pressure to drop back out of the danger zone? :thmbsp:

Any reccomendations on how long to submerge the base in ammonia? I'll give it a shot pulling it off if I can manage. I've regglued plenty of tubes, but can't say I've ever pulled one off that wasn't loose.

Well,
Was able to get the base off, there were a few broken pins, both of the heater pins looked good. Not entirely convinced that I didn't snap some of them myself when removing. If there is anything else that I should try, I'm all ears.

Zsuttle, I soaked mine overnight YMMV.
I'd go back to Squirrel Boys suggestion.

I actually wasn't that wound up...IIRC I had a spare 21AXP22 then, I knew the tube I had was toast if I did nothing, so I had nothing to loose and everything to gain, I had done a similar thing with a monochrome tube before I knew how to solder at my current level and cracked it with heat so I had a feel for max dwell time on cold glass. I had those wire stubs shiny ready for solder, warmed the glass to improve tolerance to solder temp and besides the actual soldering (which I did with calm sharp focus) I did everything slow and deliberately. I won't say I wasn't a bit tense but I was relatively calm.

2 years ago, I tried to "solder" the filament of a small B&W CRT. It succeeded, after increasing the caps value and voltage, but the filament only lasted some 10 power-ons. After that, it flashed again and opened badly.
But out there are more sucessfully cases, so is good to try, anyway