CT-100 B1001599 Failed Convergence Transformer
 

Hi All,

This weekend I began working on the CT100 again. The set was working fine through the weekend until the picture began to jump. It looked like a focus problem I cautiously powered the set. As I adnance the focus pot a dreading popping started and the higher the voltage the worse it would become.

Looks like the convergence transformer is open internally at the secondary brown lead from the focus pot. Between Blue and Yellow it measures 21kohm and the primary is okay.

I suspect a break internally due to the pitch potting. Might be lucky and be able to find the break at the connection to the brown lead.

The transformer looks like it has been replaced once in the life of the set from the look of the soldering: the replacement is a silver canned transformer.

I recall a couple of decades ago, John Folsom and others discussed rebuilding the transformer. Are any spares around?

(The transformer failed Sunday evening (December 17) which I recall was the day exactly 70 years ago the FCC adopted the NTSC standard!)

Any help or anecdotal experience regarding this problem will be greatly appreciated.

Hi to all,
Hi Penthode,

Sorry! to learn the failure of this high-stress part in your CT-100.

I searched on the ETF site in the "Classified" where John used to advertise his rewound transformer: long gone.
In the CT-100 section by Pete Deksnis, there is a photo of the transformer and John's address in Florida (see pix), maybe try to contact him & find out if he's got some spares or can rewind yours.

Alternate solution:
contact Ed Dinning in the UK, he's still currently rewinding transformers and can do with success EHT windings, Pre-War AC EHTs, etc.
Ed is in Newscastle-upon-Tyne, Northern UK.

ed_dinning AT yahoo DOT co DOT uk

Mail current as of Oct 2023,

Hope this helps,

Best Regards
jhalphen
Paris/France

Hi Jerome,

I recalled years ago John Folsom and the ETF looking at making replacements But that was about 20 years ago.

Thanks for the UK contact. I need to find out the characteristics of the old transformer. I plan to remove the potting and have a look at it.

It also occurred to me that perhaps a solid state electronic device could be designed and constructed as a substitute? I shall investigate that as well.

Cheers,

Terry

Given that the xfmr's secondary sits at focus voltage, making a ss replacement would be an interesting design challenge.

I'm watching with curiosity. My Westinghouse H840CK15 uses the same transformer and probably could benefit from a new one.

I've heard when these fail they often take the unobtainable focus pot with them, and that originals should not be used. At some point I'm going to dig out my Westy as get it the final mile from mostly working to turn it on anytime and watch status.

It would be an interesting challenge. I only wish I had the waveforms the circuit would deliver. It would make the problem somewhat simpler.

It looks like the circuit including the transformer is generating only the vertical parabola. The brown lead would be at sinal ground by virtue of the 0.01uF capacitor from the brown lead to groundwhich means the parabola would would have a slight application to the focus electrode, likely to modulate the focus perhaps? and a much greater amplitude which would be coupled through the 820k resistor to the convergence electrode. Which means a single amplifier fully sitting above ground by 4000v coupled with some voltage gain (by virtue of the step transformer) from the output of the 12AU7. Hi voltage coupling capacitors and some diode spike protection should make it work. Afterall, the dynamic convergence correction should only be slight. I am going to think on this.

I would take up Jerome’s suggestion and contact John Folsom. We obtained a replacement transformer from John 4 years ago for our Westy and more recently, I know of an H840CK15 under restoration with contacts with John about the transformer.

My understanding is John’s transformers were averaged to work with both the CT-100 and H840CK15. At the very least, John could share technical information you desire.

Hi to all,
Hi Penthode,

more info about CT-100's V convergence transformer on Phil Nelson's restoration chronicle + photo of Folsom's Xformer & EHT cage wiring diagram:

https://antiqueradio.org/RCACT-100Television.htm

about risk of Focus Pot failure & solutions, here's how Miniman82 (Nick W.) solved the issue when he restored his CT-100 in 2011:

https://antiqueradios.com/forums/vie...p?f=3&t=175030

Convergence (H & V) theory of operation - extracted from this 19 page CT-100 RCA document (page 18):
https://antiqueradio.org/art/THE%20C...20RECEIVER.pdf

"The dynamic convergence and focus modulation voltages, each having the proper waveform, amplitude, and synchronism with deflection, are produced by linear addition of a variable shape vertical parabola with a variable phase horizontal sine wave. The composite alternating output voltage is coupled to the kinescope convergence electrode and focus electrode through the respective output taps.
The vertical-deflection dynamic convergence amplifier circuit combines, shapes, and amplifies the parabola and sawtooth waveforms derived from the vertical-deflection circuit for application to the kinescope."

"Sine-wave horizontal dynamic waveform is derived from two cascaded tuned circuits excited by horizontal kickback."

+2 last photos:
V convergence waveform shape (no scope photo nor measurements values) from SAMS CT-100 SM on ETF site.

Convergence section schematic from CT-100 Service Clinic manual (page 31) on ETF web site.
Sorry! for tiny size, forum display limitation.
http://www.earlytelevision.org/tv_sc...ams_color.html

scroll down to RCA CT-100 section.

Best Regards
jhalphen
Paris/France

I think radio electronics or one of the TV magazines (probably on radio history site) has the convergence waveforms and explanation of the circuit... I'm just about sure I read the original paper about a decade ago.

You have a part number for this transformer?
Besides eft did you check Moyers and parts-link
Com
Doug

Thanks all for the advice and information.

I never gave Mouser or any other parts supplier a thought because I figure someone would have beaten me to it! But you never know. I will grab the part number and Google away...

I shall also follow up with Messrs Dinning and Folsom.

Thank you Jerome. The waveform depicted with the combined parabolas is what I would have anticipated. And the secondary is simply adding a lower coupled parabola to the focus electrode and higher level to the convergence electrodes. And as I thought, the parabolas will modulate the focus to ensure focus can be simultaneously optimized center screen to the screen edge.

The transformer appears simply the means of coupling the 12AU7 to the high voltage focus/convergence electrodes for the vertical parabola only. I remain interested in an electronic substitute eliminating the need for the transformer and will mull over a design.

Meanwhile I shall melt the pitch and see if the old transformer is salvageable. After all it is an open circuit and I may strike lucky if the break is at the winding termination to lead out.

Anyhow I will share what I find with photographs here.

HV "signal" transistors for focus and G2 control are around in the years of hi-end PC monitors, but in general, are for ~1200V, like the 2SC4632.
I noticed some MOSFET for 4500V some time ago, but I don't remember the code or maker. But it if for power switching and have considerable capacitance.

And about using a small H sweep tube for direct coupling? (well, this will probably overtaxes the heater supply and also needs a extra +++B)

Looking at that circuit I think that it could probably be made to work
with any audio transformer of the proper turns ratio and impedance by
coupling to the HV side with capacitors. Given the availability of
modern film caps of high capacitance it should work fine. You'd just need
some HV high value resistors and/or perhaps some Zener chains.

I use a Zener chain for the focus DC. The pot setting is quite stable. Thus, a 1 or 2 meg
pot and some fixed resistors or Zeners would surely work OK. The lower voltage across it would likely allow an available pot to work.

One of the common schematics has 27K for a plate resistor in the
convergence amp. Its supposed to be 270K, but 180K or 150K probably is better.

Doug McDonald

Removed the convergence transformer this afternoon and the laminations this evening.

My recipe was to bake the can in a foil pan with the transformer can opening facing downwards at 400 degrees F for 20 minutes. The pitch dripped in a pool at the bottom of the pan. Then lifted the transformer can and the transformer could be removed. Placed the transformer in a second pan and baked it another 10 minutes to remove excess pitch.

I then removed the transformer wiped it down when hot and let it sit until warm. Then washed it in varsol until clean.

After it was dry, the transformer when back into the oven at 400 degrees for 10 minutes to again heat the remaining pitch. Then while hot, sat the transformer on the open jaws of a vice and tapped out the laminations. The laminations came out quite easily when warm.

The open winding was the focus voltage secondary input. I found the wire to the open winding is the outer most winding which should raise the chances of success repairing it as it will only require the opening the attachment of the outer most winding.