A slice of pi (winding).

As some of you may know, the flyback transformer has been found to be bad on the 19CT1 Motorola set. Since it seems pigs will fly before I find an NOS replacement, I tried investigating a few places that advertise rewinding and custom fabricating transformers. One thing that seems to be a common issue is the problem that the old flyback utilizes a pi winding, which it seems nobody has the right equipment to duplicate.

I'm curious to know (I've been learning about the details as I go along), what is significant about pi-winding a flyback transformer, and why a replacement has to be pi-wound? Would there be any way to adapt a flyback from a different set to get the Motorola working again? Are there any options anyone can suggest at this point, or am I pretty much dead in the water?



Thanks,
Joe

[jstout66]

as for NOS, maybe you could try Moyers. It will be a long-shot tho. If you could get the Thordarson number maybe one may pop up on eBay. Good luck as that is a cool set!

[old_tv_nut]

The important thing about the flyback winding is its self resonance due to distributed capacitance. This depends both on the winding pattern and the insulation material dielectric constant. It is generally important to get this right because this self resonance is used to shape the peak voltage waveform that gets applied to the horizontal output and also the waveform that gets rectified for high voltage supply. The coil resonates with an external capacitor to give the basic half-sine wave flyback pulse, but the self resonance may be, for example, the 3rd harmonic, which produces a flat top or even a dimple in the top of the pulse. This reduces the peak voltage on the horizontal output. Tuning a flyback design is re-iterative work based on modifications from an existing design and requires running the re-designed winding through the potting process to get the final capacitance, and seeing how close you got, then trying again until it's right.

I think it's difficult to re-create a flyback transformer if you don't know the characteristics of the insulation as well as the winding configuration. I'd be interested in hearing if anyone has done it. How critical it is depends on how close to the stress limits of the horizontral output and other components the design is.

The pi winding is specifically used to reduce self-capacitance and increase the self-resonant frequency, compared to other winding patterns.

[above taken from long-ago memory - worth checking with someone else if you can]

[jroberts500]

Don't give up! There has got to be a way and no matter how much trouble, it will be worth it! That Motorola color set is a beautiful piece of American history and I will be glad to read that it is running.

Apparently it must be difficult to obtain the buiding specifications. Has that been tried?

[polaraman]

What is the Motorola part number of the flyback? Thordarson cross?

polaraman

If I am reading the SAMs parts list correctly (they don't actually call it by the name "flyback") it's the horizontal output transformer, Motorola part# 24D734487. I don't have access to a cross reference chart, so I'm not sure if there's an alternate replacement.



Joe

[captainmoody]

The number is in my Thordarson book but has no cross ref to a replacement.

[polaraman]

DARN! My pocket Thordarson book does not offer a cross for that flyback. Still looking to see if it does cross to thordarson. ANYBODY KNOW? Sorry, The good Captain beat me to the reply.


polaraman

[RVonse]

I have a good motorola flyback but do not knowif it is what you are looking for. My flyback comes from what looks like a late roundy or early rectangular color tube set. Is that close to the model 19CT1?

[David Roper]

Off by a bare dozen years.

[John Folsom]

A few notes about flybacks. As old tv nut stated, a flyback is designed to be a high Q resonant circuit. The period of this resonance must be short enough to cause the CRT beam to sweep back diring the horizontal retrace interval. The horizontal output tube provides no dirve during the retrace period. The idstributed capacitance of the flyback winding itself along with the distributed capacitance of the yoke, and assiciated circuitry and wiring resonates with the linuctance of the flyback windings and yoke to form the resonant circuit and determin its natural frequency. All of which makes the parameters of the flyback rather critical. And very small changes in the physical characteristics of the wire gauge, insulation, and geometry dramatically change the parameters of the flyback. So it is a non-trivial thing to wind one. A dedicated "universal" type of winding machine is require (or pi winding). These machines are complex to set up and operate.

I would like to find a machine someday and give it a try.... but so far, no luck.

There are no substitute flyback transformers for any of the early color TVs until the RCA CTC-7. A number of the earlier designs used RCA flybacks, but not all, and to get a replacement you must find an original NOS one.

Good luck to us all.

[bgadow]

One solution for this problem is for all of us to knock on doors so that no retiring tv repairman or distributor has the chance to toss a flyback in the garbage. Outside "our" circle these parts are worthless except maybe as paperweights.

[polaraman]

Has anybody tried this place? Butler Winding in Butler Pennsylvania. I snipped part of their page. Sounds like they should be in our corner. i will attempt to call them. Sorry, Forgot the link to the company.

http://www.butlerwinding.com/elelect...ack/index.html


polaraman

Butler winding can make (and has made) flyback transformers in a wide variety of shapes and sizes. This includes; various standard types of “core with bobbin” structures (E, EP, EFD, EC, ETD, PQ, POT, U and others), toroids, and some custom designs. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines.

[wa2ise]

Originally Posted by John Folsom

The idstributed capacitance of the flyback winding itself along with the distributed capacitance of the yoke, and assiciated circuitry and wiring resonates with the inuctance of the flyback windings and yoke to form the resonant circuit and determin its natural frequency.

Back about 25 years ago I worked at a small company that made boxes that would photograph video from CAT scanners. Was litle more than a CRT monitor with a camera aimed at it. The cat scanner people would have desired information close to the image edges, so overscan was not allowed. To cut the overscan we had to keep the retrace time to a minimum. One trick was to remove the high voltage winding off the flyback. HV was provided from another source. This would cut the retrace time by about half. Don't know what this did to the horizontal output transistor's collector, but we never blew one up.

[peverett]

Can you post a picture of the set? I have some Motorola Flybacks, but have not found your exact number yet. I have found some that are close.