RCA 21CT55 #8802897 is Dead

[wa2ise]

Does that coil winder machine do that zig-zag diamond shaped pattern with the wire when winding the coil? /\/\/\/\/\/\/\ Supposedly that pattern makes for lower stray capacitance. And is nearly impossible to do by hand.

[John Folsom]

Yep, the universal coil winding machine does the ziz-zag pattern ,and it IS impossible to do by hand. Here is a photo of the gears , cams, and other items used to set up the machine for a particular winding pattern. The gears range from 19 to 120 teeth, and the heart-shaped cams range from .082" to 1.5" (coil width) . The gears and cams used for a setup is dictated by the coil form diameter, the winding width, and the wire gauge.

[DaveWM]

John, how do you know how many turns between tap, and gauges of wire to use? do you need to take one apart or are there specs to be found.

btw, you going to fawg this weekend?

[holmesuser01]

All I have to add is:

I hope the set will be running again. The picture you provided of it was beautiful.

I love old electronics. I want to keep it all going for as long as I can.

Someone here said that the minute we expire, someone will let it all go for the price of copper. Maybe. In the meantime, its still mine and I am the caretaker.

Best of luck rewinding the flyback, Mr Folsom. That's one beautiful coil winder!

Bruce

[Giulio Maiocco]

Hi everybody!

I'm sorry to hear about this mishap with the flyback transformer, I think it's one of the worst "oh no!" moments.

I don't know the color flybacks, but I rewound some B&W ones without problems, and from what I saw, they are less critical than one may be led to believe. I even rewound a couple that were wave-wound in a more conventional "layer" winding, these still work flawlessly. In my Bush TV22 I rewound both the primary (bad enamel on the magnet wire crumbling away, so I decided to fix it) and the EHT winding (this one was wave wound and covered in tar, but the acid in the tar corroded the copper). As a result the TV works very good, I'm 600-700 volts short on the EHT, unfortunately I ran out of wire while rewinding. Using a test pattern, I cannot detect image distortion or ringing, so I'm really happy with the results!

A question for John Folsom, if I may ask: what kind of substance do you use to keep together the winding when you pi-wind? Now I have a wave winding machine too and experimented with some shellac, it works, but it's quite slow to set.

Thanks

Giulio Maiocco

P.S.: Sorry about my so-so English...

[old_tv_nut]

Giulio, thanks for posting this information. Your work looks good - I know handling the fine wire is very difficult. Also, your English seems fine to me.

I wonder if you have an oscilloscope and high-voltage probe, and can check the shape of the flyback pulse?

[Zenith26kc20]

I'm curious (again)and was looking at the schematic of a TS-905 Motorola regulator. These are shunt regulated high voltage supplies that, in the case of the Motorola, sample boost and that gives the 6BK4 grid bias thru dropping resistors. It seems to me a balancing act between the CRT beam current and the Shunt regulator tube to keep high voltage near constant voltage. Since high voltage was 34 KV, It makes me wonder if the higher than normal RCA HOT current came mainly from too much B+. Having no regulation would lessen the load on the high voltage as the shunt regulator would be biased to cut-off. I read the drive was OK to the tube (waveform voltage) but was the negative voltage on the HOT normal or too low. Higher RF voltage on the output would be expected but this would tend to break down the high voltage winding. The primary failed from what I read but these generally have around 6 to 8 KV RF on them so breakdown is usually rare.
Since the picture was wonderfully linear, I don't think the flyback had any shorted turns until failure.
When running before the failure, was the flyback running hot? I don't have a schematic on the CT2 series but the shunt regulators seem to be similar in the older sets.
Any opinions?

[Giulio Maiocco]

Hi again

To old_tv_nut: I'm happy you liked my work rewinding this FBT, just to let you know how boring it is, it counts 2045 turns of 31awg wire on the primary, and close to 3000 turns, 2960 to be exact, of 36awg wire on the the EHT winding, all done by hand, so not an easy task.

Yes, I do have 2 oscilloscopes, a Tek 545B and a Solartron CD1400, but at the moment I don't have the needed high voltage probe, I should indeed buy one if I can find one at a good price. As soon as I succeed at locating this probe at a fair price, I'll check the flyback waveshape, no problem!

Cheers

Giulio

[John Folsom]

DAveWM, my method is to unwind a flyback and count the turns and measure the wire gauge. And yes, I will be at FAWG.

Giulio, That is beautiful work you have done. I don't know if a layer winding approach could be used for color TV flyback transformers. At some point, I may give that technique a try, but I have many avenues yet to explore with my universal winding machine.

while the coil is winding, the ziz-zag structure is all that is needed to hold the coil together. The tall, thin high voltage secondary winding is also self supporting and stable, but I apply a topcoat of silicone RTV rubber to protect the coil and give it additional mechanical stability.

John Yurkon, I am not sure about the third harmonic tuning. I suspect you are correct, but I have no verification of this. I have a book published by Phillips which goes into great theoretical detail into the design and modeling of the windings on a flyback. And while this is helpful in as general knowledge, it does not really help me to know how to modify my recipe to achieve a particular result. I expect some trial and error is ahead of me.

[Giulio Maiocco]

Good morning

sorry to hijack this thread, if any moderator feels this is too much, please remove or move my reply!

@ John Folsom: I'm far from an expert, but I think that layer winding should not be a problem, maybe some changes are required in the circuit in order to obtain as specified performances.

Also, thank you about the info on the pi winding, and now I think I know what I'm doing wrong, the wire needs slight more tension, as until now the "layers" gives a spongy feel when touched and the width of the winding creeps up.

BTW, please have a look at the 2 files I attached, I think this is interesting

Cheers

Giulio Maiocco

[kx250rider]

I'd have to go look over the schematic to refresh my memory on what all besides the horizontal sweep and HV is unique to that chassis. But one idea might be to break down the flyback section into separate devices; perhaps a flyback from a newer color set to give the sweep, and a simple solid state high voltage supply for the 2nd anode? It should be much easier to solve this problem for this set, than for a CT-100 with the weird deflection angle and integral convergence HV feed, etc.

Charles

[John Folsom]

Giulio, thanks for those great articles. I will have to go to school with them.

I agree with Charles, it should be possible to substitute in a flyback from a later vintage set, say a CTC10 or so. It would tack some exprimenting, but I think it could bedone.

[TubeType]

You can measure the dimensions of a flyback with a time domain reflectometer (TDR), a precision digital ohmmeter, an LCR meter, and a micrometer.
The inductance of a newly rewound flyback will probably be different than the original.
You should change the inductance of your new flyback by adjusting the core gap rather than adding or deleting turns to the coil.
Remember, the inductance is inversely proportional to the gap.

[andy]

---

[John Folsom]

Andy, I know at some point, flyback designs used the layer winding approach. But all the flybacks I have seen up through the 1950 and early 60s were universal wound. When did they switch over? Did this coincide with the use of transistors?