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Old 12-22-2014, 04:59 PM
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Vertical Centering - how does it work?

Being a bit annoyed by the slight difference in the size of the black bars at the top and bottom of the screen on my CTC-38, caused me to be interested to find an answer to a question. How does a vertical centering control actually work?

Looking at various schematics of TV sets which have it doesn't seem to bring me closer to the answer.

The later "roundie" TV sets apparently have an additional winding on the vertical output transformer, which isn't really described very well on schematics. If it is out of phase, it isn't shown.. if it's in phase, then what the heck is it doing? The RCA 630 has a voltage divider which appears to be adding a little bit of signal from the horizontal circuit, again, making no sense to me. And the CT-100 and CTC-4, have a "vertical centering choke", which appears to be adding a little bit of phase shift into the waveform hitting the vertical deflection coil.

Am I missing something obvious here? I also noticed that factory service manuals which go out of their way to describe all other parts of the circuit don't touch on "theory of operation" for the vertical centering control. Also, I am getting the impression that attempting to add a centering control to the CTC-38 would entirely screw up the pincushion correction circuit.
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Old 12-22-2014, 05:42 PM
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If you want to add centering here is an old Zenith trick...On their color roundys they simply added movable magnets inside the deflection yoke (there were little fishing line wires that the service man would pull on to adjust). If you place a strong enough magnet in the right spot it will adjust the vertical centering.

On some circuits there was a centering pot that added a DC bias to the secondary winding to achieve centering.
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Old 12-22-2014, 05:46 PM
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Wait a moment, maybe I am over thinking this - all it is, is some DC bias? The CTC-2 and CTC-4 have a much more complicated circuit, and if you look at the CTC-11 - CTC-16, there's an additional winding on the vertical output transformer. A magnet would be an easy way to go about this, but would that not mess up convergence? If Zenith did it, it has got to work.
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Old 12-22-2014, 08:45 PM
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The magnet trick makes sense if the magnet is applied in the yoke so its deflection center is the same as the sweep.

Regarding the more complex electrical versions, posting a few schematics would help in trying to decipher what's going on. A DC bias makes the most sense - maybe it's just not clear where the DC current is coming from and where it is being returned.
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Old 12-22-2014, 10:48 PM
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Here is a schematic of the CTC-38 I am trying to mess with. It looks like the vertical deflection coil is actually horizontal, on the page - which is the opposite of RCA diagrams. I have no idea why Sams would do this.

I am thinking that the DC bias idea while in theory is good, may be a problem, because the DC may saturate the cores of the pincushion correction transformer.

I wonder how strong the magnet should be. Something like a fridge magnet? May be worth a try.
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Old 12-23-2014, 09:15 AM
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Quote:
Originally Posted by maxhifi View Post
Here is a schematic of the CTC-38 I am trying to mess with. It looks like the vertical deflection coil is actually horizontal, on the page - which is the opposite of RCA diagrams. I have no idea why Sams would do this.

I am thinking that the DC bias idea while in theory is good, may be a problem, because the DC may saturate the cores of the pincushion correction transformer.

I wonder how strong the magnet should be. Something like a fridge magnet? May be worth a try.
This is the schematic of a set that doesn't have vertical centering, and you would like to add it? OK, but I meant to post a schematic of a set that has it, so we can figure out how it works and suggest something to try.
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Old 12-23-2014, 09:18 AM
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Regarding a fridge magnet, I would think you need at least two to make a symmetrical field, otherwise you would mess up the deflection. Even then, may or may not work depending on how it couples into the yoke core (or doesn't). If the field is not confined to the deflection center, it could affect everything (convergence, purity) at once. Still worth an experiment.
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Old 12-23-2014, 10:50 AM
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http://www.boxcarcabin.com/rcaschematics.html

All of these sets have it. Notice how it is implemented. Let's look at the CTC-11 as an example. The voltage drop over R129 is used to bias the vertical deflection yoke. If you look at the vertical output transformer, the resistor is between the GRN and GRN-WHT secondaries, making one a different DC voltage than the other, but both have the same AC voltage. Now, this configuration won't work for the CTC-38, because of the pincushion transformer, and also the vertical output transformer which aren't designed to take DC.

I am thinking of another idea - for the CTC-38. I would much rather use an electronic solution than magnets. I will draw it out when I have a few minutes.

As soon as Electronic M said it is just a DC bias voltage I kind of had a Homer Simpson type moment and realized how this works. What threw me off is the CT-100 schematic, and how that TV achieves vertical centering... it looks a lot more complex than it is!
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Old 12-23-2014, 11:47 AM
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Everything about the CT-100 is complicated...
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Old 12-23-2014, 12:10 PM
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Originally Posted by miniman82 View Post
Everything about the CT-100 is complicated...
This is why I was confused - check it out, I stuck together the two relevant images from the CTC-2 manual. The vertical centre choke and the two pots connected in reverse were rather hard to figure out... well, it's easier now that the two diagrams are beside each other and not four pages apart.

What I am thinking of doing, is using some big (say 2200uF) electrolytic caps to isolate the vertical yoke from the vertical output stage, and then using a small (say 0-15V, 1A) power supply to bias it with DC, in series with a choke which has a high AC impedance to allow the vertical signal through, therefore keeping DC out of the pincushion transformer and vertical output transformer. If the vertical output winding is 14 ohms, I think 2200uF with a low enough ESR should be enough to avoid problems with phase shift. Maybe I will bypass it with a smaller capacitor to avoid any problems which could be caused at higher frequencies. This would have to be replicated for both sections of the vertical winding, to keep things symmetrical. Then I could dial in vertical centering to within a millimeter, and keep the RCA going well into the 21st century.

I know, I know, it is a lot of fuss to make over a 45 year old non- top of the line TV, but this thing is absolutely pristine, inside and out, and the CRT checks as new, with PERFECT colour tracking. I watch it every day, and I am always impressed at just how natural skin tones are, and the sound is also fantastic and free of any buzz or hum which plagues my other old TVs.
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Last edited by maxhifi; 12-23-2014 at 12:52 PM.
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