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I have been working on a 1953 Zenith TV; I can get raster on the top half of the screen only. If I reverse the leads on the Vertical deflection coil, I get raster on the bottom half only. So far, in addition to replacing all caps. & tubes, I have replaced the deflection yoke, vertical output transformer, and Flyback transformer. Nothing has changed as far as the raster goes. Moving the ion trap around does not help. I'm stumped.....

Any suggestions.

B.Eyer
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  • Sounds like not enough vertical deflection amplitude. Check all the tubes. Any electrolytic caps are will also be long gone, although from that vintage they are probably only on the power supply. – Olin Lathrop Mar 24 '12 at 20:46

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I've seen this with solid state drive and something similar may apply. If it uses push pull or double sided vertical deflection then the driver on one "side" may be dead. In your case presumably a pair of tubes or maybe a double triode or ... .

So ... I'd look at where the deflection drive leads come from and see if there are two identical tubes driving it - presumably via a transformer as they only make vaccuum tubes in NPN / N-Channel :-). It could be any of a good range of things causing the problem but IF there are two separate tubes try swapping them. Check plate and grid voltages. A good guess is that a grid blocking capacitor may be very leaky.

Report back ...

Russell McMahon
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Deflection coils come in pairs; horizontal deflection is accomplished by coils above and below the neck of the tube, and vertical deflection is done by coils on either side. The intent is to create a uniform field between the coils, so the two coils in a pair are wired together and driven with the same signal.

Coils can of course be joined in series or parallel, and the failure modes differ between the two. With series coils, deflection tends to either work or fail entirely. With parallel coils, it's possible to get the kind of 'half-way' fail you described. You wouldn't think a simple, non-moving thing like a deflection coil would fail, but evidently they can suffer from both shorts and opens.

So you might want to fire up the soldering iron and separate the leads of the left and right coils, and continuity check them. If the two sides ohm out differently, your problem is the deflection yoke; the two coils should have the same DC resistance. Not sure how you'd go about resolving that problem, but at least you'd have isolated the problem and wouldn't have to dig into the circuits.

See Bernard Grob's Basic Television Principles and Servicing, (4th edition ~1975, probably out of print). There's a chapter on vertical deflection circuits that specifically mentions that fail mode with vertical deflection.

JustJeff
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