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  #1  
Old 05-16-2017, 01:32 PM
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Zenith 25DC56 rolling

This TV, one of my favorites, has been out of commission for a few months after the vertical took a dump. It will not lock. You can get it close, but it still rolls really slowly. I tried a known good 9-83 module, and it did not help. After this TV broke, I put the 25CC50 tin can on top of it, and we watched that. But now we're watching the System 3 so the 25CC50 doesn't get a ton of hours.
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Old 05-16-2017, 01:38 PM
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Probably an issue with the sync sep circuit and or AGC setting. I forget what board it is on, but I should be able to get you more info off the schematic after work.
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Old 05-16-2017, 01:57 PM
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About the AGC, I did have to adjust it when we first started using the set. It would have occasional "snow outs" were the picture would be super fuzzy for a few seconds and then stop. I adjusted the AGC and it solved the problem. It worked fine for months after that.
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Old 05-16-2017, 02:32 PM
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AGC adjustment typically has a low signal (blank raster/snowy pic) range, a mid range with a sweet spot, and an overload range where sync is lost and or grossly excessive contrast/negative pic occur.

Typically you turn AGC up till synch is lost and or pic contrast becomes overloaded, then back it down until sync stabilizes and contrast is in a reasonable range for it's control....It may be worth tweaking the AGC again to see if it is having an effect.
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Old 05-18-2017, 10:25 AM
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Never had one of these roll. There are a few small 'lytics on the
main chassis. One was in the AGC. IIRC it caused snow, look
that way first.

73 Zeno
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Old 08-11-2017, 04:31 PM
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Fired the old Zenith up again today and as expected, it still rolls. Tried adjusting the AGC, and it had no effect on the vertical at all. Probably a cap...
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Old 08-11-2017, 05:09 PM
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May want to tweak the blue static convergence while your in there...
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Old 08-11-2017, 05:17 PM
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Doing any work on a TV this big is near impossible on account of the house is the size of a broom closet and there's no room to even walk. Even plugging it in was a PITA.
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Old 08-11-2017, 05:24 PM
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Vertical sycn is pulled from the 9-48 Module, off pin 3 of the 221-45 chip. Can you scope the sync there?

If I can find my Zenith Tech Topics binder (MIA right now..) there is an entire TT issue dealing with the 25DC56 and common problems. I remember scanning it, so I'll check my Canon scanner folder too...
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Old 08-11-2017, 07:27 PM
zeno zeno is offline
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The AGC on these is RF AGC. Put on strong signal & adjust
till the pix goes snow then turn back until the snow JUST clears
up. Its touchy & will cause the jitters if set to high.

BTW for you & others. Zenith made a training manual with
paper & cassettes that covers the 4 tube hybrid, flat chassis &
the CC2 upright chassis. I recommend it, I see them on e-bay
now & then. Its amazing how close they are to each other & uses
many of the same parts & design. Great way to see slow evolution.

73 Zeno
LFOD !
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Old 08-11-2017, 10:41 PM
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Quote:
Originally Posted by zeno View Post
BTW for you & others. Zenith made a training manual with
paper & cassettes that covers the 4 tube hybrid, flat chassis &
the CC2 upright chassis. I recommend it, I see them on e-bay
now & then. Its amazing how close they are to each other & uses
many of the same parts & design. Great way to see slow evolution.

73 Zeno
LFOD !
I've got the cassettes still - green and orange? body to them - covers the flat chassis, 19EC45/FC/GC IIRC, and one other chassis. I no longer have the training manual - my copy got messed up thanks to a Coke can prank gone horribly wrong.

Those Duramodule sets were the easiest to service - and no need to stock a million modules like the later System III sets.
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