Zenith 25DC56 rolling
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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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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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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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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. |
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:smoke: LFOD ! |
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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May want to tweak the blue static convergence while your in there...
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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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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... |
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:smoke::smoke: LFOD ! |
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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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