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-   -   Zenith 25LC20 vertical suddenly went crazy (http://www.videokarma.org/showthread.php?t=270136)

TUD1 02-14-2018 12:19 PM

Zenith 25LC20 vertical suddenly went crazy
 
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There I was, trying to adjust the dynamic convergence on my newly repaired Zenith 25LC20, and all of a sudden, the vertical deflection lost its marbles. The transformer started making very loud noises, and I can't get full deflection. I thought the convergence board might have something to do with it, so I unplugged it, and the noises went away, but I still don't have full deflection. It didn't start this up until I started adjusting the dynamic convergence, but this chassis has been moved around so much, I find it hard to believe that simply pulling that one board and barely adjusting anything would cause this. I don't know if I caused this, or if, by chance, I decided to do convergence at the same time this dam-ned thing wanted to break.

TUD1 02-14-2018 01:30 PM

I spoke with the previous owner of this TV, and he told me that he completely rebuilt the vertical circuit, even built new vertical integrators.

DavGoodlin 02-14-2018 04:05 PM

Check the vertical output transformer and see if the convergence winding arced over to the primary winding. Just hope its one of the new electrolytic caps instead, much easier to fix.

TUD1 02-14-2018 04:35 PM

I was going to try going in there with an ohm meter and test to see if there were any shorts from the convergence plug to the primary of the VOT.

teevee 02-14-2018 08:30 PM

In some sets the convergence is part of the vertical output tube cathode circuit, disconnecting the board would reduce the sweep size... Easy enough to check on the schematic..

zeno 02-15-2018 06:49 AM

Quote:

Originally Posted by teevee (Post 3196208)
In some sets the convergence is part of the vertical output tube cathode circuit, disconnecting the board would reduce the sweep size... Easy enough to check on the schematic..

Good point. I dont remember if it was on these BUT some of the Zeniths
had 3 'lytics in the vert out. Usually they caused lin problems
( bottom pulled up) but on one chassis one of them caused strange symptoms.
V out transformers did go in Zeniths, not common but not rare. IIRC
several here have had to change them.

73 Zeno:smoke:
LFOD !

Electronic M 11-01-2020 01:19 PM

So I bought this set back in early 2019 and the last 2-3 days have been futzing with it...
TUD1 sold it to me without a CRT and I put a jug that I've tested but never seen work in it, IIRC there was no blue lateral/purity assemblies (sourcing a lateral slowed me down a year) and missing knobs and screws (I think I still need a volume knob and 2 pencil box knobs).

In addition to vertical issues noted I've got basically no video (I can kind of percieve sync bars), brightness varying the focus/HV, and I can't do a proper setup because the guns won't fully extinguish with any setting of screens.
Gun Bias voltage seems normal except focus which is usually low when the CRT is connected. The space between the G2 and focus glows green sometimes which concearns me. The video detector showed signs of recent desoldering and I have perfect sound so I tacked in another video detector and have more distinct sync and hints of video but I suspect its in backwards. The replacement diode I scrounged had no band so I may have flipped it while waiting for my soldering iron to heat up. I plan to flip it today and see if I can get video.
If I can get test patterns on it I should be down to just vertical and brightness issues which will both be easier to diagnose with video.
Having 3 problems to chase at once instead of 1 makes this a good bit more challenging.

Electronic M 11-02-2020 09:35 AM

Well turning my replacement diode around got me weak video.

Scoping the grid of the vertical output is showing me why the vertical is so WEIRD. The very weak 120Hz sawtooth is riding on a very strong something like 3Hz sine wave. Who ever TUD1 bought this set from didn't do a stellar job rebuilding the vertical or the whole chassis for that matter. Some of the new caps are crummy approximations, and all the caps are j-hooked in (seriously it isn't that hard to solder Zeniths above chassis terminals from under chassis if your iron has enough watts for chassis work). There's also a number of original plastic caps and lytics where only a section or 2 are changed which are "make it work as cheap as possible to sell it" type fixes IMO.

Electronic M 11-02-2020 01:40 PM

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It could stabilize at a rediculously high vertical frequency and loose the 3Hz.

So after finding strange voltages on the vertical mult grid and plate that couldn't be accounted for with caps and resistors in the circuit I started paralleling caps to see if I could change the frequency...then I jumped the home made vertical feedback K network and suddenly I had normal vertical deflection that would collapse and return at 3Hz. At that point I correctly suspected that micas in the homemade network were shorted. Sure enough one had less than 1k ohm on it. Swapping it got vertical deflection back. I think the 100V micas whoever built the network used for this were a bit undersized for the application...I only had one spare so for now the other 2 caps in that network will be run to fail status.

I think I know why the HV doesn't have much oomph. The negative grid voltage on the output tube is only -34v when it should be -65v... still have to figure out the cause of that and the weak color but now I atleast have picture to diagnose by.

Also it seems like someone played musical chairs with the screen wires...green blue and red screen pots all adjust the wrong primary...time for some lead tracing.

Electronic M 11-03-2020 02:49 PM

4 Attachment(s)
Well it is as done as my parts stocks allow. The centering magnets in the yoke don't do anything despite the fishline moving the mechanism.

I'm also missing the power/volume tricolor knob and three pencil box knobs that I'm going to check my stash for.
It isn't super bright but works well.

TUD1 11-04-2020 09:32 PM

I'm glad you got this set going again, Tom. It doesn't surprise me about the J hooked caps and value approximations. The radio club here (this set came from a member there) is J Hook University.

Electronic M 11-05-2020 10:16 AM

Thanks.
It's shaping up to be a nice set...I just need to find a few odds and ends.

Fixing it and resorting my basement is shaping up to give me more floor space... something I've been desperately short on for the last year.

zeno 11-05-2020 03:35 PM

Well done Tom !
She shows a little age but still quite watchable. In the olden days the road
men loved to do aged sets on Saturday morns. When you get them going
agn the cartoons always looked great. By the time real content was on
the customer is happy as hell. A surprising amount of psychology was
used by old TV men. I could tell some stories:yes:
BTW not sure when the "war of the worlds" logo started but even if
it were originaly chrome the tri color logo is a nice touch.

73 Zeno:smoke:
LFOD !

Quote:

Originally Posted by Electronic M (Post 3228708)
Thanks.
It's shaping up to be a nice set...I just need to find a few odds and ends.

Fixing it and resorting my basement is shaping up to give me more floor space... something I've been desperately short on for the last year.



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