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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 2:02 am 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
so is it just a normal resistor or something special? how do i trace it down. i might have a chassis number or something so i can find a schematic and maybe that would say which one it is.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 2:04 am 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
so is it just a normal resistor or something special? HOW DO I trace it down? i might be able to lookup a schematic (if i can find a free one) and maybe that would tell which one it is.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 2:04 am 
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Joined: Jan Mon 18, 2010 2:13 am
Posts: 16197
Location: Dayton Ohio
Well, if were just trying to one-up each other, then I will bow out.

I dropped out of the TV service field in 2005, so all I have is 21 years, and no, I'm just a lowly tech. I never got my engineering degree.

I do remember on the earlier "Vertical fold down" Zenith chassis there was a 4.7 ohm resistor which would increase in value and affect vertical size. It was not located on the 9-92/9-147 module but on the chassis itself behind down in the corner.

-Steve

EDIT: I do have the service info, but no way to read it.
Here is a scan with my scanner at 2400dpi


Attachments:
microfische.jpg
microfische.jpg [ 80.11 KiB | Viewed 800 times ]
fische.jpeg
fische.jpeg [ 92.63 KiB | Viewed 800 times ]

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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 4:02 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
Sorry Steve that was not my intention, just trying to build some confidence! Your experience is definitely newer than mine and on the actual gear, I can only speak in generalities but by the time this TV was made this vertical circuit arrangement was pretty much universal. And this was quite a common fault.

Without a circuit you'd have to trace it back from the vertical yoke leads. Assuming that set has two plugs from the deflection yoke, the vertical one would be the smaller one.

Most likely, this plug will have two wires. One of those will connect, on the PCB, to either:

1. A resistor connected to ground, OR,
2. A large electrolytic capacitor, the other leg of which connects to a resistor to ground.

Unplug the vertical yoke plug and check with your ohm meter on the PCB to find the resistor, which should read somewhere between 1 and 5 ohms depending on the design. It's a matter of following the PCB traces back from the yoke connections and measuring.

Don't switch the TV on with the yoke unplugged!!


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 3:41 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 332
Location: New Hampshire
C2131
C2152
C2123
C2149
C2121
C2148 yoke coupling cap
C2171
R2172 V size
Looking from rear V size is on RIGHT & its lighter color than the 95V adj
RX2146 & RX2147 yoke resistors
Hope I saw them all ! I bet on a cap. Dirty V size would cause sudden changes &
be sensitive to the hammer test. If an RCA or Sylvania I would suspect the coupling
cap but never had one on a Zenith.
The hardest part is the board was NOT marked on the bottom, a PIA ! Sadly Zenith
went from an over built set to just another set in the 80's. So money savings
was important.
BTW side story. Run the set with the auto color OFF. It makes the pix orangey when on.
When these first came out the ones without the switch all looked like crap.
A trip through the manuals showed the auto color was always on. I opened all I sold
& modified them. When I told the Zenith service rep he pulled a Sgt Schultz.
" I see nothing, I hear nothing, I say nothing". And DONT let Chicago know what
you are doing !!!

73 Zeno 8)
LFOD !

73 Zeno 8)
LFOD !


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 3:46 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
yes i figured that out when i first got it the auto color makes it look like crap.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 11:39 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
Is the idea of the "Auto color" to bias the color towards flesh tones so the Hue setting isn't so critical?
Don't know, no Hue control in PAL.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sat 20, 2018 2:51 am 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
it actually is not the same issue anymore. now it when i turn it on it is about 16:9 in a 4:3 about that vertical size then when it warms up slowly gets larger until it is working fine. and then seems to be fine after about 10 minutes.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sat 20, 2018 4:21 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
Just to be certain, is there any sign of non-linearity in the picture (vertical squashing or stretching in part of it)? Are the scanning lines evenly spaced all the way down the picture? If so, the symptoms still point to that resistor, or two resistors in parallel as it seems to be.

In any case, you won't damage anything by using it, and the fault may fix itself (good) or get worse (even better, it'll be easier to find.)
A collapse down to about a 2" high raster should make it very easy to find.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sat 20, 2018 5:39 am 
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Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4482
Location: Ann Arbor, MI
To me, a slow steady change sounds like an electrolytic cap warming up. For a cap with an elevated ESR (Equivalent Series Resistance) the ESR will decrease as the cap gets warm. A slow steady change in resistance of a resistor sounds less likely,

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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sat 20, 2018 8:44 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
Yes Tom that's exactly what I would have thought too. It does sound like a high ESR gradually reducing with heat.

Only problem is, which cap would do that? Yoke coupler and bootstrap caps are inside the feedback loop and so would be compensated for until the limit is reached, at which point the amp would hit the rails and you'd see gross linearity faults.

I'd find it hard to believe a resistor could do that as well, except I've seen it happen in this type of circuit. Caught me out the first time. I still can't envisage the mechanism of the resistor fault....


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sat 20, 2018 7:05 pm 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
Just replace all the caps in the area and the resistor. They don't cost much. They do make a meter that can measure the ESR with the cap still in the circuit, but the meter costs more than a bunch of caps.

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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sat 20, 2018 11:47 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
no the picture has always looked just fine with no issues with lines or squishing or streaching. i contacted the original owner and they said that it was their moms tv set and when she passed away in 1994 it was put out of service and has been sitting ever since 1994. it has been acting fine all day today and no issues so maybe it fixed itself or not i dont want to speak too soon.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sun 21, 2018 12:45 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
Faults introduced when the shotgun approach is used can be harder to find than the original fault.

"Think with your head, not your hands"


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sun 21, 2018 4:50 am 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
You could replace them one at a time, testing in between each replacement. And you could stop when the problem is cured. But buying them all at once would save time.

It would be nice if you could know exactly which part is bad, but I don't think that we can say for sure which part that is.

Would that resistor be an ordinary resistor, possibly a wire-wound resistor?

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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sun 21, 2018 5:30 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
I did explain a method of finding the current sense resistor by tracing it back from the yoke lead, but lacking a circuit I can't do much more. I would normally use a scope to confirm a fault and only break out the soldering iron when I KNEW I had found the culprit.

Here's a circuit (NOT a Zenith, actually it's the vertical from a late 1960's B&W TV) that illustrates the principle. R527 is the one I suspect.

Attachment:
T26_vertical.jpg
T26_vertical.jpg [ 60.75 KiB | Viewed 665 times ]


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Sun 21, 2018 3:41 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 332
Location: New Hampshire
Resistors are listed as 3 ohm 1/2 watt carbon film.
I strongly agree shot gunning is risky. I always tell especially newbies one
or two at a time then test. All you learn from changing a bunch of parts
is how to solder. Of course its best to order them all at once at a
higher voltage to build a stock.

I do have the manual but no way to send. Besides I sell them for a little
beer money & to get a little back from my cost. I will always "pull one" to
help at this level though...

73 Zeno 8)
LFOD !


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Mar Tue 27, 2018 2:55 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
Not to revive a dead topic but the tv has seemed to recover because i was watching it and it made a loud pop like something shorted out and then went down to a horizontal line so i turned it off and opened it up and every thing seems fine so i turned it back on and it is working perfectly now and this has been about a week ago and nothing has happened since and it has full picture to the edges of the screen.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Mar Tue 27, 2018 11:05 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
AARRGH! The fault that fixes itself! The worst kind because you KNOW it will come back!


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Mar Sat 31, 2018 1:51 am 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 46
yep, i know if it had the issue it will come back and then be intermittent so every time i do something i think fixes it then it starts back, if it comes back i will live with it because i got a new tv to replace it anyway, (by new i mean an old curtis mathes console.)


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