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 Post subject: zenith tv has bad vertical plese help
PostPosted: Jan Wed 17, 2018 8:30 pm 
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i have a zenith model 1927w that i got for free a little while back and it has worked great for a few weeks when i first got it it had the issue that i will describe later but it fixed itself after running about 5 minutes and then now it is having the same issue but not as bad. the vertical is not taking the whole screen it is about an inch from the bottom and top but it intermittently get worse and better depending on how long it has been on also sometimes it kind of shakes from top to bottom a little but that usually stops. it seems like it is bad when first turned on then after 5 minutes stops then comes back after 15 minutes or so and does not stop. if you need pictures i can attach some. other than that it has a great picture very sharp and very bright and good colour. i have been told to get a new tv but i dont want it i want this one fixed. i dont want to keep using it with this issue because i am scared it might damage something worse.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 1:16 am 
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Location: Dayton Ohio
Hello and welcome to the Antique Radio Forums.

Is it by chance One of these? :|

-Steve


Attachments:
ZenithSA1927WTVs.jpg
ZenithSA1927WTVs.jpg [ 52.89 KiB | Viewed 1268 times ]

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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 2:12 am 
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Joined: Jan Wed 17, 2018 8:17 pm
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very similar i will post a picture of it and the issue in a second.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 2:30 am 
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Joined: Jan Wed 17, 2018 8:17 pm
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this is the picture of the set and you can see the problem is not as bad as it gets sometimes because i just turned it on but you can see that at the bottom it does not go all the way down


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 2:35 am 
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Joined: Jan Wed 17, 2018 8:17 pm
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Attachment:
20180117_201314.jpg
20180117_201314.jpg [ 39.02 KiB | Viewed 1246 times ]


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 1:57 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 362
Location: New Hampshire
First you will need the complete model number. it starts with an S then another letter then
1927W. This is to get a manual if you need it. There should be another sticker inside. The problem is
almost always a bad elecrtolytic in the vert. There are apx 5 to change. It also can be a cold
solder joint. If you can hit the set & make it come & go thats it. Any other problems are rare.

73 Zeno 8)
LFOD !


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 5:34 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
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when i first got it i could hit it and that would fix it for a second but i touched up some solder joints and that fixed that but it is still having the issue. so i will try to scorce some caps.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 5:35 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
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it is a sz1927w i think.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 6:12 pm 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
It could be a dirty control. Does this set have a height control (also known an vertical size) and a linearity control? Try slightly wiggling those and see if the height jumps around.

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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 10:11 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 362
Location: New Hampshire
ecaden1 wrote:
it is a sz1927w i think.

Yup thats a good model & right time period.
I will see if I got the manual tomorrow & give you a cap list.
If not it uses a 9-181 main board & IIRC all varieties have the same vert.
There is a size control. DO NOT turn the 95 V adjust control, its usually got
a yellow knob. Be sure you got the right one.

73 Zeno 8)
LFOD !


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 10:19 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 338
This fault sounds very familiar in the way it comes and goes. Let's see if I can help.

If it's a solid state design, there will be a resistor of about 1 to 1.5 ohm in series with the ground return and coupling capacitor of the yoke. This is the current feedback sense resistor. It handles a fairly large peak current and can go intermittently high resistance - an oxidised connection inside the resistor. Many designs used 4 or even 6 resistors in parallel or series-parallel in this position to spread the load.

It could also be that large coupling cap (usually about 1000uF) but not in my experience.

1974 production Aussie Pye T29 color chassis used a single IRC 1.5 ohm 2 watt resistor in this position and it would fail this way. A very common fault.

Even today, with surface mount components, you can buy resistors that are specially designed for current sense operation.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 10:55 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 65
it does not have a vertical height adjustment. when i said the picture was shaking... it is not anymore it just seems that when i turn it on the height is too short like i am displaying a 16 by 9 immage even though it should be full screen then slowly get better as it runs but it never gets perfect but it does nearly. it sounds like a capacitor issue but i dont know which one. i dont have a capacitor tester so i guess i could replace all the ones around the heatsinks or something but there are a bunch of caps in the set and it is not old enough to need a full recap like the real old ones. but it sounds like a bad cap in the vertical area


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 11:00 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 65
after it has been on fore about an hour this is how it looks. the picture never looses brightnes or sharpness it looks great as you can see this is about as good as the vertical gets. you can see it is still a bit off at the bottom but it looks like it goes all the way at the top. it is about an inch from the bottom and that is as good as it gets.
Attachment:
20180117_201351.jpg
20180117_201351.jpg [ 52.94 KiB | Viewed 1107 times ]


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 11:04 pm 
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Joined: Jan Mon 18, 2010 2:13 am
Posts: 16815
Location: Dayton Ohio
As i recall, the 95V adjust is a black trimmer.

The Vertical size is the Yellow.

I remember a 100uf 35V cap that was often a problem on the 9-181. It was near the control.

The Z-1 chassis is a reliable one, but has its quirks. There are a few caps on the 9-186 High Voltage "Flyback" board also involved in the vertical deflection and can cause issues.

Unfortunately all service data I had available to me is gone, except for some microfiche, which I don't have a reader… I wonder how well a 1200 dpi scanner would read them? :lol:

-Steve

_________________
Radio Interests
-Zenith
-Sparton
-Pre-War FM
Consoles and floor models, the bigger, the better!


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 11:30 pm 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 65
one thing i probably should have mentioned. the tv belonged to someone and i bought it off craigsllist it was an ad for a cash register and i just saw the tv in the background and asked about it they said it had been sitting outside in a nonheated barn for years and did not know if it worked it did of corse it is a zenith but it has this issue. and if it had been sitting in a barn with no climate control then it would be getting hot and cold and that could have affected the caps as well.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Thu 18, 2018 11:57 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 338
Very unlikely to have affected the caps in that vintage TV in my (extensive) experience. Much more likely to be that current sense resistor.
Same thing happens in some Panasonic and NEC models, same fault.

If it's a long-term intermittent, do you have access to a 2 channel DSO? Very useful for tracking that kind of fault.

If you can get a DSO and can send me a link to the circuit I can help you with a procedure that will track down the culprit.

If you can't get a DSO but can still send me the circuit, I can tell you which resistor to replace on a "most likely" basis.

Please resist any advice/temptation to replace all the caps in that vintage of TV.. You most likely won't fix it and will probably end up with more faults than you started with. If it was a late 90's build and affected by the capacitor plague issue (google it), maybe.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 12:03 am 
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Joined: Jan Wed 17, 2018 8:17 pm
Posts: 65
no i do not have a dso.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 12:52 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 338
Even so, still worth tracking down and replacing that resistor.
If it has two or more resistors in parallel, replace all of them.


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 1:28 am 
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Joined: Jan Mon 18, 2010 2:13 am
Posts: 16815
Location: Dayton Ohio
What? :o

Electrolytic capacitors were one of the biggest problems with televisions.
Very likely one is causing this vertical issue.

I worked for a Zenith service shop between 1984 and 1998. I've seen many of these come through.

-Steve

_________________
Radio Interests
-Zenith
-Sparton
-Pre-War FM
Consoles and floor models, the bigger, the better!


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 Post subject: Re: zenith tv has bad vertical plese help
PostPosted: Jan Fri 19, 2018 1:50 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 338
Steve, I'd normally agree, but in this case the symptoms disagree.

If the fault was a cap in the vertical that had gone high ESR, it would affect the linearity, causing obvious crushing as well as loss of height and/or foldover.
If the yoke coupler goes bad you will see crushing about 1/5 of the way down the raster, which gets worse as you wind the height up.
If the bootstrap cap goes bad, you will see foldover at the top of the raster.
AFAIK (and can see) none of this is happening. The symptom is intermittent height loss, without linearity changes.

It's very clearly the current sensing resistor, in this case.

My experience: 25 years servicing solid state TVs, color and mono and training technicians. Plus another 20 as a design engineer......


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