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[b]SONY TV MDL# KV-27HFR Trinitron - No Picture after 45 Sec
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[b]WebFly[/b]
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PostPosted: Tue Mar 08, 2005 9:04 pm    Post subject: [b]SONY TV MDL# KV-27HFR Trinitron - No Picture after 45 Sec Reply with quote

Dear Admin,

A bundle of thanks for such a wonderful Forum, I appreciate you guys help & courage, god bless you.

Basically I am a computer guy. I am a “Newbie” in TV repairing with a little knowledge of resistance, capacitors, transistors, ICs etc and a common sense of PCBs.

Here’s the problem:
///////////////////////////////////////////////////////////////////////////////////////////////////////////////
1) SONY TV Model # KV-27HFR Trinitron with Surround Sound capability, Manufactured: Feb 1988.////////////////////////////////////////////////////////////////////////////////////////////////////////////////
When turn ON, works fine, picture is fine, fantastic Surround Sound fine, but exactly after 45 seconds, picture starts disappearing slowly (as if someone darkens Brightness to zero), and after a minute, the picture disappears completely. Sound continues to work. Awaited for 5 minutes, no picture at all, but sound perfect. Turned OFF, waited for 5 minutes, turned ON, behaves the same mentioned above.

Suspecting Dry Soldering or Loose Contacts behind the picture tube in the laser gun, where 7-8 contacts goes through a small rectangular circuit plate, usually a copper magnetic coil appears behind it. While TV was ON with sound but no picture, without removing cabinet or chassis, I slowly hit it with my hand over it. Miracle happened, the picture appeared and stayed for about more than 6 minutes. Changing channels, no problem at all. But when turn OFF and after 15 minutes turn ON, the problem persists.


Any advice, please use very simple terms, as I am new and don’t know much terminology. A bundle of thanks in advance and your comments will be highly appreciated.

Best Regards,
WebFly
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PostPosted: Tue Mar 08, 2005 9:53 pm    Post subject: Reply with quote

sony tvs suffer from dry joints and they usually hide inside the metal canned units .but from what your saying the part you looked at sounds like the tube base check this for bad solder joints move large components to see if the solder connections are good especcially the the large transistors
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rockhound



Joined: 23 Feb 2005
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PostPosted: Wed Mar 09, 2005 2:26 am    Post subject: Reply with quote

For one thing, there is no lasers in television sets. You have a cold solder connection for the heater for the CRT (Picture tube). Resolder the connections marked H1 and H2 on the CRT socket, and also trace them down to the main board and resolder those connections as well (near the flyback transformer)
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webfly



Joined: 09 Mar 2005
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PostPosted: Wed Mar 09, 2005 7:39 pm    Post subject: Reply with quote

rockhound wrote:
For one thing, there is no lasers in television sets. You have a cold solder connection for the heater for the CRT (Picture tube). Resolder the connections marked H1 and H2 on the CRT socket, and also trace them down to the main board and resolder those connections as well (near the flyback transformer)

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webfly



Joined: 09 Mar 2005
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PostPosted: Wed Mar 09, 2005 7:50 pm    Post subject: Reply with quote

Thanks for correcting me. But what do you call that peice of glass that burns Red when you turn ON TV? and What is Flyback transformer? Is it Extra High Tension or EHT?

Thanks & regards
webfly
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PostPosted: Wed Mar 09, 2005 9:28 pm    Post subject: Reply with quote

A CRT (picture tube) have 3 electron guns,each with their own filament, (heater) so you should see all three lite up. Watch the filaments dim down when the problem occures. The flyback transformer is generally black which has a long thick red wire that goes to the anode cap of the picture tube to supply the high voltage to excellerate the ions. Most heater voltages are supplied by the flyback transformer. Any cold solder connection between the flyback and the crt would cause the problem that you mention.
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webfly



Joined: 09 Mar 2005
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PostPosted: Wed Mar 09, 2005 11:34 pm    Post subject: Reply with quote

Thanks teams for guidence. Just out of curiosity, somebody told me that black coloured trasformer (sylinderical shape) - a very thick wire stretching out from it and connecting to picture tube body with a rubber covering. So called, "FlyBack" transformer, has a huge amout of elctricity, enough for harmful shock to humans, if touched with bare hands. Is my conception correct? If yes, pls help, what precautions to take while testing or soldering such a nasty daemon....

Regards,
Webfly
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madmatt2024
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PostPosted: Thu Mar 10, 2005 3:09 am    Post subject: Reply with quote

I am not an expert but to my knowledge you probably won't get a shock from the flyback. The wire and the flyback are vary well insulated and the high power is only going through it when the set is on. I would use caution when the set is running because I think about 25,000V runs through it but still the flyback and wire are vary well insulated and I know I have brushed against the wire at least once and have never got shocked. What you should worry about are the capacitors (the round cylinderical shaped things with numbers and symbles on them.) If you come in contact with them on the underside of the board thay can give you quite a jolt. A good rule is to only handle the board by the edges and try not to touch any exposed component leads on the top or botom of the board. Also, what I always do is to unplug the tv when it is running, this should drain alot of stuff out so if you get a shock it won't be a bad one. Soldering shouldn't be a problem either because the soidering iron's handle is usualy made out of plastic. And, if you have to solder on any of the pins on the CRT socket unplug it from the CRT first but if all you have to solder is a wire going to the board that the plug is mounted on then you probably don't have to disconect it.
I hope this information helps.
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webfly



Joined: 09 Mar 2005
Posts: 9
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PostPosted: Fri Mar 11, 2005 1:10 am    Post subject: Reply with quote

Thanks madmatt2024 Idea

I think you've given good suggestions & precautions, that's very helpful & I'll do that. Now i have two questions:

1. Where you unplug the CRT from that thick wire which comes from flyback trans.. at the heater area or at the picture tube side ?

2. And how ? Because i fear, that to dig out some soldered contacts if pull.


I made a diagram to ask proper questions but, i cannot post it here due to text only restriction, and don't have my website url.

Bundle of thanks for help.
webfly[/url]
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madmatt2024
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PostPosted: Fri Mar 11, 2005 5:16 am    Post subject: Reply with quote

I think you misunderstood me about the plug. The plug I was takling about is at the rear of the CRT were it comes to an end. There should be a small board with a plug on it at the rear of the CRT. Heres a pic.
http://coinop.org/repair/monitor_tubeswap/03_TV_open.jpg
The green board at the back of the crt is what I'm talking about.
The plug is soldered to the green board and plugs into the pins at the back of the CRT. Here is the rear CRT after its inpluged.
http://coinop.org/repair/monitor_tubeswap/04_purity_rings.jpg
You should only have to unplug it if you have to solder any of the pins on the CRT plug to the board. To unplug it just wigle it back and forth while gently pulling. As for the wire going from the Flyback to the side of the CRT you probably won't have to remove it. it is generly held on to the CRT by a clip under the rubber cap and can generly be removed by squeezing the cap. Rember, I'm not a service technician just a guy who knows some stuff about electronics. Just in case your interested here's a rather long article on tv repair. http://www.anatekcorp.com/tvrepair.txt
Also, if you need to put a url in the forum just type it or copy and past it into your post. I hope this helps.
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