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Hitachi Widescreen TV C28WD2TN Red Convergence Problem?

 
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GadgetUK
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PostPosted: Wed Jan 30, 2008 6:04 pm    Post subject: Hitachi Widescreen TV C28WD2TN Red Convergence Problem? Reply with quote

Hi,

I haven't got a massive understanding of certain aspects of CRT TVs although I have fixed a fair few TVs over the years due to my involvement in PC repair - PCB repairs, power supply component repairs etc.

I am hoping that someone has had this problem before on the same model or product line...

I am not sure if convergence is the right term but I suspect it could be. If I explain the problem hopefully someone will understand what I mean...

Initially I have the power on / off problem common to this model, which I believe I can fix by replacing 2 or 3 resistors and possible dry joints on one of the ICs. The main problem that I need help with is this:-

Sometimes when TV is power on the is a red tint to the picture and you can see the pink scanlines, this can remain brightish red for a few seconds and sometimes just fades out to normal - all colours look fine. When it stays tinted red it doesn't appear to affect the picture colours much but you can see red 'striations' to the right hand side of any bright white or bright coloured objects displayed, ie. red bars that start from the right hand side of anything bright on the screen and travel to the very right of the screen. It seems to come and go dependant on how warm the TV is. Sometimes it's not there at all, othertimes its there for an hour or 2.

An example would be when displaying a game on my Wii, the black loading screen is shown with an animated dolphin in the bottom left of the screen. There is a red bar that travels from the dolphin all the way to the right hand side of screen. The red bar is the full height of the image, yet the rest of the screen looks normal, including the dolphin itself which looks to be the correct colour.

Is this a convergence problem, has any had problems like this before on the C28WD2TN, maybe with a different gun, B or G? Does it sound like it's a problem driving the guns, or on one of the signal processing circuits before we get to lineoutput? I hope it's not the tube, the picture is nice and sharp and appears to be OK.
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Gravityboots



Joined: 09 Jan 2008
Posts: 38

PostPosted: Thu Jan 31, 2008 5:13 am    Post subject: Not sure your model is affected Reply with quote

Not sure your model is affected but,

Certain Hitachi projection TVs had a problem with the connections cracking on the yoke PCBs. The process to get at them can be very involved. This occurs where your 4 pin and 7 pin connectors are attached.

Again, I have no written documentation on this particular model so a grain of salt is needed with this advice.
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gary



Joined: 05 Apr 2005
Posts: 584
Location: MANCHESTER, uk

PostPosted: Thu Jan 31, 2008 9:16 am    Post subject: Reply with quote

set is fitted with a philips crt, notorious for failing, usually causing variations on the fault you have, to be sure disconnect cathode feed resistors of red and green guns, use wire to link red feed resistor to green cathode, and green feed resistor to red cathode, picture will look very odd but if pink flyback lines are still present then the crt is definetley u/s. fault is not convergence, that would result in misaligned colours.
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PostPosted: Thu Jan 31, 2008 12:38 pm    Post subject: Reply with quote

gary wrote:
set is fitted with a philips crt, notorious for failing, usually causing variations on the fault you have, to be sure disconnect cathode feed resistors of red and green guns, use wire to link red feed resistor to green cathode, and green feed resistor to red cathode, picture will look very odd but if pink flyback lines are still present then the crt is definetley u/s. fault is not convergence, that would result in misaligned colours.


Thanks for the advice guys! Very very much appreciated. Will swap red and green feeds around and post back.

Cheers!
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GadgetUK
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PostPosted: Sun Feb 03, 2008 4:18 pm    Post subject: Reply with quote

Thanks for the pointers! I didn't need to swap the red and blue cathode feeds I did a visual inspection first and noticed the red feed resistor burnt out, although it still measured 560 ohm. Replace the feed resistor, still no joy. Quickly checked the associated transistor (pair, darlington?) the secondary transistor BF422 was open circuit across B->E or C->E (can't remember which).

Swapped the B and R transistors around and I get the same display issue with Blue, so looks like it was:-

560 ohm resistor
BF422

I suspect from what I have read that there may be some future problems related to the red cathode... I don't full understand CRTs, anyone know what actually happens to / with the cathode connections that causes this kind of problem? Just interested in the theory more than anything.

I will post back once I have confirmed it's working but the red seems fine now it has a new resistor and transistor.

One final thing, anyone know where I can get a BF422 (or equiv) in the UK? Going to have a look at Maplin and CPC... And, does anyone know of online equivelance guides for replacement transistors? When I was fully in the electronics arena we used to use guide books for finding equivements, there must be something online these days?
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GadgetUK
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PostPosted: Sun Feb 03, 2008 4:46 pm    Post subject: Reply with quote

I have managed to order them from CPC, still interested to know where else I could have got one, CPC seem a bit poor compared with when I used to use them 10 years ago. ie. very limited resistors available...
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GadgetUK
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PostPosted: Tue Feb 05, 2008 8:29 pm    Post subject: Reply with quote

Replaced the BF422 and 560 ohm resistor, well I kind of had to make a resistor... Couldn't find a supplier of a 560ohm resistor so put to ceramic 1K resistors in parallel, now measuring 550ohms (probably due to tolerance, hopefully they won't change too much under load).

It seems perfect, infact it looks better than I can ever remember... Only been powered up for 30 minutes so time will tell.

Thanks to Gary and GravityBoots! Very much appreciated, your skill and input on forums like this really is very valuable!
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