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Old 19th Jan 2023, 1:30 pm   #1
Paul Godley
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Default Philips KT3 Colour Bleed Problem - 2 chip decoder

Hello, I wonder if anyone can help with this?

I have a Philips KT3 Chassis Tv (20" version) which has a colour bleed problem. That is when the Colour is delayed in respect to the luminance. I understand this was a problem with the early 2 chip decoder and some mods exist? does anyone know what these mods are? The later single chip decoder does not have this problem.
Any suggestions welcome, I have tried 2 different decoder panels (both 2 chip versions) and the issue remains.

Many Thanks

Paul G
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Old 19th Jan 2023, 2:50 pm   #2
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

There was a mod but I can't remember what it was. Something tells me it was a resistor change......I no longer have any of my old Philips information.

By the way, you can't swap a two chip decoder for a single chip or vice versa. There are quite a few changes in the external circuitry. I know someone who did it but it was a lot of messing about and hacking the print.....The single chip decoder was fitted to KT3 MkII
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Old 21st Jan 2023, 10:47 am   #3
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Hello,

I noticed in the supplementary service Information section of Radio & Television Servicing, 1980-81, a service brief from Philips.

Luminance/chrominance panel change. C3219 on the luma/chroma board is changed in value from 4u7F to 1uF to reduce colour flash. This mod was in addition to a change of IC8367 from TDA2571Q to TDA2571AQ and associated component changes for field jump on channel change.

Not sure if the change of C3219 will cure this fault though.


Regards,
Symon
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Old 21st Jan 2023, 12:31 pm   #4
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

No...the colour flash was just that....changing channels produced a brief (less than a second) increase in saturation as could a sudden scene change.
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Old 27th Jan 2023, 6:24 pm   #5
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Hello,

Thanks for all the comments so far. Much appreciated. Hopefully there will be a philips bulletin for this problem hiding somewhere!

Kind Regards

Paul
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Old 28th Jan 2023, 10:08 am   #6
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Hello
I seem to recall the chroma bandpass on this chassis was peaked rather around 4.43 MHz. A slight adjustment to broaden the response with the coil associated with shaping the response at the chroma input to the decoder should improve luma/chroma registration. Without access to the manual I don't have circuit details. Hope this may help
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Old 28th Jan 2023, 2:42 pm   #7
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Hello
I seem to remember the chroma bandpass coil U195 was rather peaked on 4.43 MHz. A slight adjustment to broaden the bandwidth often corrected the chroma/luminance misregistration.
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Old 29th Jan 2023, 12:56 am   #8
Philips210
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Hi

I'm not sure if the fault is a colour fit problem between luminance and chrominance signals. If so, the following may be helpful which is taken from the Service Bureau, Television magazine, July 1984:

PHILIPS KT3 CHASSIS
There's misregistration of the colour with respect to the
luminance, seen most clearly with saturated colours.
Colour from a left-hand object spills about 4mm across to
the right. Otherwise the colours are correct, there are no
blinds and all is well.

The colour fit can be improved by realignment of the
chroma coupling circuit on panel U3430. Use a test
pattern - Ch. 4 is ideal. Screw the core of the 2.2MHz trap
U194 right down then back half a turn. Then tune the
bandpass circuit U195 for best transition between the
green and magenta bars without producing ringing. The
effect of this is to widen the chroma response without
spoiling the pulse response of the luminance channel.


Regards,
Symon
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Old 29th Jan 2023, 3:44 pm   #9
Philips210
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

I hadn't noticed Cornwalltone's posts before replying in post #8 but I don't think they were there when I replied. I think they may have been added after my reply by a moderator, perhaps due to approval of posts from a new member?

I thought I'd mention this as it looks as if I've not acknowledged his reply which is along the same lines as mine.

Anyway, Cornwalltone's points tie in with the TV magazine Service Bureau request.

Regards,
Symon
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Old 29th Jan 2023, 4:05 pm   #10
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

I can confirm that new user posts have to be approved by a moderator. They appear in the order they are posted, but are not visible to members until they are approved.

Cheers

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Old 29th Jan 2023, 4:07 pm   #11
Philips210
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Thanks for that Mike.

Regards,
Symon
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Old 29th Jan 2023, 7:49 pm   #12
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Hi,

I thought folk may be interested in this.

in 1983 I got a job at Leicester University in the Computing Studies Unit (later became Computing Studies Dept)

Computer monitors at the time were expensive and the likes of Microvitec etc.not having a particularly good definition.

The Hardware guy I worked with had just designed a plug in board using the TDA3560 decoder chip. This allowed the input of digital RGB - whereas the original 2 chip decoder didn't.

The definition of the KT3 was far superior to many of the computer monitors available at the time - and at half the cost!

The CSU approached Philips with the design but they weren't interested.

When the Philips CTXS came out with the 3560 decoder it was easy to make a daughter board to plug into the 3560 socket and use that model as a computer monitor too. Much easier.

Andy
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Old 31st Jan 2023, 11:21 am   #13
Paul Godley
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Quote:
Originally Posted by Philips210 View Post
Hi

I'm not sure if the fault is a colour fit problem between luminance and chrominance signals. If so, the following may be helpful which is taken from the Service Bureau, Television magazine, July 1984:

PHILIPS KT3 CHASSIS
There's misregistration of the colour with respect to the
luminance, seen most clearly with saturated colours.
Colour from a left-hand object spills about 4mm across to
the right. Otherwise the colours are correct, there are no
blinds and all is well.

The colour fit can be improved by realignment of the
chroma coupling circuit on panel U3430. Use a test
pattern - Ch. 4 is ideal. Screw the core of the 2.2MHz trap
U194 right down then back half a turn. Then tune the
bandpass circuit U195 for best transition between the
green and magenta bars without producing ringing. The
effect of this is to widen the chroma response without
spoiling the pulse response of the luminance channel.


Regards,
Symon
Hello Symon

Many thanks for this. I will try it and report back. That description is exactly the symptom.

Kind regards

Paul
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Old 31st Jan 2023, 11:22 am   #14
Paul Godley
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Default Re: Philips KT3 Colour Bleed Problem - 2 chip decoder

Many thanks to all who has assisted with this problem, I will try these adjustments and report back.

Kind Regards

Paul
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