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Forum 135

CTV Pye CT71

Colour Television

Last Post by The_Teleman 2 years ago

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The_Teleman

(@the_teleman)

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This set a PYE CT71 dual standard colour set had worked perfectly well then the overwind went , after a bit of a chew trying to fit another one the set worked but with this unusual fault of what I can only call wobbly verticals, it varies in intensity with brightness the brighter the screen the less it does it , also I’ve found if I turn the horizontal shift to the extreme left it stops altogether

im convinced it’s something I have done but I’m darned if I can find out what

as they say man made faults are always the worst to find

if anyone has any idea it would be greatly appreciated.

An example of the fault can be viewed on the VRAT Youtube Channel (see below). Apologies for the quality of the video, it was sent to Chris via a text message, so I think the quality was lost. It should still give you a reasonable idea though.

Posted : 09/05/2022 9:45 am

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PYE CT71 CTV Dual standard

PYE625

(@pye625)

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That is an interesting fault. It looks as if the width is being modulated somehow by ripple. But looks more like a 100hz frequency, rather than 50.

I wonder if a capacitor failed when the overwind gave up, possibly due to momentary overload if it developed shorted turns?

It may not be a man-made fault.

To understand the black art of electronics is to understand witchcraft. Andrew.

Posted : 09/05/2022 7:39 pm

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Nuvistor

(@nuvistor)

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It’s as though the flywheel sync is hunting, I would scope the supplies first then check the feedback circuit from LOPTX for the sync timing.

I only ever changed one LOPTX in this chassis, over wind fail, 50 years ago. Something in the back of my mind about 3rd harmonic tuning required setting on early dual standard sets, it’s another place to check perhaps.

The PD500 will perhaps need setting up as well.

All guess work I am afraid.

Frank

Posted : 09/05/2022 8:27 pm

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PYE625

(@pye625)

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I think Frank is more on the ball with this one than I am, especially with the hunting and feedback.

How does it behave on 405?

To understand the black art of electronics is to understand witchcraft. Andrew.