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Old 30th Jan 2007, 11:54 am   #1
Ghostuser
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Default Cracked Pye V4 LOPT core

My Pye V4 LOPT has a visible crack across the ferrite core. The picture shrinks by about an inch in width as the set warms up. Does the air gap from the crack matter and could the faults be linked?
To the moderator: Sorry that this duplicates part of my last posting on Pye V4 Restoration.
Nigel
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Old 31st Jan 2007, 7:46 pm   #2
Heatercathodeshort
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Default Re: Cracked LOPT Core

Hello Nigel,
If the transformer has not been dismantled I certainly would not worry about it. The crack will be a perfect fit and should not cause a problem.
The problem with all these very old flyback transformers is generally breakdown of the interleaving tissues that are wound on between the layers of windings. They were never designed to last for 50 years! The less you 'touch' it the better. I have discovered that large numbers of vintage flyback transformers suffer from 'core saturation' resulting in the width and eht reducing say after 30 minutes use. I intend to investigate this during the coming months. A possibility is deterioration of the core gapping paper tabs placed between the cores of early Ferroxcube designs. I don't think the crack will be the cause of your problem.
Regards John.
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Old 31st Jan 2007, 8:42 pm   #3
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Default Re: Cracked LOPT Core

Dear John,
Thanks for your insight. The crack is wider at one side than the other, possibly pointing to mechanical stress. It is certainly wider than a hairline. I will be very interested to know what you make of the core saturation issue as I have two sets suffering from reducing width and EHT after thorough warm up. It is interesting that the cores were originally gapped.
I am naturally reluctant to have the transformers rewound until I am sure that insulation failure is the root of the problem.
Regards
Nigel
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Old 31st Jan 2007, 9:02 pm   #4
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Default Re: Cracked LOPT Core

Hello again Nigel,
So you have an actual gap..Thats not good and may well be the cause of your trouble. The gapping papers are very thin and usually consist of small circles of paper about the thickness of a cigarette paper.
I used to make line output transformers as a young lad of 13. I still have my notes that are now over 40 years old but the Pye V4 details are not to hand. The problem is that if you found a scrap transformer, the original would not stand the core swap. Regards John.
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Old 1st Feb 2007, 2:46 pm   #5
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Default Re: Cracked LOPT Core

Dear John,
I have three LOPTs, from either V4 or VT4. I obtained a complete VT4 and a scrap VT4 from 'BRC 3000' and now have a my original V4 and a VT4 working, apart from the picture width issue. Part of this was caused by fried linearity coils and I am having one rewound. Substitution of one of my three lin coils for another had proved that there was a linearity coil problem - adjustment of one made no visible difference to the picture, but the swap gave me more width and some control of linearity. As to LOPTs, two of the three show signs of melting of the pitch at some time. On one, the pitch has run down the side of the transformer from the primary winding. I will not disturb anything until I have covered all other possible causes of problems as rewinds are not cheap. I guess there is a possibility that primary overheating could have been caused by excess LO anode current due to line oscillator failure at some time. Several line oscillator components had been changed on that chassis in the past, but the LOPT seemed no worse than the one from the scrap chassis when I tried a swap.

I expected TV restoration to be a pleasant challenge and I am being rewarded!

Cheers and thanks again for your interest.
Nigel
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Old 1st Feb 2007, 5:18 pm   #6
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Default Re: Cracked LOPT Core

Quote:
Originally Posted by Heatercathodeshort View Post
I have discovered that large numbers of vintage flyback transformers suffer from 'core saturation' resulting in the width and eht reducing say after 30 minutes use. I intend to investigate this during the coming months. Regards John.
Probably worth moving this, John, when you do this.
This may have some bearing on it, and I am sure you have seen this many times!

The DC feed chokes, sometimes called desaturation chokes in the Philips G6 LOPT, failed regularly, giving lack of width, and Philips replaced them with a larger diameter ferrite core.
We investigated those that failed, and though there were no shorted turns at all, the ferrite core had a much lower resistance than a new one (cannot the remember values we found).
Could this be the same reason as your old LOPTx symptoms?

For an explanation for others, the choke was connected across the PY500 cathode and PL509 anode and carried the entire PL anode current giving the LOPT some relief.
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Old 2nd Feb 2007, 10:49 am   #7
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Default Re: Cracked LOPT Core

Hi Mike,
The Pye V4 series have the Line Linearity coil between the Booster diode cathode and the Line Output anode, so it may also have a similar role to the desaturation choke you describe. When I have had mine rewound, I will report back on the results. I will also try swapping the adjustable ferrite cores to see what happens.
Regards
Nigel
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Old 2nd Feb 2007, 7:48 pm   #8
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Default Re: Cracked LOPT Core

Hello Nigel,
Thats exactly what I had in mind. Possibly isolating the line output valve anode from the transformer and coupling it via a capacitor. A wound choke similar to the Philips G6 could then be connected between the boost diode and line output anode to maintain The DC connection. I have a Ferguson 998T that will be easy to play with. Regards John.
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Old 2nd Feb 2007, 9:04 pm   #9
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Default Re: Cracked LOPT Core

Here's the circuit of the Pye V4. It does not isolate the LOPT from anode curent, but does have the series choke in the anode feed.
Nigel
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