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Old 31st Jul 2006, 10:45 am   #1
Giulio Maiocco
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Default LOPT question - end of overwind O/C

Hi,

I'm trying to save a LOPT for a friend's TV set, a Philips very similar to the one I posted here a while ago. The problem is the overwind is O/C on the last few turns, as someone poked around with a screwdriver and ruined the winding. The question is: if I remove the few turns needed to have the continuity again (430 Ohms worth of winding, very very close to the results I obtain on my TV which employs the same LOPT) could the transformer work again? I removed about 10 turns, so I don't think that would be a problem...

Thanks

Giulio Maiocco
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Old 31st Jul 2006, 11:08 am   #2
ppppenguin
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Default Re: LOPT question

Assuming there is no other damage, removing a few turns from the thousands in an overwind should make no difference.
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Old 31st Jul 2006, 11:12 am   #3
beery
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Default Re: LOPT question

Hi Giulio,
It won't make any noticable difference.
E.g. on the TV22 10 turns are used to provide 6.3V for the EHT rectifier.
Say for instance that you had a 110 degree vinatge TV with a transformer producing 1volt per turn with a total of 10KV. 10 turns would equal 10 volts and this would represent 0.1% of 10KV which is nothing when compared with the EHT regulation in these sets.

Cheers
Andy
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Old 31st Jul 2006, 12:20 pm   #4
Giulio Maiocco
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Default Re: LOPT question

Thanks,

this is a very welcome news! I think Philips used about 900-1000 turns in their overwind, so what I unwound is about 1% of the total winding, so no problem should arise... Even if the EHT is a bit low, I don't think this is a problem, as the CRT mounted in my friend's TV measures over the full scale of my CRT Tester (over 1000uA). The CRT, btw, is a MW43-64.

Just another question, if I may: How do you recoat the overwind, I tried wax, hot glue and non acid silicone, but I cannot do a clean work... What is the secret to obtain a factory-like finish?

Thanks a lot

Giulio Maiocco
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Old 31st Jul 2006, 6:27 pm   #5
tvden
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Default Re: LOPT question

the factory ones were dip coated in pitch but I dont think this is obtainable now I suppose you could try potting compound.
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Old 31st Jul 2006, 7:24 pm   #6
Giulio Maiocco
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Default Re: LOPT question

Hi,

Thanks for your answer, I would like to point that the coat on the overwind here in Italy were usually done with some kind of hard wax, the biggest problem remaining is how to apply this coat so to have an "out-of-the-factory" look

Thanks

Giulio Maiocco
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Old 31st Jul 2006, 7:46 pm   #7
Heatercathodeshort
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Default Re: LOPT question

Hello,
The only way I know of restoring a 'tyre' on a EHT overwind will result in a possible risk and I would not suggest this unless you can afford to lose the transformer!
An old tin is required about 3" deep. Two slots are cut in the sides opposite each other around 1 1/2" deep.
A length of dowel, slightly thinner than the winding former is placed through the winding assembly having removed it from the transformer [this is the risk]. A quantity of candle wax is melted in the tin with GREAT CARE, the dowel complete with overwind is now placed in the slot and turned in the molten wax. When set the process can be repeated until a suitable build up of wax has been obtained. The winding can then reinstated in the transformer cores and after rewiring to the tags, refit in the receiver.
The risk will always be present that wires may snap off or you damage the overwind by rough handling but if you have some knowledge of line output transformer construction, gap papers and insulation cautions you should be ok!

Regards John.
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Old 31st Jul 2006, 8:09 pm   #8
af024
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Default Re: LOPT question

Have a look at this link Giulio:-

https://www.vintage-radio.net/forum/...ead.php?t=1874

In the end, I used Bitumen paint made by a company called Palace. It was brush on and dried after several hours. I gave it several coats. Before I applied it to my exposed winding I melted some candle wax over the winding (just in case there was a nasty reaction - and to build the exposed area up back again to the corect surrounding depth). So far so good - there is no insulation breakdown, no nasty chemical reactons and it's not melted off. It does pong a bit when it's been on a bit though. If you like tar-type smell well that's a bonus perhaps? I have no idea whether I did the right thing, but judging by the results (and looks) I am very pleased.

As regards what the proper stuff was, well I can only but assume that it complied with BS 1858 (I've been looking at some old British Standards). This is called, 'Bitumen-based compounds for electrical purposes'. Not entirely sure of course, but it's the closest thing I've seen thus far.

Best of luck.

Regards,


Andy
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Now where on earth did I remove that from?
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Old 31st Jul 2006, 8:33 pm   #9
Giulio Maiocco
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Default Re: LOPT question

Quote:
Originally Posted by Heatercathodeshort
Hello,
The only way I know of restoring a 'tyre' on a EHT overwind will result in a possible risk and I would not suggest this unless you can afford to loose the transformer!
I old tin is required about 3" deep. Two slots are cut in the sides opposite each other around 1 1/2" deep. A length of dowl slightly thinner than the winding former is placed through the winding assembly having removed it from the transformer [this is the risk]. A quantity of candle wax is melted in the tin with GREAT CARE, the dowl complete with overwind is now placed in the slot and turned in the molten wax. When set the process can be repeated until a suitable build up of wax has been obtained. The winding can then reinstated in the transformer cores and after rewiring to the tags, refit in the receiver.
The risk will always be present that wires may snap off or you damage the overwind by rough handling but if you have some knowledge of line output transformer construction, gap papers and insulation cautions you should be ok! Regards John.
Hello John,

In order to fix it, I had to disassemble it, so I could follow your method to apply a coat!! I must confess it was not an easy repair, the wire is very thin and easy to break, without considering how the EY51 is anchored to the "tire", a disaster waiting to happen!! I should save your precious idea!

Thanks a lot! (don't care my poor English!)

Giulio Maiocco
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