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Old 26th Oct 2017, 5:37 pm   #41
Niechcial,Steve
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Default Re: KB VC1 width problem

I've decided it's too risky to try and take the core apart without dissolving all the wax etc on the main windings. The ferrite core is very brittle and may fracture if I pull on too hard against the waxed- in formers. I could cut off the overwind and try the transformer again. But if I do that that might make it difficult for Mike Barker to do a rewind.
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Old 26th Oct 2017, 6:10 pm   #42
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Default Re: KB VC1 width problem

Good luck with it! Well worth the effort to restore a VC1. John.
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Old 26th Oct 2017, 11:19 pm   #43
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Default Re: KB VC1 width problem

Quote:
Originally Posted by Niechcial,Steve View Post
I've decided it's too risky to try and take the core apart without dissolving all the wax etc on the main windings. The ferrite core is very brittle and may fracture if I pull on too hard against the waxed- in formers. I could cut off the overwind and try the transformer again. But if I do that that might make it difficult for Mike Barker to do a rewind.
It is sensible to be cautious. And sometimes that old ferrite is very brittle. Can you post photos of the transformer, I might be able to make a suggestion about a way to tackle it if I could see it.

Another idea in the meantime:

Do you have, or can you get any other similar lopty that is known to be ok (with or without an overwind) so that you could subs it into your set, so as to measure the stability of the B+ boost voltage ? Just as an additional check that the problem resides in the transformer you are working on. (I agree that very very likely it does) its just another level of certainty, then if you had to make a risky move with your existing transformer you would feel better about it.
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Old 27th Oct 2017, 8:20 am   #44
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Default Re: KB VC1 width problem

Sadly I don't have a suitable transformer. Does anyone have a VC2 transformer or later they could lend me? I think that could be rigged up without using the VC2 feedback/stabilisation to confirm that the problem is the transformer.
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Old 27th Oct 2017, 9:19 am   #45
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Default Re: KB VC1 width problem

Have a look at this web site, there are 18 pages of them, perhaps for much later TVís though. Unfortunately not cheap.

http://markhindes.easywebstore.co.uk...ut+transformer
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Old 27th Oct 2017, 9:44 am   #46
Niechcial,Steve
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Default Re: KB VC1 width problem

Thanks- I've already tried that site but nothing comparable. However, progress made after conversation with Mike Barker. Mike has rewound two VC1 LOPTS and believes LOPT is definitely the problem. However he also says problem may be the main windings rather than the overwind. On one he did the insulation between layers had disintegrated and reacted with the wire to strip it of enamel in places.
So it's eventually off to Mike for a rewind. Mike has details of the overwind, so I can safely cut it off and run the set to see if the problem persists. I'll let you know the outcome.
Many thanks to all for all the thinking so far!
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Old 27th Oct 2017, 10:29 pm   #47
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Default Re: KB VC1 width problem

Good evening,
I have been following this thread with interest! John's experience with the Ferguson 998 transformer and the conclusion that the pitch/ wax was lowering the 'Q' of the transformer was also intriguing.
Now 'Q' isn't my thing so I'm on shaky ground with this (it's part of the 'dark arts' along with such things as inductance & reactance)!!
Would there be any way of measuring the 'Q' of a LOPT and assessing whether it was below par? I've done 'ring' tests on LOPT's which was useful but I guess that would only show up shorted turns rather than more subtle things like overdamping.
I assume a transformer needs to resonate at the line frequency in order to transfer maximum energy into the overwind. It would be great to have a test rig to see how good a LOPT actually was.
Whether any meaningful results could be obtained, I don't know.

Steve, would it not be worth trying to remove the pitch/ wax using the dissolving in white spirit method outlined by Argus? It would be an interesting experiment before you hack the overwind off and there is nothing to loose!
Just a thought.
By the way, is this a single or dual standard set?
All the best
Nick
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Old 28th Oct 2017, 12:54 pm   #48
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Default Re: KB VC1 width problem

1100 man, I'm not sure about "white spirits" in the UK , sometimes that is another petroleum product, the solvent to dissolve the pitch must be "mineral turpentine " (and also not vegetable turps either)
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Old 28th Oct 2017, 1:13 pm   #49
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Default Re: KB VC1 width problem

Hi Argus,
For many years in the UK, domestic paint thinners was just called 'Turps'. As far as I know, this was 'wood turpentine' obtained from trees. Gradually this was replaced with 'turpentine substitute' which was white spirit.
Wikipedia says that 'mineral turpentine' is also known as 'white spirit' in the UK.
Whether we are talking about the same stuff is a different matter entirely!! White spirit we can get easily but anything else would be tricky. We don't have particularly free access to chemicals in the UK!
Cheers
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Old 1st Nov 2017, 4:45 pm   #50
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Default Re: KB VC1 width problem

It was tempting to try the pitch removal route, but to be honest, I am limited in the time I want to spend on this machine. So I went down the hack-off the overwind road. This did not cure the slowly dropping boost problem. So pray the fault is the main windings else I have made an expensive mistake. We won't know until early New Year because Mike Barker won't be able to rewind it till then. I'll keep you informed and thanks for all the help.
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Old 1st Nov 2017, 4:46 pm   #51
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Default Re: KB VC1 width problem

PS. It's a dual standard Nick- same problem on both 625 and 405
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Old 3rd Nov 2017, 11:17 am   #52
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Default Re: KB VC1 width problem

Quote:
Originally Posted by Niechcial,Steve View Post
It was tempting to try the pitch removal route, but to be honest, I am limited in the time I want to spend on this machine. So I went down the hack-off the overwind road. This did not cure the slowly dropping boost problem. So pray the fault is the main windings else I made an expensive mistake. We won't know until early New Year because Mike Barker won't be able to rewind it till then. I'll keep you informed and thanks for all the help.
Unfortunate indeed. You've probably already replaced the coupling capacitor between the line oscillator and PL36 and checked the line drive minus voltage at the control grid of the PL36 before and after the fault develops.

The screen grid feed resistor to the PL36 could also change value when hot.

If all is well I would suspect the primary. Don't be too despondent in hacking off what appeared to be a good overwind. From experience if one set of windings fail the others fail not long afterwards.

Presumably you're having the complete LOPT rewound?

Cheers
Brian
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Old 3rd Nov 2017, 12:45 pm   #53
Niechcial,Steve
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Default Re: KB VC1 width problem

Yes, checked all that and more. I'm not worried about losing the overwind as
the intention was always to get the whole thing rewound.
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Old 3rd Nov 2017, 1:18 pm   #54
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Default Re: KB VC1 width problem

Hi Does the primary winding get hot when the fault occurs?
I have a Bush transformer with the same problem. On that the overwind heats up when the fault occurs.
I had a KB VC53 transformer doing this a few years back that was definitely the primary as it got hot after a while, I had a second transformer with a suspect overwind so I swapped the overwinds over (these were the later plastic potted type ) That cured my problem confirming my suspicion that it was the primary on the first transformer that was at fault.

Rich.
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