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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 6th Apr 2009, 5:37 pm   #1
Boom
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Default Testing LOPTs for shorted turns

I did look through the forum and can't see any mention of this method for testing LOPTs for short circuit turns so I hope I'm not teaching anyone to suck eggs. It is a little trick I learnt when I first left school in 1966 and have never known it to fail. Maybe it might be helpful to someone?

Connect the 'scopes square wave calibration output to one end of the LOPT and the input of the 'scope to the overwind output. No other connections are used.

What should happen with a good transformer is that it will 'ring' and the resultant sine wave will gradually die out as in the picture marked 'good'. This picture was taken with a good transformer from a TV22.

In the picture marked 'bad' to simulate a transformer with a shorted turn I wrapped a piece of wire around the transformer. The resultant waveform is shown. The sine wave dies out much quicker and is heavily damped.

The 'scope is a Tektronix 475A but as it is only running at 1kHz any 'scope should do fine.

Dave
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Old 6th Apr 2009, 6:44 pm   #2
Colin
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Default Re: Testing LOPTs for shorted turns

Thanks for posting Dave ... really useful tip.
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Old 6th Apr 2009, 9:34 pm   #3
ParcGwyn
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Default Re: Testing LOPTs for shorted turns

A good tip. Television magazine published a design or LOPT tester in the late 70s or early 80s based on this. I built one and if my memory is correct it was simply a transistor multivibrator feeding an emitter follower to supply the square wave.

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Old 6th Apr 2009, 10:12 pm   #4
Lloyd 1985
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Default Re: Testing LOPTs for shorted turns

Hi,
thanks for posting this, i have a pye LV30 which I don't know whether the lopt will be any good or not, so I shall give this a go!

regards,
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Old 7th Apr 2009, 9:01 am   #5
kalee20
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Default Re: Testing LOPTs for shorted turns

It's a good tip, however, the rate at which the damped oscillations die out is heavily dependent on the design of the LOPT. For instance, if the EHT overwind be wound with resistance wire (as was occasionally the case), I'd expect the ringing to die out pretty quickly. On the other hand, with a transformer intended to be used in a circuit with some sort of external damping, I'd expect the ringing to continue much longer.

So, as a means of comparing a suspect LOPT with a known-good transformer, it's going to be a pretty useful trick. If there's nothing to compare against, I'd be a bit dubious!
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Old 7th Apr 2009, 11:47 am   #6
Mike Phelan
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Default Re: Testing LOPTs for shorted turns

I always did this years ago!
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Old 7th Apr 2009, 11:59 am   #7
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Default Re: Testing LOPTs for shorted turns

I used to do this when I first started in the trade. Some transformers give a really good 'ring' whilst others not so much. You can get some faulty transformers where the ring collapses almost at once and others give a dampened ring as shown. It's a good guide however and it might be worth trying a few transformers to see different results.



Rich.
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Old 7th Apr 2009, 6:05 pm   #8
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Default Re: Testing LOPTs for shorted turns

For many years I have used the Bob Parker LOPT tester (see http://members.ozemail.com.au/~bobpar/fbt.htm) which does the ringing technique in a novel way by lighting a line of LEDs to show the number of ring cycles. I have found it a pretty reliable means of finding duff LOPTXs (and other transformer like those in SMPSUs). You do need to watch if there is any other sort of load on the transformer pins and sometimes need to isolate them. It won't though flag shorted diodes in the EHT stack, which is a fairly common problem with today's transformers. You will see guidance on there on how to build it yourself (the original kits are NLA) or to buy the new Anatec version of it.

At one time I did have one of the HR testers (HR is the Spanish firm who makes pattern LOPTX). This allowed you to test the EHT stack as well. But it is not cheap and I always fell back on the Bob Parker one as much easier to use, so eventually sold the HR one.

Dave
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Old 8th Apr 2009, 8:27 am   #9
Mike Phelan
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Default Re: Testing LOPTs for shorted turns

I should have mentioned that when I used the 'scope method, if there was not a very definite ring, a single turn of wire around the core would show me if there was already a shorted turn present.
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