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Old 25th May 2017, 12:39 pm   #1
1100 man
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Default Testing 405 line LOPT's

Good morning,
I have spent some time cleaning up the LOPT for my 'Pilot' project. This got me thinking about testing it for shorted turns before refitting it to the chassis.
Many years ago I built a tester which I think was in 'Television' magazine. I seem to remember this worked pretty well but I can't remember much about it.
I have heard about 'ringing' the transformer using a 'scope, but have never tried it.
Looking through past threads on the forum, I came across an article from the May 66 Television magazine describing this in detail.
Does anyone else have practical experience of doing this and the sort of results that are obtained from actual transformers?
At the moment, I suspect this transformer will be OK as all the wax on the overwind is intact so it hasn't been running excessively hot. Mind you, I don't think it's seen power for 40 years or so.
It would be handy to test it before powering it as I will then have something to compare it too when it does inevitably develop shorted turns
Cheers Nick
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Old 25th May 2017, 1:52 pm   #2
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Default Re: Testing 405 line LOPT's

chris "crusty" has an article on his radios-tv w/s on ringing, a fella down here has or had an old boxed SEME tester - must get hold of him !
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Old 25th May 2017, 2:34 pm   #3
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Default Re: Testing 405 line LOPT's

Hi.

I haven't had experience testing 405 line LOPTs but have had good results using the ringing method for checking 625 line LOPTs. I built the tester from 'Television' magazine which if my memory serves me correctly was in the September 1993 issue. It was a very useful piece of kit to have in the workshop and I remember one occasion where a very expensive Panasonic LOPT was suspect. The tester, in conjunction with the oscilloscope revealed a somewhat over damped sinewave oscillation. The replacement LOPT thankfully cured the fault.
Many years ago, I also built the design in May 1982 'Television' which also worked well. That used just an LED for determining whether the transformer would ring. I preferred the unit that uses the scope as you can actually see the damped oscillations and get a feel of what's happening.

I would have thought that any ringing type tester could be equally used for both 405 and 625 line LOPTs.

I can scan the 1993 article if required. I'll try to locate it in one of my many boxes of mags. PM if interested.

Regards
Symon.
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Old 25th May 2017, 3:16 pm   #4
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Default Re: Testing 405 line LOPT's

Link to Chris's page.
https://www.radios-tv.co.uk/333-2/

I used to ring 405, 625, scan coils etc to test them.
The main problem is having the knowledge of what a good inductor rings like. Differently LOPTX etc can give different results.
Frank
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Old 25th May 2017, 3:46 pm   #5
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Default Re: Testing 405 line LOPT's

Quote:
Originally Posted by Philips210 View Post
I built the tester from 'Television' magazine which if my memory serves me correctly was in the September 1993 issue.
Assuming this is the one, your memory serves you well:
Attached Files
File Type: pdf transformer tester Sept 1993.pdf (920.5 KB, 278 views)
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Old 25th May 2017, 3:48 pm   #6
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Default Re: Testing 405 line LOPT's

Thanks Pastyboy & Frank: that looks like an interesting read. I have quite a selection of LOPT's, some known to be working and at least one known to have a dead overwind so I can do some testing and get a 'feel' for the results.
Symon, 1993 is more likely to be the one I built but I can't recall much about it. I would be interested to re read the article so have sent you a PM. I rather foolishly gave away all my copies of TV mag 10 years ago!
Cheers Nick
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Old 25th May 2017, 3:51 pm   #7
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Default Re: Testing 405 line LOPT's

I have a JABCO LOPT tester I acquired which I've never used. Would that do the job, do you think - or was it meant for diode-split transformers?
Glyn
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Old 25th May 2017, 3:52 pm   #8
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Default Re: Testing 405 line LOPT's

Symon, PM not sent after all as Dave Moll has posted a PDF of the article in question. Many thanks anyway!
Cheers Nick
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Old 25th May 2017, 3:54 pm   #9
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Default Re: Testing 405 line LOPT's

Quote:
Originally Posted by Dave Moll View Post
Assuming this is the one, your memory serves you well:
That's the one Dave, and thanks for uploading the article.

Regards
Symon.
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Old 25th May 2017, 3:55 pm   #10
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Default Re: Testing 405 line LOPT's

Hi Glyn,
I've never played with a commercial tester so have no idea if it would be suitable. I rather like the simplicity of the ringing with a scope method so will have a play next week when I am near my workbench.
Nick
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Old 25th May 2017, 3:58 pm   #11
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Default Re: Testing 405 line LOPT's

Quote:
Originally Posted by 1100 man View Post
Symon, PM not sent after all as Dave Moll has posted a PDF of the article in question. Many thanks anyway!
Hi Nick.

That's OK. It is certainly worth the effort to build the tester as it will come in handy for checking other types of transformers and chokes.

Regards
Symon
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Old 25th May 2017, 6:01 pm   #12
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Default Re: Testing 405 line LOPT's

I will add a 'like' for that transformer tester. I built it shortly after the article appeared and it has proved very useful over the years for testing LOPTs, chopper transformers, etc.

It was easy to build and the components are common. I guess the only downside is that you need a 'scope but it will work with just about any 'scope I think. Certainly every 'scope I've tried (CRT based and digital) has worked.
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Old 26th May 2017, 12:03 am   #13
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Default Re: Testing 405 line LOPT's

Thank you to everyone who has responded. I now have enough info to do a few experiments- initially with the ringing method using the x output on the 'scope. Depending on the results, I may build the tester providing I can find a 4001 in my odd bits.
I'll post some results next week.
Cheers Nick
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Old 26th May 2017, 5:02 am   #14
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Default Re: Testing 405 line LOPT's

The 4001 is not hard to get. Seeing that all 4 gates are strapped as inverters, I suspect many other CMOS logic chips could be used. I've not tried it, but I would guess a 4011 would work without changes, or maybe 4 gates from a 4069.
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Old 6th Jun 2017, 10:11 am   #15
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Default Re: Testing 405 line LOPT's

Digging around in my scrap box, I found a 4001 on an old board, so I knocked up the tester in 'skeletal form' from the article in post No5 above. It's shown in the photo hooked up to the transformer from an Ecko T344 (note the decayed LOPT shroud!). The result on the 'scope is pretty much what the article leads me to expect to see- a decaying 'ring' that lasts for a complete period of the applied square wave. (about 90Hz in this case).
Placing a shorted turn around the transformer core stops the ring completely, just leaving a short spike on the screen.
I've tested 4 different 405 tx's and they all give pretty much the same result, including the one from the Pilot so that's encouraging!!
Interestingly, the Tx from my VT161 which I had condemned as having a faulty overwind also tests out ok so I don't know what is going on there- more investigation needed before getting it re-wound.
Increasing the frequency of the drive to 10Khz and monitoring the output of the EHT overwind can give an indication of how well the transformer will actually work. (According to the article).
I did this to all 4 TX's and the results were remarkably consistent- about 170V p-p on the 'scope.
So all in all an interesting couple of hours and an indication that the Pilot LOPT might be functional- at least until it has to withstand 16KV
Cheers Nick
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Old 6th Jun 2017, 6:28 pm   #16
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Default Re: Testing 405 line LOPT's

I have a 'ScanTest' produced by Direct TV Replacements in the early 60's. It's good but not conclusive.

Tip. If you have handy ANY 405 line flyback transformer a simple test can be set up by simply removing the top caps to the boost diode say PY81 and line output valve PL81 and substituting the equivalent connections from the spare transformer. Good EHT should be produced from the test transformer and if the EHT rectifier is in place, this should give a good glow. Give it a go. John.
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Old 6th Jun 2017, 9:39 pm   #17
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Default Re: Testing 405 line LOPT's

Hi John,
That seems a good idea- I'd never thought of doing that! Surely though, the lack of scan coils, capacitors etc would upset the LOPT to the point where it would not operate efficiently enough to light the EHT rectifier heater?
Rather embarrassingly, I don't currently have any working 405 sets which I can use for tests. The ones that did work 10 years ago have suffered due to poor storage- the rest were never finished!! I have a very large 'to do 'pile
Cheers Nick
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Old 6th Jun 2017, 9:46 pm   #18
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Default Re: Testing 405 line LOPT's

It works very well Nick with receivers that use a line blocking oscillator independent of feedback from the lopt. I used it quite a lot in my early days in the trade.

It is probably unsuitable as a test in your Ultra/Pilot due to the feedback arrangement [C44 47pf] but worth keeping in mind. J.
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