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Old 23rd Mar 2024, 12:07 pm   #1
paul farn
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Default LOPT Testing (Split diode)

What would be a suitable check for intl diodes diode-split device? No reading on tester at eht outlet(transformer out of cct). Should be about 30V if OK. Primary seems to be ringing as viewed on 'scope.

Last edited by Radio Wrangler; 14th May 2024 at 7:12 am. Reason: less general title
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Old 26th Mar 2024, 11:50 am   #2
Welsh Anorak
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Default Re: Lopt

In my experienec there's no easy check for a shorted diode other than replacing the transformer.

I assume the LOPT was deemed faulty when in the TV?

What TV is it out of? That might help. For example the old Thorn 1691 mono portable was able to be repaired by the addition of a series diode following the internal shorted component - a common repair back in the day. Of course in a large screen colour set that's not an option.
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Old 26th Mar 2024, 5:19 pm   #3
paul farn
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Default Re: Lopt

Quote:
Originally Posted by Welsh Anorak View Post
In my experienec there's no easy check for a shorted diode other than replacing the transformer.

I assume the LOPT was deemed faulty when in the TV?

What TV is it out of? That might help. For example the old Thorn 1691 mono portable was able to be repaired by the addition of a series diode following the internal shorted component - a common repair back in the day. Of course in a large screen colour set that's not an option.
Yes, doubt if these are obtainable now. From a Toshiba 28n 03b. Looks like end of the road but I'll keep the chassis for spares, if anyone wants some. Tube was OK.
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Old 28th Mar 2024, 11:19 am   #4
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Default Re: Lopt

Always worth trying HR Diemen - they still hold some stock for older items.
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Old 29th Apr 2024, 10:58 am   #5
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Default Re: Lopt

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Old 4th May 2024, 4:56 pm   #6
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Default Re: Lopt

Thread reopened by request.
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Old 4th May 2024, 5:56 pm   #7
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Default Re: Lopt

Thank you!

I wanted to add that HR also did LOPT-testers. Small yellow cases with a few crcodile clips to attach to the primary winding and cold side of the EHT winding, and a slot to insert the EHT connector. When you connect the transformer, some reference value is displayed on the LCD display and you can look up in the book what the value should be if the transformer (including rectifier) is working correctly. I think they use a microcontroller with proprietary software, but it should be possible to use a 16kHz low voltage pulse as the input and a voltmeter to compare the output between a working transformer and the transformer under test.

I don't know if this scales linearly. If it does, a 12V input might generate 2 to 3kV output. Ideally you want to keep the output under 1kV so it can be measured with a normal voltmeter.
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Old 13th May 2024, 12:12 pm   #8
paul farn
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Default Re: Lopt

Interesting,but haven't got a good one for comparison.Neither had HR when I looked.
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Old 13th May 2024, 1:50 pm   #9
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Default Re: Lopt

Hi.

Regarding testing line output transformers, there was a really useful project in Television magazine, Sept 1993. This tester is used together with your oscilloscope for checking how well the windings ring. I built this tester many years ago and it saved me a fair amount of money in not having to fork out for a new LOPTx.
Obviously it won't be a 100% test as that needs to be done around its normal operating voltages, but it will show a winding suffering from shorted turns by displaying a heavily damped sine wave. A healthy winding will ring with a characteristic exponential decay. The tester uses non critical parts and is cheap to build.

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Old 13th May 2024, 6:18 pm   #10
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Default Re: Lopt

There were several such 'ringing tester' in Television magazine over the years. I am not sure if the one I built is the one 'Philips210' is refering to, it used a common CMOS logic chip (4011?) to make the oscillator and an easy-to-find (at the time) line output transistor and not a lot else.

I've used it a lot, not just on line output transformers but also on switch-mode power supply transformers. It works very well.

However, it will not pick up faults in EHT rectifiers built into line output transformers, such as the diode split types.
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Old 13th May 2024, 6:45 pm   #11
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A copy of the original article is attached. I think this is the same one that Tony built by the sounds of it. Unfortunately the forum software has reduced the resolution due to file resizing. Alternatively, the article can be found here: https://www.worldradiohistory.com/UK...on-1993-09.pdf

Regards,
Symon
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Old 14th May 2024, 4:42 am   #12
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Default Re: Lopt

Yes, that's the tester that I built and use. Assuming you have a 'scope (and just about any 'scope will do, it doesn't need much bandwidth, or to be calibrated), it's well worth building this unit. Several times it's proved that the cause of a transistor blowing is/is not shorted turns in the transformer.
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Old 17th May 2024, 10:30 am   #13
paul farn
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Default Re: LOPT Testing (Split diode)

Doesn't appear to be a tester for diode-split types. Anyone Know why? Previous 'Television' designs are for transformer only.
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Old 17th May 2024, 10:47 am   #14
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Default Re: LOPT Testing (Split diode)

As said, for a DST test you need to drive the transformer and measure the output voltage. You can then make a ballpark estimate and weed out hard short circuits, but to know for sure you have to compare to a known good transformer. HR had easy access to all this data because they already reverse engineered every single transformer, so they could easily include comparative measurements in their tester.
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Old 17th May 2024, 12:25 pm   #15
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Default Re: LOPT Testing (Split diode)

Quote:
Originally Posted by paul farn View Post
Doesn't appear to be a tester for diode-split types.Anyone Know why?Previous 'Television' designs are for transfmr only.
Hi

The ring testers work fine with diode split LOPTs. The same applies to other wound components such as chopper transformers.
To make it clear, with ring testers you're not actually measuring the output from the transformer. It's only a test to check for shorted turns in a winding. It's unlikely to pick up inter winding shorts.
The ring tester is normally used on the primary winding on a LOPT ie where the line output transistor collector is connected. You should isolate this transistor before doing a ring test on a transformer that's in circuit. A short in one of the diodes connected to the LOPT can also heavily damp the ringing waveform.

Regards,
Symon
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Old 18th May 2024, 11:48 am   #16
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Default Re: LOPT Testing (Split diode)

To clarify, a ringing tester will detect shorted turns in a diode-split LOPT. What it won't do is find faults in the diodes.
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Old 21st May 2024, 10:14 am   #17
paul farn
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Default Re: LOPT Testing (Split diode)

I'll see if I can get hold of a used one (any make) for comparison.Looks like fault on diode(s) so far.Boohoo.
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Old Yesterday, 2:04 am   #18
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Default Re: LOPT Testing (Split diode)

Ideally the transformer should be the same or similar, but if you know the turns ratio or voltage ratio (the latter would be in the HR book or sheets) you could compare with different transformers.
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