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Old 3rd Dec 2017, 5:24 pm   #1
idlerbiker
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Default What are the chances of a modern LOPT failing?

Hi Guys,

Just a little question,
What are the chances of a LOPT from the late 70s, 80s and 90s failing?
Are they easily replaced/repaired?

Just that they are usually encased in plastic, is it difficult to get to the components inside?
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Old 3rd Dec 2017, 5:40 pm   #2
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Default Re: What are the chances of a modern LOPT failing?

There were a few bad ones out there.
One of the Thorn B/W portables had square EHT diode in the potted part that got a bubble under it causing overheating.
VGA screens suffered with bad ones as well even the big brands.
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Old 3rd Dec 2017, 5:52 pm   #3
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Default Re: What are the chances of a modern LOPT failing?

It would depend on which windings have the highest voltages. The EHT over-wind for certain. But how many CRT TV's are there these days for us to discover if dipping LOPT's eliminates moisture ingress?
I never had a modern colour telly with a bad LOPT. The most modern big-screen TV I had was of the Thorn 8500 series. It was usually failing tube emission that sent them to the tip in the end.
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Old 3rd Dec 2017, 6:18 pm   #4
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Default Re: What are the chances of a modern LOPT failing?

There were still plenty of bad lopt's out there in the 70's and 80's. Some sets were notorious for it. Although generally the reliability was a whole lot better than the 405 era. I've never tried cutting (a dipped) one open to examine the windings. Is there a particular set you need a lopt for?
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Old 3rd Dec 2017, 6:18 pm   #5
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Default Re: What are the chances of a modern LOPT failing?

I've replaced very many from the 80's and 90's particularly, mostly the modern-type diode-split types.

I suppose it's what kept people like Konig, Eldor and HR Diemen in business. The TV spares catalogues (remember them!) had large sections on lopts, mains switches and lop transistors, so make of that what you will!
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Old 3rd Dec 2017, 6:29 pm   #6
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Default Re: What are the chances of a modern LOPT failing?

In times-past I've found old EGA/VGA computer-monitors to be a good source of LOPTs: you may need to fiddle around with the capacitors that tune the LOPT to resonance.
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Old 3rd Dec 2017, 7:10 pm   #7
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Default Re: What are the chances of a modern LOPT failing?

The LOPT in my Beko C7 certainly failed fairly early in its life:-

https://www.vintage-radio.net/forum/...ad.php?t=56469
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Old 3rd Dec 2017, 7:29 pm   #8
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Default Re: What are the chances of a modern LOPT failing?

Quote:
Originally Posted by McMurdo View Post
I've replaced very many from the 80's and 90's particularly, mostly the modern-type diode-split types.

I suppose it's what kept people like Konig, Eldor and HR Diemen in business. The TV spares catalogues (remember them!) had large sections on lopts, mains switches and lop transistors, so make of that what you will!
There was one made by philips used in lots for set toshiba matsui just to name a few . Some times it would s/c betweenpri winding and sec winding . dead ease to fault find loptr collector would very low resistance to chassis , other time on one trippler doides would go sc in that case burning smell> We must have had lots of troubled with them as they became stock carry on the cars .
Some of the bigger sets like Itt & Some Grundig the case would split near the point were the eht lead came out causing arking .
regard Derrick
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Old 3rd Dec 2017, 9:50 pm   #9
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Default Re: What are the chances of a modern LOPT failing?

The ones going short between primary and secundary low voltage winding, were the small grey ones for 90 degree deflection sets throughout the 1990's. I think they were in the AT2079/AT2094 series and the fault was common in a few variations of these for several years before the design was presumably changed.

Another notable problem were insulation faults in the EHT section, giving sever flash overs and resulting damage. Orega DST's fitted mid to late 1980's to the ICC5 chassis, particularly in the Telefunken version I think, and around 1992 Eldor DST's fitted in the Philips GR2.2 chassis were prone to this.

Apart from that, there was always a few percent chance of a LOPT failing in general. Mostly developing shorts in the EHT secundary or rectification.
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Old 4th Dec 2017, 12:48 am   #10
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Default Re: What are the chances of a modern LOPT failing?

Sony LOPT's were notorious for breaking down, sometimes taking many other parts with it.
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Old 4th Dec 2017, 12:55 am   #11
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Default Re: What are the chances of a modern LOPT failing?

When I was employed by a firm that specifically serviced EGA / VGA monitors, the replacement of LOPTs in Sony monitors was quite common. Often caused by the line O/P transistor going collector-emitter S/C.

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Old 4th Dec 2017, 1:00 am   #12
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Default Re: What are the chances of a modern LOPT failing?

Quote:
Originally Posted by Maarten View Post
The ones going short between primary and secundary low voltage winding, were the small grey ones for 90 degree deflection sets throughout the 1990's. I think they were in the AT2079/AT2094 series and the fault was common in a few variations of these for several years before the design was presumably changed.
Was that the one used in the Anubis chassis?

I also encountered many mid 80's Blaufpunkts with defect DSTs.
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Old 4th Dec 2017, 1:00 am   #13
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Default Re: What are the chances of a modern LOPT failing?

Anubis and others, indeed.
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Old 4th Dec 2017, 1:01 am   #14
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Default Re: What are the chances of a modern LOPT failing?

Quote:
Originally Posted by Skywave View Post
When I was employed by a firm that specifically serviced EGA / VGA monitors, the replacement of LOPTs in Sony monitors was quite common. Often caused by the line O/P transistor going collector-emitter S/C.
Could that have been the other way around?
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Old 4th Dec 2017, 1:07 am   #15
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Default Re: What are the chances of a modern LOPT failing?

On some of the better no brand name VGA screens the power supply used to trip saving the transistor. A scope would show the severely clipped collector waveform.
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Old 4th Dec 2017, 1:17 am   #16
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Default Re: What are the chances of a modern LOPT failing?

Quote:
Originally Posted by Skywave View Post
When I was employed by a firm that specifically serviced EGA / VGA monitors, the replacement of LOPTs in Sony monitors was quite common. Often caused by the line O/P transistor going collector-emitter S/C.
Quote:
Originally Posted by Maarten View Post
Could that have been the other way around?
Yes, possibly. But when faced with a pile of monitors to fix and a 'daily target' to meet, there wasn't the time for an in-depth analysis of the various failure modes. I worked there for 6 months - and then left to get a less demanding job that was better paid and much more satisfying.

Al.
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Old 4th Dec 2017, 2:05 am   #17
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Default Re: What are the chances of a modern LOPT failing?

Quote:
Originally Posted by G6Tanuki View Post
In times-past I've found old EGA/VGA computer-monitors to be a good source of LOPTs: you may need to fiddle around with the capacitors that tune the LOPT to resonance.
The lopt and the associated yoke inductance is tuned to "a resonant frequency" (by the capacitor on its primary) so that a cycle of this resonance is typically in the range of 50kHz to 60kHz vicinity.

A half a cycle should be in the order of roughly about 10uS to 12uS duration.

The tuning capacitor, if increased in size will simply lengthen this time (the flyback time) and lower the peak voltage on the horizontal output transistor's collector.

Or if the capacitor is made smaller the flyback time will shorten and the peak voltage will be higher.

This is because a fixed amount of magnetic field energy from the lopty & yoke (for any picture width) is transferred into the capacitor as electric field energy which peaks at 1/4 cycle of the flyback oscillation, which is half way into the flyback period, and the energy stored in the capacitor is CV^2/2, so if the capacitance is lower, the peak voltage is naturally higher.

The main thing to watch out for is that the capacitor is not too small in value and the peak voltage too high or it will take out the horizontal output transistor (HOT) by exceeding its collector voltage.

So if you are experimenting with non standard loptys for some set, it is best to temporarily double up on the pre-existing capacitor value, it will make the flyback time long and encroach on picture time, but then you can slowly reduce the value in steps until the flyback time just does not encroach on active H picture time, and the peak voltage on the HOT will be as low as it can be.

If you have a failed lopty due to a single failed EHT rectifier (leaky or shorted) inside the molded lopty assembly it can work to leave that as it is and string another EHT rectifier in series with the output. Doesn't work for voltage multiplier types.
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Old 4th Dec 2017, 9:06 am   #18
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Default Re: What are the chances of a modern LOPT failing?

My mid 80s Hitachi lost its lopt due to the pins burning off , poor soldering on oversize holes in the pcb and line o/p transistor went west with it.
I stripped the other smaller Hitachi down to find that it was starting to do the same. Soldering it all up properly kept it going till digital changeover.
Other than that, I don't think I have had a trany fail since I was a post war kid. The Deccas we had in the late 70s and early 80s were very reliable all round.
I could never understand why rentals failed more than private buyers' sets, or it that just an illusion?
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Old 4th Dec 2017, 2:17 pm   #19
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Default Re: What are the chances of a modern LOPT failing?

I'm surprised at people saying they rarely had a diode-split LOPT fail. Along with most others in the trade we certainly did!
Top of the list? Philips CP90 and the similar grey ones as Derrick mentions. A squealing CP90 always had me looking at the shelf before the back came off.
Runner-up? Green spot TX100 (we bought these in tens) and the red spot one fitted to the ill-advised 20" TX90.
Then the Hitachi CPT2174 series - we changed loads of these, possibly as we were (in fact probably are!) Hitachi dealers. The one fitted to the 2476/8 was much more reliable. However we never had to change one (except by mistake) in a CPT 2196 series, though I bet they were made by the same manufacturer.
Best ones? G11 and K30 - K40 without a doubt (if we exclude the EHT lead) and similar cream coloured ones in B&O and GEC sets. Also TX9 and 10s were good.
I still have a pile of yellow HR and Konig boxes with new LOPTs I'll never use. Oh well....
Glyn
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Old 4th Dec 2017, 6:37 pm   #20
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Default Re: What are the chances of a modern LOPT failing?

Yes they fail.... but I don't have the same memory recall as Glyn and can't say which brands if any were better. Also have quite a few HR/Konig etc boxes of new ones on the shelf which are worth zilch these days.
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