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Old 10th Jan 2018, 11:39 pm   #1
1100 man
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Default Dismantling LOPT's

Hi,
TV's from the mid '50's to the mid '60's mostly have what I would consider a 'conventional' LOPT. That is an auto-transformer design with two bobbins mounted on each limb of a ferrite core comprising two 'c' shaped pieces held together by long brass bolts. One bobbin is the EHT winding and the other bobbin is all the rest of the windings.
Most of the TV's I've ended up with seem to be of this type.
My question is, how are the two halves of the ferrite core joined together?
The transformer pictured below is from a Marconiphone VT161 17" tv from 1959. The EHT winding is arcing to it's former and sending up a plume of smoke!
I have removed the connection board and the through bolts but the two halves of the core don't want to part even with moderate force. Are they glued together?
I've recently had exactly the same issue with the LOPT from my Pilot which I have been re- insulating. Again, the core did not want to part even when it was quite hot.
I would be very interested to know how they are constructed and how to get them to bits!
The third picture is the Pilot LOPT just to illustrate the type of construction.
Many thanks
Nick
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Old 11th Jan 2018, 2:09 am   #2
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Default Re: Dismantling LOPT's

Yes, they are glued. Some do just fall apart, most don't!
As some of these predate superglue, I suspect it is clear epoxy resin. If so it will withstand more heat than you would wish to use.
Best remover was the now banned paint strippers. Soaking in Acetone may work eventually but will destroy most of the rest of the transformer in the process.
Sorry, I have no other suggestions.
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Old 11th Jan 2018, 7:01 am   #3
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Default Re: Dismantling LOPT's

I have never known the two 'U' ferrite cores to be actually glued together. They are normally just clamped together with threaded rod. There are very small paper gaping paper discs between the core faces.

They are usually stuck very tight with hardened wax and heat from the transformer itself. Take great care, it is very easy to snap off one of the arms if handled with force.

When I was a young lad I used to make Ekco and many other line output transformers including rewinding during the period 1961-66. [D&B Television. Wimbledon] No adhesive was employed and the transformers used to fall to bits once unclamped. This was the best part of 60 years ago and they are all now seized solid!

I can only suggest warmth and general easing. Good luck with it. John.
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Old 11th Jan 2018, 8:00 am   #4
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Default Re: Dismantling LOPT's

I have taken a few apart, one fell apart and the other two were glued with epoxy.
The only way I succeeded to part them was to put the core on the cookers hotplate. Protect coils with foil and turn every 10 seconds until its reached about 150 degrees, pull apart with oven gloves.
There was some melted plastic on the former as a result but it did need the temperature.
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Old 11th Jan 2018, 10:17 pm   #5
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Default Re: Dismantling LOPT's

Hi,
Thank you, Sam, John & Stephen for your interesting replies. I wondered if they might have glued them to prevent them from squealing? I was wondering how the people that rewind them would attempt to separate the core without damage?
I've not tried heat on this one yet, but the hotplate idea, whilst a bit scary seems like it might be the best way to go. I do have access to non-diy paint stripper which is quite potent, but getting it down the tiny gap between the former and the core would be difficult.
It certainly doesn't want to fall apart
Cheers
Nick
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Old 11th Jan 2018, 11:26 pm   #6
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Default Re: Dismantling LOPT's

Well what do you know? Having looked at it fiercely, in case that might frighten it, I picked it up tonight and found the core was loose!! Very strange as I'd pulled the two bits of core quite vigorously last night and there was no movement!
Anyway, I now have the EHT winding in a position where I might be able to do something with it. The problem is where the lead in wire (top cap of PL81) enters the winding down the side of the former. There was an arcing like the sort you get from the top cap of the line o/p valve and a quantity of smoke. Rather strange it was coming from the start of the winding rather than the high voltage end. I wonder if the lead in wire is not making good contact with the actual winding wire.
I need to remove the thick coating of black wax so I can see what is going on.
I wonder if I can effect a repair?
All the best
Nick
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Old 12th Jan 2018, 5:39 am   #7
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Default Re: Dismantling LOPT's

If you just want the ferrite core, you could try removing all windings by brute force, and all hardware, and then putting the core in a microwave oven with half a mug of tap water on the other side of the oven plate to act as a ballast.
Remove when cooked.
Salt and pepper to taste.
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Old 12th Jan 2018, 6:49 am   #8
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Default Re: Dismantling LOPT's

Ludens does a very good website on transformer winding. I contacted him on how to get a pot core apart without damage. One recommendation was to boil it for a while. Hard boiled perhaps.
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Old 12th Jan 2018, 12:54 pm   #9
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Default Re: Dismantling LOPT's

Has anyone tried putting steel fencing wire through where the brass bolts run and powering it from a couple of turns of thick copper wire around a transformer?
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Old 12th Jan 2018, 1:53 pm   #10
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Default Re: Dismantling LOPT's

I have one which snapped in the middle of the limb while trying to get it apart. They seem to be very fragile.

My question is, would it be ok to superglue the 2 halves back together or would the transformer be affected by the break, and also is it likely to squeal?
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Old 12th Jan 2018, 1:58 pm   #11
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Default Re: Dismantling LOPT's

I once put a pot core back together with epoxy and it has been working fine for years.
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Old 13th Jan 2018, 10:01 am   #12
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Default Re: Dismantling LOPT's

You can certainly repair a broken core with a minute smear of adhesive if it becomes broken. Providing the broken limb fits perfectly [practically invisible if held together] the clamping plates/rod will keep it tightly in place without the adhesive. You don't want to introduce extra gaping to interfere with the core's magnetic efficiency.

Cores are often stuck unintentionally to the actual winding bobbin either during the assembly process, exuded hard wax or resin with later models such as the RBM A640, Philips 210/300 etc, then sets solid over decades of heating and cooling locking the whole lot solid! The actual bobbin can also go out of shape with years of heat.

Rewinding line output transformers is an art believe me and I take my hat off to anyone that can do it successfully and reliably.

Tiny, usually circular gaping papers are placed between the cores during manufacture and it is possible that a smear of glue such as the type that used to back postage stamps is dabbed on to stop the tabs from falling off during manufacture. This glue would have dried out forming a flimsy film that would have given way with gentle persuasion.

I can only say that I carried out large numbers of rewinds of pre 1960 line output transformers and never had a problem releasing the Ferroxcube cores. Transformer manufacture during the late dual standard period was changed slightly due to the elimination of pitch, tar and wax from the manufacturing process,

I would add that there were many differing types of Ferrite core make up, the types identified by the letters FX prefixing the type number. They were a product of Mullard.

An early better quality core material more suitable for 10kc/s was 'CASLAM' manufactured by Plessey, introduced in the period between metal laminations and Ferrite. When cleaned up it has a shiny appearance almost like Chromium plating but can be easily damaged so if you come across this, take care. It's a bit like 'Monkey Metal'. This dates from around 1949-51 and will be discovered in ETRONIC receivers and a few others.

I think the best approach to the problem as stated is gentle heat and patience but you may well discover that the breakdown was instigated by rotting interleaving tissues, an age problem that even Philosan could not fortify! John.
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Old 13th Jan 2018, 11:40 am   #13
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Default Re: Dismantling LOPT's

In the 1950s, in the Netherlands, Philips used paint to glue the core halves (and air gap disc) together.
This method may not have been used in the UK.

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Old 13th Jan 2018, 12:19 pm   #14
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Default Re: Dismantling LOPT's

The LOPT shown is in fact a transformer made by Ferguson, They started to make sets for Marconiphone in the late 50s up to some time in the 60s.
I have repaired a few of those transformers quite successfully, the problem is that the start of the EHT winding is pulsed to the full potential of the line output valve anode, perhaps as high as 3.5KV.
Simply replace the inner insulated tube under the EHT winding, if you lose a layer of the start of the winding it will not matter. I can't remember but the core may need gappng with paper or mica to reduce core saturation. Just reassemble it the way it came apart The core material needs to be non absorbent and be highly insulating with low dielectric constant. I hope this helps and that the winding has not become carbonized at the edge.
Victor.
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Old 15th Jan 2018, 10:56 pm   #15
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Default Re: Dismantling LOPT's

Quote:
Originally Posted by Heatercathodeshort View Post

I would add that there were many differing types of Ferrite core make up, the types identified by the letters FX prefixing the type number. They were a product of Mullard.

but you may well discover that the breakdown was instigated by rotting interleaving tissues, an age problem that even Philosan could not fortify! John.
Hi John,
As usual, an interesting insight into the world of '50's television technology:- if you could just download the contents of your brain onto this site, that would be great! I had to Google 'Philosan' and, being born in the '60's, am too young to remember such things!!
This core, like that of the Pilot transformer, has an FX number. The core dimensions are very similar. Interesting that it's a Mullard coding system. There was a rectangular mark on the end of each core leg, suggesting there had been something like a paper spacer, but there was nothing obvious left apart from some while powder: maybe that was all that remained of it!

I had good success with my Pilot transformer removing the wax with white spirit so I will try the same technique with this one. Hopefully I will then be able to see where the failure is and maybe effect a repair.

English Victor: Thank you for your input. I had a feeling somebody ( undoubtedly HCS!) had mentioned that it was a Ferguson chassis. I had wondered myself about removing the coil former as that seems to be where the problem is. I will know more after I have removed all the wax.

All the best
Nick
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Old 16th Jan 2018, 12:03 pm   #16
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Default Re: Dismantling LOPT's

The Marconi VT161 can best be described as a 90 degree [AW43-80] hand wired version of the later, by only a few months, very popular Ferguson 500 series.

It is incredibly reliable and gives a super bright picture. It is also known as the British Relay Wireless model BR217.

I think downloading my brain to this site would never work as I can't even download a driver for my film scanner to Windows 10.

Good luck with your transformers. Happy Days at D&B Television, 131 Kingston Road, South Wimbledon S.W.19. [I was still at school! John.
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Old 17th Jan 2018, 11:23 pm   #17
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Default Re: Dismantling LOPT's

Quote:
Originally Posted by Heatercathodeshort View Post
Good luck with your transformers. Happy Days at D&B Television, 131 Kingston Road, South Wimbledon S.W.19. [I was still at school! John.
Hi John,
To have been involved with a local repair shop whilst still at school would have been a dream come true for me. However, being a lad in the early '80's had its compensations. There was an endless stream of old stuff to be had for free:- 405, dual standard and early colour TV, open reel VTR, early VHS & beta machines along with Philips 1700, Grundig SVR and Hitachi video disk players. Radiogrames, record players, tape recorders and all manner of other obsolete bits of equipment! I dragged it all back up to my bedroom much to my mothers dismay
I would have loved to have been in the trade in the '60s,70's & 80's, but I only got the scrag end of it in the '90's!! Still, I thoroughly enjoyed it until widescreen came out: that was the end of it for me. Short, fat people on Tv's that I couldn't lift- no thank you!
Thank you for your interesting glimpses of the trade from those earlier times- its always fascinating to read.
All the best
Nick
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Old 19th Jan 2018, 2:13 pm   #18
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Default Re: Dismantling LOPT's

Quote:
Originally Posted by 1100 man View Post
This core, like that of the Pilot transformer, has an FX number. The core dimensions are very similar. Interesting that it's a Mullard coding system.
Philips Components must have supplied Ferroxcube cores to other manufacturers, but they certainly also supplied complete transformers. Either from the catalogue or custom made if necessary.
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Old 20th Jan 2018, 10:22 am   #19
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Default Re: Dismantling LOPT's

Philips certainly did supply later type diode split transformers not all of which were robust.
Toshiba used them in their portable range from 1989 and I replaced large numbers supplied free from Toshiba even when out of guarantee.

Mullard certainly supplied Ferroxcube cores to most makers together with scan coil core assemblies in pairs, in fact just about every transformer lamination employed in high frequency television receivers was superseded by Ferroxcube by the mid 60's. 'Iron' was still in used in mains/sound/frame output transformers probably due to the low frequency and high currents associated with these circuit requirements.

The cores were supplied from Mullard in boxes each core separated by a cardboard rectangle to prevent chipping and damage. There were many differing shapes but the majority of the LOPT cores looked identical but had very different characteristics.
Using the wrong core would result in all sorts of weird effects. The FX number was stamped on the long arm of the 'U'.
All this takes me back 57 years when I used to make EKCO transformers from scratch.

Cut newly fabricated Paxolin sides and top from large sheets with a lethal band saw.
Rivet the 4BA studs and anti corona cups to hold it all together.
Layer wind the underwind [primary]each layer interleaved with special tissue. [on the Ekco a separate winding]
Wave wind the overwind,[EHT] both windings on Paxolin bobbins.
Vacuum impregnate windings.
Form a [TYRE] around perimeter of overwind to seal off and prevent flash over.

This was simply done by employing an old style OXO tin with a couple of slots cut in the sides. This was filled with pitch and heated from below by a small gas flame to a depth that was just enough to allow the overwind to just dip into the pitch with the dowel resting on the slots. The overwind was taped to a short length of dowel and 'spun' in the bath forming a neat tyre around an 1/8th of an inch in depth.

Assemble Ferroxcube cores and windings not forgetting the gaping papers.
Fit assembly into Paxolin case using threaded rod to hold it all together.
Solder lead out wires to anti corona cups.
Fit Paxolin top and secure with 4BA nuts.
Loop two turns of EHT cable around underwind to supply EHT rectifier U25 heater [2V]
Picture test in Ekco television chassis. [Long leads and crocodile clips]
Finally dip top in hot pitch to help prevent tracking under very damp conditions.
There was of course many other steps in manufacture, mostly minor mechanical such as attaching a more substantial lead from the high end of the very thin overwind to the anode cup of the EHT rectifier. A bit fiddly but you soon got the knack of it!

I know one forum member that has a TMB272 with one of my transformers still working well!

Happy days. John.
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Old 20th Jan 2018, 10:39 am   #20
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Default Re: Dismantling LOPT's

Making LOPTXís for Ekco TVís, it must have been cost effective for you to do it.

Transformers were readily available from Ekco and easily replaced without major disassembly in many models. The Achilles heel was the casing, the windings gave little trouble apart from the heater winding, again easily replaced.

I seem to remember the casing was available, though we normally just changed the complete LOPTX.

What was the saving, once replaced the Ekco manufacturers LOPTX lasted the life of the TV?

Very interesting anyway and hats off for your ingenuity.
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