Dismantling LOPT's

[Heatercathodeshort]

They were cheap to manufacture and sold in large quantities. Ekco would not supply spares to non Ekco dealers so were difficult to obtain. I think they sold for 35/- [£1.75] back in those days but you will have to check the D & B full page adverts in Practical television to confirm. [1962/66]. The genuine Ekco transformer was double that price. D & B sold large quantities to TAM [Television Audience Measurement] a company that supplied special EKCO receivers to the public that were able to record which channels were being watched, so that advertising space could be charged accordingly.

Remember a 10/- [50p] profit on a component back then represented a fair return.

Many of the cores were obtained very cheaply on the manufacturers surplus markets and insulating materials such as pitch were provided free from companies such as Berry Wiggins providing we filled in a 'performance sheet'. I well remember the ancient 8 wheel Foden flatbed lorry that used to deliver the heavy cans of solid pitch.

I have scanned a few pages from my rewind book together with my rather crude jottings. I was around 14 at the time. John.
Oh dear..Was it really that long ago?
Just to add. D&B Television were spares suppliers to the trade and retail. They also sold reconditioned receivers.

[Nuvistor]

Thank you for that insight, just had a look at a PT for 1961, D&B are in there with various items for sale, new and used.
Interestingly the advert states “ We are open from 10am to midnight”.

[Peter88gate]

I remember D & B Television who were near South Wimbledon Station and Papworth Transformers who were also nearby. I used both at various times in the late 1960s and early 1970s to obtain LOPTs for B&W TV repairs. Another outfit in the same area was Harverson Surplus who did various audio amplifiers, tape recorder spares etc.

Advert for D & B Television attached from May 1966 Practical Television. Also an advice slip from a LOPT supplied by Papworth Transformers.

Peter

[Jac]

Very interesting John.

Could you tell a bit more about the vacuum impregnating?
How it was done exactly, what material were the coils impregnated with, etc?

Thanks!
Best regards,
Jac

[Heatercathodeshort]

Interesting to see how critical the gap was. You can see how heavy glue would have messed it up.

The sheet was supplied with transformer 'inserts', the actual windings themselves. You dismantled the old transformer and fitted the new windings yourself. A very cheap and satisfactory answer for obsolete transformers. John.

Papworth Transformers took over D&B Television and moved to Merton High Street around 1970 I guess. John.

[Heatercathodeshort]

Hello Jac, There was a small high pressure vessel with a screw top that held a number of finished windings. Two pipes were connected rather like a modern fuel filter on a car. Both had stop valves.

A container of RS insulating varnish thinned with meths was connected to one pipe and the valve closed. The other pipe connected to the vacuum pump was opened. A high vacuum was created in the vessel and after a few minutes the valve was shut off retaining a very high vacuum.

The other valve was then opened allowing the insulating varnish to fill all air gaps in the windings. They were then recovered from the vessel and allowed to drain.

To be honest the transformers were so generously constructed that it was not necessary to vacuum varnish and after a while we discontinued the time consuming procedure in all but the most difficult rewinds,[Philips 1768 etc]. It was a long time ago but I can still remember the procedures. John.

[Jac]

Thanks John.

Most interesting!
In some old Philips documentation I read that Philips in NL around 1950, used Zophar wax for impregnating the coils. I couldn't find more about it than that it was the name of a company in the US that made materials like compounds and impregnating waxes.
Unfortunately I have no documentation about the actual impregnation procedure that was used during the manufacture of the LOPTs.

Jac

[AJSmith625]

Hello John (HCS), I've found your posts on this subject extremely interesting and informative. Having fitted several pattern part LOPTs over the years, what I would like to know is a bit more about the reverse engineering of the windings. OK it only has to be done once for each model/chassis, but it must be a painstaking job counting number of turns on each winding plus gauges of wire etc.

Alan

[Heatercathodeshort]

It was quite simple really. Most of the line output transformers were of fairly simple construction. It was easy enough to dismantle them leaving just the underwind and overwind complete with their formers to deal with.

I would mount the overwind on the coil winder, set the counter turns indicator to zero and literally pull of the winding off slowly. Care had to be taken, breaking the wire was to be avoided as it could be difficult to locate the new start! A baby Belling was used to melt any stubborn pitch or wax.

The same applied to the underwind [primary] easier because the wire is much thicker.

A micrometer was employed to gauge the wire diameter and interleaving tissue thickness. Turns per layer had to be counted. You had to get this spot on or the bobbin would not fit between the cores!

Notes and pictures of connections etc were drawn and listed in my 'rewind book'. [Ha ha!]

The overwind was often wound with Eureka resistance wire. Probably done to reduce capacity. It was not necessary to use this but it had to be remembered that the resistance would be much lower than compared with the makers service data.

When checking LOPTs it is pretty useless comparing resistance readings with makers data as one single shorting turn can damp the transformer to such as extent that the EHT voltage collapses.

Testing was usually carried out with an old Ekco T231 under working conditions. Most transformers could be tested with the very versatile Ekco but I seem to remember I also used a couple of other chassis for the self oscillating types.

95% of the transformers I rewound were from the period 1952-60. As I said earlier, most of them were run of the mill. Anything earlier such as the PYE B18T were being scrapped in large numbers and I must admit I don't think I had the skill to rewind such a messy beast. The sectionalized EHT winding in particular was very compact and I don't think the Douglas wavewind machine could have coped with such close spacing between the windings.

To give you some idea I have scanned a very tatty page from my rewind book together with the school boy jottings. Not sure what make the transformer is intended for. It has a fuse on the front panel. Probably an obscure Argosy or similar. John.

[AJSmith625]

Thanks for that John. That as answered my question a treat.

Alan.

[Maarten]

Wow, really professional especially for a school boy.

[Heatercathodeshort]

I can assure you I was a horrible child!

I make it sound easy but believe me it is a dirty, time consuming job. Nothing can be overlooked or rushed. It would soon bounce back if you let standards slip.

This was when the transformers were probably less than ten years old. 60 years on they are corroded, damp, mildew and rotten. I'm afraid most reinsulating jobs are only a temporary cure but certainly sufficient to get the timebase running to evaluate etc.

Rewinding specialist transformers today is a very difficult task. Coil formers, wire, insulating materials are not easy to source and the actual process is incredibly time consuming.

The cost of professionally rewinding a transformer must be one of the few true value for money deals of the day.
Look at it this way. Considerably less than two tanks of diesel in my Ford Ranger, ten packets of fags that won't last a week, A few bottles of 'designer' wine, and last but not least, one daily cup of coffee for just five weeks at £3 a cup and to say nothing of that must have horror, the blueberry muffin..

Regards, John.

[1100 man]

Hi John,
Many thanks, as always, for sharing your experiences of LOPT rewinding! I had a go at one when I was a lad. Dad had brought home a Decca Brunswick 22" delta tubed colour set given to him by a work colleague. We could never get the line stage to work and condemned the LOPT.
I unwound it, layer by layer, counting the turns and making notes. AFAIR, I did manage to rewind it, but the bobbins were a bit fat!
The set still didn't work so I'll never know if my transformer was functional or not!

Anyway, back to the transformer in my original post. The wax has now been removed and the EHT winding (I hesitate to use the term 'overwind', as it's a separate bobbin on the other limb of the core, rather than wound on top of the primary) can now be examined under a lens.
The problem was arcing somewhere near the entry to the winding and a small plume of smoke.
The plastic former was easily removed from the winding and as can be seen in the picture, a black carbonised mark was evident. The former has a crack running through it and a carbonised mark on the inside. The inside of the winding, which seems to be an SRBP type material, also has a carbonised mark.
It would seem that the input to the winding has been arcing through its SRBP former, then through the plastic former to the ferrite core of the transformer.
So hopefully, with some careful cleaning and re- insulation, the winding can be saved.
I must say I will be well chuffed if I can repair it as I had condemned this transformer as needing re-winding a couple of years ago.
Compare how the winding looks now to how it did in pictures 1 &2 in the first post of this thread when it was covered in black wax. It's now half the size with no wax coating!
All the best
Nick

[1100 man]

Good evening,
After finding the damage pictured in my last post, I was pretty convinced that was the reason for failure rather than actual winding damage. Before proceeding to do a full repair and varnishing the winding, I wanted confirmation that it was actually going to work. I decided to temporarily cobble it back together and test it out in the set.
This I did with the transformer balanced on a pile of books and hooked up with longer wires- It was a real lash up but was only to prove a point!
With the EHT winding exposed to the air, I was expecting some arcing & corona so I powered the HT from the variac to keep the energy in the transformer as low as possible.
As it turned out, it all worked perfectly and there was no discharge even at full HT! On top of that I was rewarded with a fantastic picture- pin sharp and high contrast.
I had forgotten just how good the picture used to be on this set- Most of my other sets have Mazda tubes but this Mullard AW43-80 knocks spots of them!
This set has had a bit of a chequered history whilst in my possession for the last 20 years, so it deserves a proper overhaul. I'll start a thread about it in the next few weeks.
All the best
Nick