LOPT 'saturation' Ferguson 998T

[Heatercathodeshort]

Having a few hours to spare this afternoon I decided to have a go at finishing a 998T 12" Ferguson [1953] that has been hanging around in bits for around a year. The picture is brilliant for around a quarter of an hour and then the dreaded lopt saturation takes place resulting in a slow decline in the eht voltage and due to this sets self oscillating line output stage, loss of hold range. After around 30 mins the picture has faded away and very careful adjustment of the brilliance control will reveal a very poor dim raster.
Now this is a very common fault with vintage receivers that did not occur when these sets were in daily use. As a lad I handled many of these sets from just about every maker and I never came across this strange fault. The transformer was either s/c turns or it worked perfectly so what has changed?
The transformer in this set does not overheat but just gets warm as you would expect it to. I think I can rule out s/c turns. leaving the set to cool down results in the whole performance repeating itself. I played a hot air gun on the transformer and within 15 seconds the picture had vanished. Spraying freezer on the windings resulted in it slowly returning to full performance.
I can only conclude that the wax used to vacuum impregnate the windings 60 years ago has gone the way of the wax capacitors. It has either become damp impregnated or more likey changed in chemical make up resulting in a change of 'Q' of the transformer as the wax heats up.
Its very frustrating as a rewind is simply not worth the bother with a set of its type and a replacement transformer is unlikey to turn up and if one did would probably be in a similar state. I do have a Douglas wavewinder [underwind and overwind are wave wound] but to be honest I can't see myself stripping the transformer and taking all the data etc. Its just too much hassle, 30 years ago when I was young and eager, maybe. Regards, John.

[thermionic]

Hi John.
I am not familiar with this set or the construction of the transformer. Is it possible to remove all the wax impregnation by cooking in the oven for a while?

With the wax totally removed, try spraying the windings with WD40, letting this soak in for 24 hours, then drying off the residual WD 40 with a hair dryer.

After all this, spray the windings with a high voltage conformal coating ( RS used to sell this - I think it was made by Electrolube) and see what happens!!

This treatment was meted out to a very old Neon sign transformer many years ago as it was breaking down internally due to moisture ingress. The above worked in this situation - much to my surprise.

Good luck! Simon.

[llama]

If it's the dampness option you suggest, how about popping the transformer in the airing cupboard for a couple of weeks and then spraying it with something that won't attack the wax but seals against moisture? Vinyl emulsion followed by car spray?
Just an idea.
Graham

[Chris55000]

Hi!

This is a very strange fault that I can't say I've ever come across before!

The likeliest causes that spring to my mind are:-

1) Faulty LOP valve running into grid-current or alternatively overheating due to other causes! Make sure that any cathode-bias resistors or wirewound width controls are the value specified in the original circuit diagram.

2) Leakage in EHT rectifier;

3) Inadequate HT voltage resulting in the LOP stage having to work unnecessarily hard to maintain output.

4) Change in value of line-hold constants in LOP grid-circuit causing inadequate drive or negative self-oscillating GB to be developed.

5) Faulty linearity damping componenrts across LOP transformer secondary.

Check these before condemning the transformer itself - it doesn't necessarily suggest yours is dud!

Chris Williams

[ronbryan]

John

I owned a 14" console Ferguson 994T schedule C as my bedroom set in the 1960s/70s (it did have a band III tuner added!) and it was my favourite mono tv. As I'm sure you remember, this model in its original schedule A version had a self oscillating line timebase, which was replaced in later schedule letters by an ECL80 multivibrator line oscillator driving the PL81 line output stage. The Trader sheet 1131 quotes poor EHT regulation in the self oscillating version as the reason for implementing the modification.
I am assuming that the 998T uses a similar line output stage, but could the problem be attributable to a lack of line drive as the transformer warms up and becomes less efficient? Presumably there must be a cumulative effect as the losses mount and the positive feedback from the LOPT becomes less and less.
Is there any space on the chassis to try a separate line oscillator stage, to see if a constant line drive signal gets around the problem?

Ron

[Steve_P]

Change everything in the area, and the valves. Also a quick check on the Power Supply.

What would happen if you replaced the winding + EHT valve with a tripler circuit? From the Anode of the Line output valve to tube.

Cheers,

Steve P.

[Heatercathodeshort]

Thanks for all your suggestions but alas it has all been done! I have had this fault on many sets and I'm surprised it has not come up before. [I'm pretty sure it has in fact.]
Hello Graham, I had thought of that one re the airing cupboard. A mate of mine keeps his magneto from his Fordson tractor in the airing cupboard over the winter and never has any trouble with it! Hello Steve, I think the problem is in the overwind as you suggest.
Hello Chris, Thanks for your input. All done!.................
It looks in good shape as far as the windings go but I'm still thinking that wax has something to do with it. I have changed the valves and all the components in the line output stage. These sets did work very well and were one of the most reliable sets produced at the time. DER rented thousands of them. The line sync tended to 'trigger' a bit on odd signals but was otherwise stable in a good signal area. It may have caused problems in high interference areas. I think I'll stuff it back under the bench for another year or so......It might vanish..Regards and thanks, John.

[chipp1968]

I recon try removing the wax etc . If it works properly after that then It will have been a good experiment

[murphyv310]

Hello John.
Indeed this is a common problem with vintage sets and as you say I am surprised not often mentioned on the forum.
My TV22 did have a similar problem but it was due to rusty laminations, but leaving the LOPTX in a bucket of WD40 for two weeks cured the problem.
The V4 Ive just done also had the problem you have and with this I removed the transformer and melted off the pitch very carefully, after that the transformer was put in the oven for 4 hours at 90 degrees "C" the transformer then was soaked in WD40 for 48 hours. I then refitted it and the set run on test for a full day with no problems, I then re-pitched the transformer and have had no problems since.
Trevor

[dominicbeesley]

I think this might be what's happening to my TV24. Do I need to strip off the pitch before putting in the WD40 or will it disolve away - I'm scared to death of tearing off a winding by accident!

Dom

[Heatercathodeshort]

Thanks again for your interest. Its definately the overwind that is effected as it gets warmer than it should but the winding is in very good condition. I will have a go with the WD40 as suggested. Nothing to lose and if all else fails I will have a go at rewinding the overwind. It may take some time so don't expect a result overnight.....I'm glad that I am not the only one to suffer this problem. I'm still certain that its a damp problem that has actually changed the character and make up of the type of wax used in the 40's and 50's.
I have attached three pictures with a time clock so that you can see the effect if you have not experienced it first hand...You will...
Ist pic is at switch, on EHT 7kv. 2nd after 15mins, width is starting to decrease. EHT 6kv. 3rd pic just after 25mins. Width and picture collapsing, EHT 5kv. It remains at a similar level after this period. The CRT is a Mullard MW31-74 and like every other component and valve in the set was original up to the point of repair by myself. The PY81 is the early type with the ceramic cathode that takes up to 5 mins to warm up! Regards, John.

[chipp1968]

John ,It would be good if you could do the experement with the lopt .if it works ,it will be good ref for future problems that people will encounter

[dominicbeesley]

Hello all,

I'm going to try this out on my TV24 loptx this weekend and will report back with success or failure:

1) Remove pitch
2) Bake in oven to remove any wax
3) Bathe in WD40 for 1-2 days
4) Dry off
5) Spray with anti-corona lacquer RS 569-313

Please, before I break or irreversably damage anything any advice:

1) How should I remove the pitch?
I was going to try heating up to ~50 centigrade then peel it off. Then wash off any residue with petrol - will petrol damage anything else, should I not bother with petrol?

2) What temperature and for how long to bake in the oven, I want to make sure I drive off as much moisture as possible *without* worsening any dodgy insulation

3) Do I do all this with the core laminations in place?

4) How dry does it need to be before spraying with the lacquer?

Any other suggestions on how to proceed would be good.

Dom

[murphyv310]

Originally Posted by dominicbeesley

Hello all,

1) How should I remove the pitch?
I was going to try heating up to ~50 centigrade then peel it off. Then wash off any residue with petrol - will petrol damage anything else, should I not bother with petrol?

Dom

Hi Dom.
I removed the pitch with a good hairdryer at maximum heat, it does take some time but the majoriity will come off this way, WD40 also helps to dissolve it.
My oven was at about 95 degrees and the last residues dripped off, while the tranny was hot it was then submerged in a pot of WD40, good motor factors will sell you 5 litres of WD40 for around £15.
If the set is kept in a dry place the repitching isn't so important and will also keep the windings cool, if you do re-pitch only use a very thin coating evenly applied this will stop cracks appearing. I have also used waxoyl underseal it looks good and seems to work ok.
All the best.
Trevor

[HamishBoxer]

Forget the petrol,dont sound like a good idea at all.Put loptx in the oven at low and try bring up temp till pitch melts might be safer.

David

[Heatercathodeshort]

Hello David,
Thanks for the suggestion. Thats exactly what I had in mind and will have a go soon. If all else fails I will leave the windings in a vat of 'Duck Oil' and clean it up before refitting. Regards, John.

[Heatercathodeshort]

Hello again,
Over the weekend I did a lot of experimenting with the 998T lopt.I have been convinced all along that the problem is damp in the windings, particularly the overwind. This model uses a very well constructed transformer and was known as the 'coffin' to those that serviced these receivers. It consists of a black bakelite box shaped exactly like a coffin complete with clip on lid with the EY51 laying in state underneath! [tombstone shaped into the bargain]
The overwind is impregnated with softish wax similar to the type used in our little wax capacitors that we are all so familiar with. The overwind is finished with a Pitch 'Tyre' to prevent flash over from the top of the winding. Flyback transformers were working on the edge of technology at the time and anything that damped the operation at 10kcs caused a total collapse. I had doubts about that Pitch tyre and wondered if it had obsorbed dampness over the years and if this would act like a shorting turn around the winding.
At this point the madness started and I decided to see if I could introduce the fault by other means. I cut a small length of cotton material and soaked it in salt solution wringing it out to leave it damp. I then with considerable difficulty....wound it round the Pitch tyre tightly.
Switching on resulted in a picture lacking width by about 1" either side and replacing the cloth tape with a single shorting turn of wire produced a complete collapse.
This seemed to prove that as little as a dampened tape could reduce the efficency of the transformer to show the fault in the pictures.
I removed the transformer from the set and with great care played the heat from a hot air paint stripping gun over the tyre removing it as the Pitch began to soften and 'glaze'. I managed to cover the entire tyre and at one point I thought I had overdone it as the Pitch started to smoke a little.....
I fitted the transformer back in the set when it had cooled....Low and behold even after several hours use its still holding up with 7kv EHT!
Time will tell and this seems to back up the comments that Trevor and Dom have experianced.
Hope this is of interest.
Regards, John.

[chipp1968]

Thats interesting .Alot of people may overlook such a simple solution to an elusive problem ,which is why this sort of info is very important

[Heatercathodeshort]

Hello finally with this saga,
100% proved! The Ferguson 998T LOPT has been well tested quite unintentionally...... I turned the set on in the workshop this morning to see how it was holding up, that was at 07.30. I completely forgot about it and went off to the garage as normal....A mate of mine came around this evening and as he was about to leave he looked into the workshop window and commented on the good picture on the old telly...... Yes it has been on for over 16 hours and remains brilliant! I could hardly believe it and if he had not said anything it would probably have been on all night and most of tomorrow! It was only the fact that the area was in complete darkness that he noticed it.
I have just rushed a few pics to prove it [10.15] It looks like the reason for LOPT saturation has been proved. I have slightly offset the focus adjustment to reduce strobing from my digital camera. The EHT is now a constant 8kv and you can see the raster now over scans the screen. Regards, John.

[chipp1968]

Brilliant John . Good job done ,and a point proven