KB VC1 width problem

[Heatercathodeshort]

Some years ago I did a lot of experimenting with a Ferguson 998T transformer that was suffering from this fault very badly. It is still working OK today after having been dried out over a period years back. I can't find the thread in the search but to be honest I didn't try hard enough!

It appears that the overwind absorbs dampness over a very long period of time lowering the Q of the transformer.
On examination the transformer overwind is insulated with pitch with of course is hygroscopic. It must be something to do with the transformer losses that cause the overheating which if left unattended can reach such levels that the overwind breaks down.
A single turn of wire placed around the core of the transformer and shorted will have a similar effect.
It may also be due to changes in the interleaving tissue between the winding layers if employed in the overwind which I believe in the VC1 they are.
It's a combination of age, dampness and changes in Q [efficiency] of the transformer over many decades.
A transformer rewind cures the problem. John.

[Argus25]

Originally Posted by Heatercathodeshort

It must be something to do with the transformer losses that cause the overheating which if left unattended can reach such levels that the overwind breaks down.
A single turn of wire placed around the core of the transformer and shorted will have a similar effect.
John.

It would be good to figure out what was happening to these old transformers.

You mentioned the word "saturation" on a previous post but were non specific. With magnetic B-H saturation that doesn't happen easily with ferrite cores especially if the core halves have a gap too. For electrical saturation then over the course of the line scan, the rise in current with time would come non-linear toward the right side of the scan and there would be insufficient inductance. But if the linearity on the right was ok, that doesn't seem likely, so maybe saturation is not the word.

If it looks as though its acting like it has shorted turns, but the effect happens slowly, it can't actually be that because the effect would be staccato not gradual.

In this case also, heating makes it worse with time.

If rewinding the transformer fixes it and as you say the overwind breaks down, then perhaps its energy absorption due to absorbed water in the overwind and nothing to do with any saturation issue but as you say it would lower the Q, that would tend to have even more effect on the EHT than the scan width.

When transformers are coated in old pitch, you can drop them into a container of mineral turps. Each day brush them with a fine soft brush. After a few days all the pitch melts and washes off. Then it can be dipped a few times in marine grade varnish to give it a thick coating. I did this for my BUSH TV22 transformer some years ago and its been perfect since that time and it also gives the transformer an excellent appearance and protects the windings from infiltration of water.

[Niechcial,Steve]

Thanks for the comments. Just to recap-
HT, drive waveform and drive bias remain constant.
Boost falls with the width from about 650V to 560V,
All resistors and caps. (including boost cap) checked by substitution.
Boost feed to brightness and focussed networks disconnected on rare possibility something there dragging down the boost.
It really can only be the LOPT, but I would like to understand the mechanism. I've tried disconnecting the overwind input and the boost still goes down. The overwind reads correctly as per the manual when cold- 60R.

[Argus25]

Can you post the schematic of the line output stage ?

[Argus25]

I just had a look at the varnished transformer in my TV22, it still works fine. I wonder if the problem might simply be the pitch itself, which could turn into a lossy dielectric with time. If that is so then it wouldn't be necessary to rewind any of these old Loptys, just replace the pitch with varnish using the method I did for my TV22.

[Heatercathodeshort]

Originally Posted by Niechcial,Steve

I've tried disconnecting the overwind input and the boost still goes down. The overwind reads correctly as per the manual when cold- 60R.

The overwind has to be removed from the transformer, not just disconnected to prove the saturation problem.

I have had a couple of KB transformers fail on the underwind or better known as the primary winding, two in recent years a RV10 and a RV20. They use a similar transformer to the VC1 construction wise. There may be a problem here but it is far less common.

Resistance readings can vary widely with LOPTs even when operating normally so unfortunately correct readings can be misleading

Knowing the ITT/KB chassis from the VC1-VC100 there is very little to go wrong and the reliability factor is beyond reproach. Other than a few resistors that go high in value and the odd s/c cap they soldier on for ever.

I would like to know how much the EHT drops. I guess around 2-3 kv over say an hour.

As I mentioned earlier I never experienced this drop off when servicing any make of what we now call vintage receivers in their hey day. The fault simply did not occur.

It is an age related fault that I feel is due to long term storage under poor conditions. Internal rust, breakdown of interleaving tissues, damp ingress due to the hygroscopic pitch insulation, disintegrating core gap tabs or changes in the characteristics of the core material, likely with pre 1952 pre ferrite construction or even the wire itself.

I am not a scientist or have the instruments necessary to dissect a transformer but can only put to 'paper' what my personal experiences have been over many years.

Of course Australia has a much drier climate than our own rather cold damp one! This frustrating symptom may never occur in a hot dry climate.

The UK receivers were dried out with daily use and it was only when they were stored for decades under poor conditions such as lofts and wet sheds that the fault reared it's ugly head.

Circuit of the VC1 timebase below. I think you have checked/replaced most of the relevant components Steve.

One resistor that is frequently overlooked is the anode load to pin 6 of the ECC81 line oscillator 56K. It can go high resulting in low drive but this is usually displayed as slightly low width that does not vary with time.

Regards, John.

[Heatercathodeshort]

Just to add I have boiled off the pitch at high temperature when experimenting with the Ferguson 998T transformer. It did work very well but unfortunately it is not a similar story with all makes of LOPT. It really is a matter of luck.

There are many makes of old LOPT's that have survived without any problems. Early Philips 1100 [1951] right through to the dual standard models up to the Style 70 and any Ekco produced at the Southend factory.

Ekco did not use pitch. They were absolute experts in plastic materials [they did have a problem with some transformer cases but the windings themselves were 100% reliable]

The EHT overwind in Ekco receivers was finished with a very small plastic 'tyre' eliminating the pitch problem.

[Argus25]

John,

Thanks for posting that schematic.

Also I agree that the overwind would have to be removed, not just disconnected to see if it was the culprit and absorbing energy.

I also would assume that both the line output tube and efficiency diode would have been replaced twice, with at least one attempt with NOS parts to confirm the ones in these are ok.

Also has the line output tube's screen resistor been replaced with a known good resistor ? Any even small variation over time of the value of this part would lower the scan width, if it went a little high with increased temperature.

At least with these pitch coated transformers, its very easy to get the old pitch off in a mineral turps bath without damaging the transformer at all, its just a matter of being patient and waiting over a few days for the pitch to melt into the solvent.

[peter_scott]

Originally Posted by Heatercathodeshort

Some years ago I did a lot of experimenting with a Ferguson 998T transformer that was suffering from this fault very badly. John.

I wonder if this was the one:
http://www.vintage-radio.net/forum/s...highlight=998T

[Argus25]

While we are waiting to prove that the overwind is the trouble...Another thing that might not be too obvious:

Since with time the width only comes in a little on each side (I'm not sure about the size of your screen) it might be that if the peak drive voltage to the line output tube control grid was in fact changing over the 20min, it might not have been very obvious on a scope trace of the grid drive voltage.

Also any very small leakage in the input coupling capacitor to the grid of the line output tube could result in a current via the lopty that would alter its properties. It might just be worth double checking those things too, just on the off chance.

[Argus25]

Originally Posted by peter_scott

I wonder if this was the one:
http://www.vintage-radio.net/forum/s...highlight=998T

I just looked at that great thread and it appears that the correct chemical solvent for pitch, which is mineral turps, was not tried. Its much better to dissolve it off slowly with this than heating it in attempts to melt it off. Also the turps bath removes all the pitch as it migrates out of the windings and other hydrocarbon like wax substances that have impregnated the windings diffuse into the bath. Then it is best to apply varnish to protect and insulate it, not new pitch. You can use transformer grade varnish with repeated dips to get a thick coat, but I found that marine grade spar varnish works perfectly well.

Also, as I recall, Steve McVoy of the Early Televsion Foundation reported that the overwinds on EHT RF power supplies used in a lot of American electrostatic TV sets using the 7JP4 CRT suffer a similar problem, and the EHT falls away.

[Nuvistor]

A very interesting and successful repair of a RBM LOPTX in this thread on the Chris’s archived radio and TV forum. Lots of skill and patience went into the repair.
http://www.forum.radios-tv.co.uk/vie...hp?f=5&t=11844

[Niechcial,Steve]

I have tried to post the schematic, but the system is telling me it's too big for the forum to upload it as an attachment. Is there another way to do it?
As has been pointed out, the VC1 does not have the stabilised line drive feedback network which are on the VC2 onwards, so the circuit is quite simple.

I will try and post a picture of the LOPT later. There is are visible joins on the core, unless the joins are hidden by being inside the two coil formers

[1100 man]

Hi ,
That thread of HCS's about the Ferguson 998 LOPT makes interesting reading. Interesting too that just a lightly damp piece of cloth round the TX has almost the same effect!
I like Hugo's method of dissolving the pitch in 'mineral turpentine'. This would appear, after a quick internet search, to be what we know as common or garden 'white spirit' over here.
All good food for thought!
All the best
Nick

[Heatercathodeshort]

Originally Posted by peter_scott

I wonder if this was the one:
http://www.vintage-radio.net/forum/s...highlight=998T

Hello Peter and thanks for finding that thread!

I can understand Argus's doubts about the cause of this. It may never occur in a very dry country and I doubt if many vintage receivers exist in Australia having set up a television service in the mid 50's. By this time all LOPTs were constructed with high efficiency ferrite cores.

It really is a frustrating fault but at least we have Mike Barker to carry out first class rewinds to a higher standard that new for us. Regards, John.

[dazzlevision]

Originally Posted by Niechcial,Steve

I will try and post a picture of the LOPT later. There is are visible joins on the core, unless the joins are hidden by being inside the two coil formers

The core consists of two “U” sections, which abut under the two windings.

[FERNSEH]

The first attachment shows the circuit diagram of the line timebase in the Kolster-Brandes VC1 chassis as fitted in models VV10, VV20 and VV30, also in many WVxx models. The second attachment shows the stabilised line timebase employed in the VC3 chassis. VC2 is similar.

DFWB.

[Argus25]

Originally Posted by Heatercathodeshort

I can understand Argus's doubts about the cause of this.

I don't doubt the cause at all, especially after reading about the multiple cases of it that have been reported in vintage UK made sets. I think though that if the pitch and other waxes are dissolved out with the turps, then it may not be necessary to have to do the rewinds. Its best to keep the transformer undamaged and original, if possible.

When I got my TV22, the pitch on the transformer was very dried and cracked, I wasn't game to even power the transformer for a test because of the deep cracks and the surface of the pitch was infiltrated with dust & dirt. So I never got the chance to see this fault myself first hand, because this got sorted out before I powered the set.

[Niechcial,Steve]

Fascinating debate! Thanks for the core info Dazzlevision. Should I just be able to pull it apart?

[dazzlevision]

I would refer you to post #15.