TV22 lopty restoration

[Argus25]

I just put a new speaker in my TV22 and was able to take a photo of the line output transformer. (attached)

At the time I worked on this transformer I had no idea of the troubles these were giving folks restoring these sets in the UK, with energy absorption issues.

Of course like all of these it was covered in old dry pitch which had deep cracks and crevices in its surface. I had tried melting pitch before on other components and knew that wouldn't be a good method without risk of damage. The pitch was also very brittle and picking it off wasn't an option.

However, I knew from my early experiences with pitch, on old car battery tops, or tar I scavenged from the road for various projects in my childhood, that pitch dissolved well in mineral turpentine. (Apparently this is called white spirits too). Also mineral turps is fine on all the substances in the transformer but it will dissolve many modern plastics.Turps is also a common solvent and compatible with many varnishes and oil paints as a solvent.

So I placed the transformer in a bath of turps. Over about 2 to 3 days the pitch melted off. It could be encouraged with a soft artists brush, but putting it in a fresh bath it was easy to see the pitch leaving the transformer into the solvent and the process appeared to speed up with time. Then I let the transformer dry for a day.

Also years ago I had seen an EHT overwind in a Marconi monitor that was heavily varnished and I liked the look of it, especially compared to how horrible pitch looks.

I knew the pitch would have to be replaced with a good coating to prevent corona discharge on the overwind's surface. So I dipped it repeatedly in Marine grade spar varnish, which I knew was ok, as I'd used it in many transformers & high voltage applications before.

The attached photo shows the transformer in the EHT cage. Looking closely at the overwind there are still some black flecks or staining of the pitch there, in the overwind's crevices, but obviously over 99% of it has gone.

(The extra item in the cage is a filter cap on the EHT , I put that there because the replacement CRM type CRT had no external aquadag)

This transformer has been working flawlessly for years. So I think that I inadvertently discovered the best way to restore these to keep the transformer original, without requiring a rewind and without risk of damaging the transformer.

[HamishBoxer]

That is so clean.

[peter_scott]

Originally Posted by Argus25

At the time I worked on this transformer I had no idea of the troubles these were giving folks restoring these sets in the UK, with energy absorption issues.

Did your overwind have energy losses that were resolved by stripping the pitch and varnishing?

Peter

[Argus25]

Did your overwind have energy losses that were resolved by stripping the pitch and varnishing?

Since the pitch on the transformer and many other parts in my set such as the caps etc were in such poor condition I wasn't game to power the set until after it was restored. So I do not know if my particular transformer had the absorption issue prior to replacing the pitch.

My approach for it was a total re-build rather than powering it initially and tracking down defective parts. One reason for this was significant chassis corrosion, nearly all the wiring had crumbling insulation, the valve sockets were corroded and most of the caps defective.

However it certainly wouldn't have been a wise proposition to have powered it with the cracked pitch anyway even if the rest of the set was ok, so the lopty needed some sort of treatment.

Given that all the transformers I have heard of so far that get the energy absorption issue were pitch coated (is that correct is it only pitch coated ones that do it ?) its probable then that my transformer could have got the issue too.

Does any one have an idea roughly what % of these transformers get the energy absorption issue ?

[Al (astral highway)]

Hey Hugo,

I checked out your link on another thread to your finished TV22 restoration and it looks factory-gate pristine and just incredible. It must be wonderful to wait and wait and slowly do all the things needed, without being tempted to rush, and then finally to get to this level of perfection.

Wonderful work indeed.

[Philips210]

Argus25, that LOPT cage looks immaculate, I bet that's cleaner than when it originally left the factory!

I find the worst part of restoring old equipment is dealing with nicotine tar staining, it's such a mucky nasty old job.

Regards
Symon.

[Ed_Dinning]

Hi Argus, the TV22 overwind is generally pretty good on these sets. the main mode of failure is the primary windings overheat and develop inter-turn shorts.

Rusty lams do not seem to give problems, but the primary winding layout is quite critical of the transformer tuning is all to hell.

Ed

[Argus25]

Ed,

The interesting thing about the idea of shorted turns being the cause for the energy absorption would make more sense if there was a reduction in EHT of some fixed amount.

But some of these transformers have gradually altering performance depending on how long the set is switched on ? (that is what I had heard)

It is hard for me to say because I have never investigated the effect myself. If I did I probably would put a shorted loop around the transformer to see it it caused a similar effect. In these ones the magnetic circuit is not closed, there is a very high reluctance between either end of the core. So the effect would vary as to where the shorted turn was placed with respect to the length of the core and the position of the other windings, unlike the situation with a closed magnetic circuit.

[Ed_Dinning]

Hi Argus, yes the open magnetic circuit will alter things a bit.
The effect may be carbonised paper whose resistance drops with heating (time). It certainly reduces the EHT but has little effect on the sound of the LTB.
It could be that the transformer just becomes very lossy and goes "off tune". Rewinds are certainly very critical of position of windings with respect of each other.

Earlier transformers had "T" shaped lams, later ones straight lams, no difference seen with the fault on either lam type.

Ed