A modded Cossor 54

[`FRANK.C]

Hi Mike
I tried plumbers PTFE tape years ago. But the result was not good.
One of the problems was that the tape was soft and the ECW tended to cut through it or bury its self into it reducing the thickness of the tape doing the insulating.

Again years ago. Before the internet and before I had access to proper tapes. I did some tests on the breakdown voltage of PCB tapes. There was a huge difference from brand to brand.
I found one that was good and used it on a pulsed EHT transformer to insulate between layers. The transformer worked 24/7 for years before eventual the PVC tape broke down. At the point of the breakdown there was about 3.5KV across 4 or 5 layers of tape. I think part of the cause of the breakdown was the ECW over time sank into the tape reducing the thickness of tape between layers.

Frank

[Mike Watterson]

I suspected PTFE pipe tape is too soft. Ordinary PVC tape is a bit mis-named as "insulating tape"!

[Freya]

My rewinds have been successful using 0.03 mm Kapton tape, used it on all sorts including overwinds.
Dad used to work for the company way back, producing PTFE on reels we know today, he solved the toolmaking to allow this to happen, so I'm told.
It was amusing as a child as we had five or six black bin bags in our loft full of prototype metal reels full of the stuff, it rarely came off correctly but sometimes we could run down our very long garden streaming the stuff as we went.

[`FRANK.C]

I have ordered some ECW for the EHT winding and while waiting for that to arrive I am getting on with some other things.

Over the weekend I built the EHT reservoir and smoothing capacitors. I for the most part followed the way Brian Cuff built his in this thread.

I added 44M (2 X 22M 3.5KV 1W) bleed resistors within each capacitor. I kept the resistance high as I didn't want them to unduly load the supply.  It will take a good number of seconds for the capacitors to discharge via the internal bleed resistors. They are really there as belt and braces as the divider chain for the focus control is the main EHT bleed. 

The originals were replaced with Visconols. They both should be 250nF. The reservoir was the correct value but the smoother was replaced with a 100nF and the resistor connecting reservoir and smoother was increased from 250k to 500k. As I go through it I will restore all to their original values.

Frank

[Freya]

Nice job on those capacitors.

[`FRANK.C]

Hi Stephen
They look more acceptable that brightly coloured modern ones.
The ones that I fitted were 6KV 0.22 uF + 0.033 uF.
The 0.22 uF cap is the largest film cap that I have seen. I had to shave the corners off to get it to fit inside the 75 mm down pipe that I used for the replicas.
Photo below of one shaved capacitor that will just about fit and another waiting to be done.

There are three 0.025 uF 4500V capacitors that will need replacing. Two look original even though one it nearly twice the size of the other.
These two can be restuffed. The other one will need replicating as it was replaced with a Visconol.

Frank

[`FRANK.C]

I got around to finishing the EHT transformer this week. It has been a long time since I started it but just hadn't a lot of time to spend on it.
The laminations were quite rusty. I knew as they were, they wouldn't all fit back. So I removed the rust and in doing so removed the insulating layer. So I gave each lam a light dusting of black spray paint. It took a considerable length of time to do all that.
All that time I was considering how to wind the EHT winding. I was leaning towards building a multi section former for it as it would avoid having to use insulation between layers. But in the end I decided to wind it as it had been wound in a single section. I wound it with 0.08mm ECW and used Polyester tape between each layer. I built a former from Perspex. The inside width of the former work out at 25mm which was convenient as the tape width was also 25mm.  It was a slow job. I would do a few layers in each session and a few sessions each day. I got it completed in 3 days.
After finishing the winding I dipped it in transformer varnish and while in the varnish placed it in a vacuum chamber. This helped the varnish to penetrate deep into the windings.

The heater winding for the EHT rectifier is also on this transformer. As it sits at EHT potential I though it prudent to replace it. It was replaced with some narrow 10kV wire that I had.

I reassembled it and everything seams to be OK.
I need to camouflage the new winding but I will leave that until the varnish fully hardens.

The photos below are of the painted lamination's, the former, part way through winding, the completed winding before dipping, My vacuum chamber made from a pot and perspex sheet and the last two are of the completed transformer.

Frank

[ppppenguin]

Now that's a real labour of love. I would have asked Ed Dinning or Mike Barker to do the job and paid them accordingly.

[Jac Janssen]

Excellent work Frank!

Jac

[`FRANK.C]

Thanks Jeffrey and Jac
Hats off to Ed and Mike what a fantastic service they provide. I don't think we realise fully the amount of work that can be involved in a rewind.

Frank