A modded Cossor 54

[peter scott]

That looks really superb Frank and certainly deserves to work really well.

Peter

[Murphyv310]

Amazing Frank.
I love the vacuum pot. It's so simple but no doubt very effective.
Well done.

[`FRANK.C]

Thanks Peter and Trevor

The first attempt at a vacuum chamber didn't work out. I used what I thought was a sturdy bowl but the vacuum pump made short work of it.
   

I was a bit concerned using the vacuum chamber. I thought that the air trapped between the windings might force the windings towards the edges of the layers. The wire could then drop down a few layers and cause trouble. Because of that I let the vacuum build up slowly. I don't think any of the windings have displaced. At least I cant see any at the edges.

One thing that was of concern was immediately after finishing winding it I measured the winding resistance which was 10k. After it coming out of the vacuum chamber I measured the resistance again and it had dropped to 9.7k. The first reading was taking at the end of a long day in the workshop which was good and warm. The second reading was taken the following morning when the workshop was much cooler. I am aware that the resistance of copper changes with temperature but this seamed too big of a change to be put down to temperature difference. I was afraid that while in the vacuum chamber some of the wires got crushed together and caused them to short. But testing the transformer showed no faults.

Over night I had left the transformer in a cooler room. This morning it measured 9.5k. I then left it in front of a fan heater until the core was warm to the touch. Measuring the resistance of the winding again it had risen to 10.8k. So I looks like it is down to temperature. I am just surprised at how much the resistance changed with tempature.

Today I tackled the HT transformer. The insulation was crumbling on its leadout wires.
Unlike the EHT transformer the laminations were varnished so there is very little rust on it. I had thought that I would have to remove the laminations to work on it. But there was sufficient distance between the windings and laminations to allow the removal of the insulation and cloth tape holding the wires in place. I replaced the wires and also the sleeving  covering the heater wires.
All windings test OK but I will give both transformers a long soak test.
   
Frank

[ppppenguin]

The temperature coefft of copper is approximately +0.4% per degree C. 9.7K to 10K is 3%, so a change of 7.5 degrees will give this.

[`FRANK.C]

Thanks Jeffrey for doing the calculation. That confirms it. There would be easily be a temperature difference of 7.5 degrees in the workshop between evening and morning.
A good lesson. At one point I was sure I had the winding damaged with the vacuum chamber and nearly wrote it off.

Frank

[`FRANK.C]

Just a bit of an update.
I have been working on the tag board that is mounted on the tube support at the rear of the chassis. The components are all at EHT potential.
This board had a hard life and was in a complete mess. There was components missing and some were sniped at one end. There was also strings of 1M resistors used instead of individual resistors.
I decided it was best to strip it and rebuild.

The capacitors were made from brown card. Which was printed on, sealed with french polish rolled up and then glued together with wood glue. hot melt glue was used to fill the ends.
I moulded and painted some dog bone resistors. Each one contains a 3.5 kV 0.5W resistor.
The yellow blob is a ceramic capacitor that was missing from the board. I could only find a surface mount type that had a high enough voltage rating. So I soldered on some wires to it and encapsulated it in hot melt.

Frank

[Murphyv310]

What an amazing Job you've done of that board. Are the resistors 3d printed and the replacement resistors just slid inside? If so that's neat.

[peter scott]

Really nice!

Peter

[`FRANK.C]

Thanks Trevor and Peter.
I made silicone moulds from the old resistors using this mould making rubber
https://www.ebay.ie/itm/254658945216
I placed the new resistors in the moulds, extending their leads if necessary and filled with this resin
https://www.ebay.ie/itm/254659007928
Both rubber and resin work well without the need for any special tools or vacuum chamber.
But it is slow with painting and everything.

Frank

[Jac Janssen]

Great job Frank.
Looks fantastic!

Jac