Mains transformer winding tails with perished rubber insulation

[BillDWVA]

Hello, I am attempting to restore and re-use a mains transformer that has flexible stranded wires leading from its end cover grommets. Now, I’m sure that most will tell me to just weigh it in for scrap as it is from a Vortexion tape recorder and these are known to be prone to becoming toast.

However this transformer is actually OK at the moment and the primary windings have  been flash-tested to the frame before disassembly and show no breakdown. 

The question is about trying to cut and strip back the brittle, and in some cases, softened rubber and to clean up the stranded copper tails enough to accept new pvc wire soldered on and sleeved with heat shrink. In the attached picture I have done one of these tails.

As the rubber has perished it seems to have leached a corrosive chemical that has made the copper brittle, or is is just from the heat in the transformer? I have noticed that some of different coloured rubber is more prone to this effect than others. Black insulated rubber being the worst - in this transformer and elsewhere in the set the black coloured wire literally crumbles.

I am trying to solder onto a fragile and crumbling stump of tail - i’m guessing that it is in this state deep into the encapsulated windings? Maybe this is why these Vortexion transformers go pop?

Has anyone got experience of attempting this (bodged?) repair? Or should I just bin it?

I have fitted my old WVA recorder with another non-Vortexion mains transformer but it was too big to fit in the chassis and is mounted in the case under the motor board so the machine is not original now.

Bill

[Refugee]

Can you find something that dissolves the goo?

[BillDWVA]

The rubber insulation has actually become brittle and cracks and splits if the wires are flexed rather than turning to goo. Sometimes just disturbing them causes cracks and exposed copper to appear. The trouble is that the insulating properties of this rubber is questionable now. I am hoping to solder on new tails as close to the windings as I can and sleeve as much of the original tail as possible.

Bill

[Ed_Dinning]

Hi Bill, it is sometimes possible to oversleeve the wires with modern heatshrink sleeving which cures the peoblem of brittle rubber. Hopefully there is no internal movement on the rest of the wire inside the teansformer.
If all else fails the transformer can be rewound.

Ed

[Refugee]

I was thinking of injecting insulating varnish down the sides of the tails to stop the old rubber causing any more corrosion of the coils.
That should stop the old rubber getting into contact with the windings.

[boater sam]

A ticklish problem.
I would try to stabilise what remains of the tails by drawing a diagram of the position and where they connect, then cutting them back to short stubs, just enough to solder to. 
Use either liquid epoxy applied 2 or 3 times or pot the whole transformer in polyurethane varnish by dipping again 2 or 3 times.
Then you should  have stubs to solder new thin insulated wires to that won't bend and break off.

[BillDWVA]

A final instalment and update on this transformer which I got around to finishing a repair on. Following advice I carefully soldered new tails onto the stumps of the old and over-sleeved them with heat shrink. The wires follow the colours of the originals but are now multi-stranded equipment wire of 0.5mm sq. section.

I’m going to get it flash-tested before putting it into service. I estimate that this transformer is approximately 100VA which is a bit on the low side for the loads from the chassis, in my estimation. Maybe another reason why these transformers have a reputation for failing?
   

[Diabolical Artificer]

Nice job Bill. I would have had a go at annealing the the old leads before cutting and soldering, too late now though.

Andy.