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Old 19th Feb 2021, 11:13 am   #333
beery
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Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
Default Re: Baird T5 restoration project.

Hi all,
I've now repaired the Baird T5 line output transformer.

The outer (synch injection) winding had been chewed by some wee beastie.

I took off the laminations to get to the windings. The laminations are T and U stampings, which were alternatively arranged as in a mains transformer, this is unusual in a line output transformer where we would expect the laminations grouped together and gapped.
The outer layer consisted of 10 turns of 36SWG (0.19mm) SSC (Single Silk Covered wire) which was no enamelled. A few of those turns were broken in the same place.
I unwound the damaged layer and found 33 healthy turns. 3 turns from this layer were unwound to provide a good lead in to a wire splice. I then covered the good layer back up with craft tape, cut back the wire to about 1 turn and spliced on some new 36SWG enamelled wire. I then added another 12 turns to get back to where we were before. As it was the final layer, there was space to spread the wire out a bit, which would give more even support to the lead out terminal.
Some transformer tape was used to stabilise the windings, but I was careful not to let it go too near the edges of the craft paper. So that the bright yellow tape would be hidden when the repair was completed.
A layer of craft tape was added and the terminal was connected and secured with fibre glass tape. More craft tape was added over the top.
Finally about half of the original varnished cloth outer insulation was glued back on. The damaged insulation on the top part of the transformer was replaced by similar material that was recovered from a scrap GPO 3000 type relay.
I will post the transformer back to David next week.

Some observations on the transformer design:-
Very unusually proportioned laminations made for a narrow, but thick coil, i.e. many layers, each with a small number of turns - this makes for a transformer with low inter-turn capacitance, great for responding to the rapid flyback of the line scan. Single Silk Covered wire has a slightly larger overall diameter than enamelled copper wire and this will have assisted in reducing the winding capacitance. For VHF coils often Double Silk Covered wire was used in pre-war sets.
Anyway, the use of a narrow but tall winding Single Silk Covered wire can be considered an alternative to wave winding a line output transformer. As I had only to replace 12 turns on the outer layer then enamelled copper wire slightly spaced out will be fine. If I had to rewind the whole thing then another approach would have to be considered. Synthetic silk covered wire is available, but it is very very expensive.

Cheers
Andy
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