Further to my report on the failed combiner/output transformer in my
Decca QRO Tx, I have dismantled the transformer. The really good news
is that the huge (85mm dia) ferrite pot was not glued at all, and was
easily unclamped. The internal wires were more difficult, being
wrapped with cloth tape that seems to have been soaked in hard glue.
I finally got to the secondary coil and found the shorted turn. That
coil comprises three layers wound on top of each other, but the cloth
wrapping was inadequate in one place, resulting in one turn from the
top layer being adjacent to one on the bottom layer, and thus
providing a higher voltage gradient than intended and shorting
several turns.
So all I have to do is to improve the construction (to avoid the
voltage gradient) and re-assemble. I will wrap some sleeving over the
slightly charred wires where the short occurred.
I need some advice on what to use to replace the glue-soaked cloth
tape. It needs to fix the turns and provide a high degree of
insulation. I could use ordinary plastic tape but it may suffer from
possible high temperatures (It is physically difficult to check
whether the transformer gets hot whilst transmitting for long
periods, but it's certainly likely).
I have looked at the Maplin catalogue and they do "Silicone Self
Fusing Repair Tape" which seems to resist high temperatures (up to
260 deg C) and high voltages (it actually says "7,000 volts/20mil
(0.51mm) dielectric strength", which I don't really understand).
Should I replace the wire, or perhaps just its insulation? It is
solid, enamelled and insulated with some brown cloth or paper
(similar to the glue-soaked cloth) and is in good condition away from
the fault. Outside diameter is 2mm.
Any suggestions or advice would be gratefully received.
Mike, G3XDV
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