Quote:
Originally Posted by Helder Crespo
I understand that this can also help with heat dissipation, but I still find the pitch removal procedure a bit risky. I've read reports of two different approaches: thermal (put it in the oven) and chemical (dip it in some solvent).
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As pitch ages the lower molecular weight hydrocarbons leave the pitch via evaporation. This shrinks the volume of the pitch and causes it to be less viscous and more brittle. So it hardens and cracks. These cracks & crevices in the material allow atmospheric water vapor to enter the windings. It also encourages corona discharges.
In addition, another reason for failure of this LOPTY is that the PL38 can acquire a positive grid bias due to the g1 coupling cap going leaky. This further heats the transformer.
Once the pitch has lost its low MW hydrocarbons and has become brittle, heating it won't cause the pitch to melt or flow. If you stick the transformer in the oven, or use a heat gun on it, all you will achieve is to further harden the pitch and bake it on and heat stress the transformer's insulation.
It is much better to place the transformer in a bath of solvent (mineral turpentine is the superior product) and let the pitch soften and dissolve over a few days, and then use a multiple varnish dip after that, either professional transformer varnish, or I have found that marine grade spar varnish is perfectly fine. (PS olive oil will not dissolve hardened pitch in your lifetime)
Never pick at the pitch, it will damage the windings, especially on the fragile EHT winding or "over-wind".
It is possible that the pitch coated transformer in your set might not need any restoration, check it for crevices and cracks in the pitch surface.