Re: [TCML] suggestions for nst cracks in potting

[bturner@xxxxxxxx]

Best thing is to simply unpot the thing. Get a disposable foil roasting
pan, and a cheap baking shelf. Put the baking shelf in the tray, set the
NST on the shelf upside-down, stick the whole thing in an outdoor BBQ set
on medium heat, and wait... The tar will melt and ooze out into the pan.
Let things cool down, then it's pretty easy to clip the transformer itself
out of it's box. Use another roasting pan, a cheap paint brush, and laquer
thinner to dissolve off the remaining tar coating. If you want, paint the
exposed HV windings with a good HV corona dope.

Often what happens is that the winding itself doesn't break down, rather,
a conductive path occurs in the tar itself, the current then chars the
tar, creating carbon, which then acts like a low-resistance short across
the windings.

Note, this only works for the transformers potted in the gooey tar stuff.
More recent NST's (such as Actown and TransCo) use an epoxy-based material
that won't melt.

- brent




>    Have a couple of nsts12/60 and 15/60. Opened the top and there are
> fissiors and cracks in tar. Probably due to the tar drying out over time
> and shrinking.  these cracks seem to be an easy path for arcing across
> windings internaly. Read Barts and others experience about repotting,
> etc.  Im interested in getting the most life out of them as they don't
> grow on trees!$!  Heating them up in a rod oven seems like the simplest . 
> Any more suggestions would be appreciated.
>   PS
>  Have built nice t-filter for the 15k that will be used on new TC.
>                                                                         
> Thanks, Bob
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