[Boatanchors] Baking Transformers

[Carl]

I use a similar method on transformers over around 20#.

I place an ampmeter in series with a short across the winding that appears 
to be at the core, often the primary. And then run the variac on the 
secondary slowly up to about half the current ratings. This drives moisture 
out from the center. Then reverse the procecess if its a big transformer.

The last big one I did was a 7500VCT  1.5A 150# hunk of iron built in 1946 
and most recently stored in an open shed for 25 years. I let it cook totally 
for 4 days, hipotted it and then applied power with a 500ma load and 
amazingly it was fine.

Heated it up again, hoisted into a homebrew pressure pot, poured in 
transformer varnish, secured the top and pressurized to 15#. Its been runing 
my AM KW+ for almost 2 years now.

With something in the 30-50# range it takes about a day each direction. 
Thats probably overkill but its not something I have to watch or wait for 
the family to leave so I can use the oven. Naturally both windings are 
fused.

Carl
KM1H


----- Original Message ----- 
From: "John Lawson" <jpl15 at panix.com>
To: <boatanchors at mailman.qth.net>
Sent: Monday, November 02, 2009 7:32 PM
Subject: Re: [Boatanchors] Baking Transformers


>
>
> On Mon, 2 Nov 2009, Mark K3MSB wrote:
>
>> Recently acquired an SB-200 parts unit from a 10 year stay in a pole 
>> barn.
>> Power tranny tests OK for shorts and continuity etc.
>
>
>   You might want to consider removing the rectifiers, and putting a
> suitable power resistor across the HV leads, then running the unit up on
> the variac at a percentage of full input power, and let the transformer
> bake itself. The resistor will load the HV, and the remaining tubes will
> load the filament windings.
>
>  I've used this method several times on large HV power transformers;
> afterward they all showed perfect on the hipot tester.
>
>
>   Saves putting the rig in an oven and endangering the "soft" parts,
> drying out already-old rubber, etc...
>
>
>   Just my 200 millidollars...
>
> Cheers
>
> John KB6SCO
>
>
> PS: the filter caps are most likely toast - kits are available, or Digikey
> or Mouser have (much smaller) replacements, and I took the opportunity to
> use units with appreciably more capacity.
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