[AMRadio] Drying out HV transformers / Chokes


Larry Will lhwill at verizon.net
Sun Sep 18 19:49:23 EDT 2005


Don,

I found a source for transformer varnish on the internet.  That 
varnish is very volatile and you may have trouble getting it as an 
individual.  (the flash point is around 70 degrees with a spark as I 
remember and it has a short shelf life according to the mfgr.). I had 
no trouble but I am a licensed Professional Engineer.  An option for 
the units is to find a motor rebuild shop.  They usually have a 55 
gallon drum of it that they dip the rewound motors into when they are 
done.  I sure they would do it for you for a modest fee.  I have no 
source on the tar or fish paper etc.

Larry

At 01:00 PM 9/18/2005, you wrote:
>Hello Larry and All,
>I have several 2.5v and 10v rusty,dirty, and rattling UTC small can 
>S series filament transformers that I want rebuild and would like to 
>know a source for transformer varnish and potting material, wax,tar, 
>or whatever.
>On pulling the cover off the most rattling around one I found only 
>sections of corrugated cardboard to be the 
>"packing/potting"  material !!. I would like to use something better 
>and improve the heat transfer as these seem to be really small 
>construction for the rated current. I intend to use the 10v units 
>for 813/814/805 family.
>Any suggestions would be appreciated.
>Bill KB3DKS/1
>
>-----Original Message-----
>From: Larry Will <lhwill at verizon.net>
>To: w8hrq at lemleys.org; Discussion of AM Radio <amradio at mailman.qth.net>
>Sent: Sun, 18 Sep 2005 10:41:41 -0400
>Subject: Re: [AMRadio] Drying out HV transformers / Chokes
>
>
>Don,
>
>Here is the section in my article from ER Magazine on rebuilding an 
>RCA BTA-1R1 on the homemade over I used with great sucess. all you 
>need is a thermostatically controlled hotplate a thermometer and a hood.
>
>.from my RCA BTA-1R! article in ER.
>
>A check of all the iron with a 1000V 1000 megohm ohmmeter showed 
>excessive leakage (less than 1000 megs) in the modulation reactor, 
>the driver plate transformer, and the control transformer. The 
>control transformer makes 110VAC for relays and lamps from the 240V 
>input. The smaller transformers were baked in the kitchen oven at 
>140 degrees F for 5 hours. For the very heavy 50 henry modulation 
>reactor, I fashioned a homemade "oven" using a thermostatically 
>controlled single burner hotplate and a hood made from an old water 
>heater jacket (Figure 1). This allowed me to cook the transformer 
>out in the garage near the rig. An oven thermometer allowed me to 
>set the oven temperature to 140 degrees F and after 25 hours of 
>"baking", the leakage was cured. All of these transformers and 
>chokes were then dipped in transformer varnish to re-seal the 
>winding from new moisture incursion.
>
>Regards,
>
>Larry W3LW
>
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