[AMRadio] Drying out HV transformers / Chokes

GBrown gkbrown at gwi.net
Mon Sep 19 16:24:35 EDT 2005

Regular floor varnish works just fine. You have to thin it with Zylene. The
motor repair shop is a baking varnish. I use nomex paper when insulating
rewinds. Its expensive but very very reliable. The "tar" is nothing more
than the stuff roofers use on flat roofs. Its applied hot and cools down to
a durable application. Should be able to chip a pound or two of one of the
plugs of tar. I bet they would give it to you. The nomex paper is about
$12.00 for a small sheet. There are different thicknesses.
----- Original Message ----- 
From: "Larry Will" <lhwill at verizon.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Sunday, September 18, 2005 7:49 PM
Subject: Re: [AMRadio] Drying out HV transformers / Chokes

> 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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