Quoting ed-at-alumni.caltech.edu (Edward V. Phillips):

 Ed> Richard:

> As comprehensive as your writings on neon sign transformers
> have been, there are some points you have either not covered  
> or which I have missed. A number of years ago my carpool      
> buddy and I were driving home from work and noticed several   
> neon sign transformers lying in the iceplant next to a        
> freeway on ramp.  We came back to investigate and found a     
> half dozen FRANCEFORMER 12 kV, 60 ma power factor corrected
> transformers, which we split down the middle.   There was     
> also a 9 kV, 60 ma transformer which I got.  To make a long   
> story short, all of these transformers, which had obviously   
> fallen off the back of a truck as it went round a curve, had  
> more or less damage to the insulators.  I have been able to   
> use two of my 12 kV transformers, but am now interested in    
> rehabilitating the third, if possible.  Here is the problem:  
> One insulator is completely broken and gone, together with    
> the stud which ran through it.  When I look into the empty    
> hole I see tar with no wire visible.  Questions:

> 1. Is it likely that when the stud broke off it pulled the    
> attached lead out of the transformer so that winding is now   
> NG?

It is very unlikely the windings are damaged or destroyed. This
same thing frequently happens in the course of a normal un-
potting. With a little bit of experience in unpotting a
particular make and model of transformer, the builder learns the
location and orientation of the spring steel retaining clip used
to hold the HV bushing (or insulator) in place. With this
knowledge it is fairly easy to dig a small hole in the asphalt
with a screwdriver, pop the retaining clip out of the potting,
and remove the insulator without damaging the porcelain or the
lead wire. Without this key little bit of knowledge, the builder
usually ends up cracking the insulators off with a hammer, and
the thin HV lead wire is frequently cut, sheared, or pulled out
of the potting. All is not lost, in fact the setback is minor.

> 2. If there is hope of salvaging the transformer, I am left 
> with the alternative of peeling off the case, as you recommend,
> or attempting to melt out the tar so that I can at least get 
> access to the winding whose lead no longer gets through to the
> outside world.  I kind of favor melting out the tar, as I would
> like to reuse the case.  Questions with regard to that are:
> 3. What is wrong with melting out the tar?  In some ways it is
> less messy than chiseling off the case and the tar.

As you already suspect there is more than one way to "skin this
cat". Regardless of the particular method actually used to
physically remove the asphalt potting (it is not really tar), I
still advise removal of the case before doing anything. Melting
out the asphalt potting without removal of the case means removal
of the lid and placing the case either on it's side or in an
inverted position in a pan, on a rack, etc., while heating. When
the asphalt begins to flow out of the case it exposes the
delicate windings, then the entire core will suddenly drop out or
roll sideways in the case and damage the exposed windings. 

On the other hand, you can split the corners of the case clean
down with a hammer and chisel, beat the asphalt block with a
small sledge even if you desire, and the windings are safely
protected by the potting. Once the block is freed it may be
placed in a pan and heated, and the block will settle gently on
the windings where they will not be damaged.

> 4. Are the windings attached to the case, or is the transformer
> just sitting in the tar? 

The entire core is really just sitting on the bottom of the case.
There is a small steel strap which electrically connects the core
to the case at the same location where the grounding lug is
screwed in. Once the grounding lug is removed the only physical
connection that exists between the core and the case is through
the lead wires. Once the potting is removed, the case is just
useless junk. It serves no purpose other than to hold the

> I am thinking of putting the whole thing in an oven I have,  
> and letting the tar melt into a pan.  If this resulted in the 
> transformer falling out of the case and ripping off the 
> remaining leads the whole operation would be a big and very 
> messy waste of time.

> Any comments or suggestions?

Go ahead and bust the insulators to dust with a hammer, then
chisel the case off by splitting the welds/corners from the top
down to the base. If the asphalt block still does not come free
then pry the base of the case away with a hammer and screwdriver.
Don't spare the force and damn the lead wires. Just watch the
fingers and hands, I have gashed myself more than once doing
this. Heavy gloves would be advisable. Someone mentioned using a
cutting wheel on a dremmel tool, I have used air chisels,
whatever works.

Toss the block into a pan and heat at 375 - 400 degrees until the
asphalt begins to soften and run (about 30-40 minutes), then
reduce the temp below 300 or so and cook carefully until done.
There are spliced connections where the windings are wired to the
lead wires. These splices are well protected as they are located
deep in the potting. Once the core is freed from the potting the
old lead wires can be easily replaced.

The reason I am not really enthusiastic about melting the asphalt 
out is that the melting temperature of the potting and the
smoking/burning temperature of the transformer fall in a very
narrow range. The temperature range varies from core to core
depending on the age and quality of that particular potting. Some
cores with nylon wedges pinning the windings in place can become
a molten plastic mess if the nylon melts, mixes with asphalt,
then later cools on the sides of the HV windings. The clean-up
requires hours with a dental pick.

By far the cleanest and simplest way to unpott cores bedded in
asphalt is to soak the core (sans case) in a medium sized (not
kitchen size) rectanglar household polyethylene trash can. One
gallon of diesel, kero, or gas will remove all the potting in two
- three days if the solvent is kept between 40 - 50  degrees in a
garage or outside porch. The unpotted core is then split, the
windings removed, and everthing can be finished up with a few
minutes of scrubbing with an old toothbrush and a cup of clean
solvent. The waste can be then be poured into a couple of one
gallon plastic milk containers which will hold up long enough to
take it to the gas station, oil change station, muni recycling
center etc., where it can be safely disposed of in the waste oil
sump.  The windings will dry thoroughly after about a 7 - 10 days
laying out on some clean newspaper. Turn them every day or two. 

Richard Quick

... If all else fails... Throw another megavolt across it!
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