Repairing neon xfrmrs
The following is a guide written by Richard Quick for repairing failed
neon transformers - I mentioned that I would post this if anyone was
interested (see "Re: Coiling in South Africa").
Original guide follows:
If you are interested in making a high voltage, high current,
power supply, I can tell you how to do it almost for free....
First call the local neon shop(s) and tell them that your working
with Tesla coils. Ask them to hold all of their failed xfrmrs so
you can pick them up. Make sure you talk to the boss or foreman,
and tell them that you want dead units. I have never had a shop
turn me down for free cores. They are happy to get rid of them.
There are two types of failed neon xfrmrs: warranty units, and
old junk. The local shop must return units that fail within the
two year warranty period back to the distributor for credit. Old
junk (older than two years) you can pick up for free right from
the local shop, but I also scavenge from the distributor. Ask
where the failed warranty units go.
If you can locate the distributor who sells wholesale, and
handles failed warranty units, you have found a gold mine of high
voltage xfrmrs. The distributor removes the PLATE from the xfrmr
for return to the manufacturer, and throws the unit away. The
manufacturer credits the distributor for the plate, as the
shipping is too expensive. The cores go to the dumpster.
After locating your source of failed units, be selective. Try to
bring home the high current units. Ratings commonly used are 9
kv, 12 kv, & 15 kv, with common current ratings of 30 & 60 ma.
Once in awhile you will come across a 120 ma unit. I grab all
of the high current units (60 ma+) I can get in these voltages.
If you can't find any 60s, then grab all the 30s.
First test your units. Use wire with a 15 kv rating or better.
This wire can be obtained where you pick up the transformers.
If you ask they will usually cut you off a few feet for free.
I prefer using the solid polyethylene core from RG-213 or poly
core RG-8 coax, as it will withstand the voltage with gobs of
extra safety margin. Draw an arc from the HV bushing to the case,
one at a time.
About 50% of the "failed" units I pick up are just fine and need
nothing other than a clean up. There is nothing wrong with them.
Often shops get these units from signs they have dismantled, and
they just toss them into the junk pile with the rest. The other
50% are bad. Either one, or both, of the HV windings have broken
down. These units can frequently be repaired:
Remove all hardware, and insulators if possible. Take a hammer
and a chisel and remove the cases by splitting them down the
corners. Break off any stubborn insulators, but try to preserve
the lead wire. You are left with a block of asphalt. Set the unit
outside when it is very cold, and let it freeze solid overnite;
or place in a freezer or deep freeze until frozen solid. The next
morning, short the high voltage lead wires with a clip lead, and
connect 110 volts across the primary. Since the cores on these
transformers are shunted, they may be shorted without harm or
blowing fuses. Let the unit cook for 15-30 minutes.
Disconnect your leads, and with the chisel and hammer, chip a
groove around the block. You want to score a groove lengthwise
that will allow the block to cleave in two. Then starting from
one end of the block, chip until you hit the core, then do the
same with the other end. Pry and chip the asphalt away from the
core until the xfrmr is free. The core may then be disassembled,
and the windings removed and examined. Kerosene and a stiff brush
will clean up the windings and core of any remaining asphalt.
The "cold-cook" method is fast, clean, and works very well. Since
the asphalt is frozen and brittle, it chips away cleanly. The
"cooking" softens up the asphalt around the core, allowing it to
release. The other ways to free the cores are long soaks in sol-
vent such as kero, diesel, or gas, (the nasty waste makes a good
driveway crack filler and roofing patch), or melting out the
asphalt with external heat from a hot plate, oven, or fire.
Solvent soaks are the best done outside in summer. Get a rect-
angular kitchen type trash can. A plywood board may be used for a
lid. Fill the container with about two gallons of solvent (enough
to submerge the xfmr completely). Prepare the transformer as
outlined previously by removing the case and hardware. Submerge
the alphalt block in the solvent for several days. Remove the
unpotted core and clean up with kero and a stiff brush. This
method works best in warm weather; when it gets cold unpotting a
core this way can take a week or more. The solvent can be reused
to unpot three to five cores before it must be replaced.
Melting out the asphalt is best done outside in summer. You will
need two matching galvinized steel pans, such as those used for
catch pans during oil changes. A turkey roasting pan with a lid
will also work. Place a hot plate on some ceramic tiles. Set the
hot plate on high and center a metal pan in the heating coil. Set
the asphalt block in the pan, then cover. It will be necessary to
turn the asphalt block several times. Depending in the wattage of
the hot plate, the temperature may also need adjusting if the
asphalt begins to boil, smoke, and char. You can use a small
charcoal fire in place of a hot plate, but I feel this method is
excessively dangerous: simply set the pans up on bricks, and
build a small charcoal fire under the pan.
There is a new type of neon potting that has emerged recently.
This new potting is a cream colored opaque polymer of some type.
It is not affected by the solvents normally used to unpot neon
cores. It is recommended that this material be removed by the
"cold-cook" method. Freezing makes the material very brittle and
it will chip away easily, the warm core will allow the polymer
to release from the windings without too much difficulty.
Most neons fail when the high voltage breaks down the asphalt
potting. The resulting carbon track bypasses and shorts out the
winding. Simply removing the carbon rich asphalt brings them back
to life. Other times the coils break down internally. In this
case I discard the winding after disassembling the core, and
replace it with a good winding from another unit of the same
model with the same type failure. Sometimes all that is wrong
is a faulty ground connection between the winding and core.
While the core is apart, you can beef up the current output by
removing a few of the shunting plates between windings. Never
take out more than 2 or 3 of these plates per side, as the
additional power output will burn out the secondaries. Generally
I get about 70-75 ma out of 60 ma units after I have finished.
Rebuilt units need a little protection from the high voltage
secondary outputs. The first thing I do is solder on a new lead
wire to the high voltage windings. The HV secondaries are wound
with very fine magnet wire, in the 30 ma units the wire is not
much thicker than a coarse hair. Once a good solder connection
is made, bed the connection and the first 1/2 inch or so of lead
wire to the top of the HV winding with hot glue or clear epoxy.
The lead wire need not be anything special, any thin insulated
stranded wire may be used. Heavy wire increases the chances of a
failed connection due to mechanical stress. When setting the unit
up to fire you simply have to route all high-voltage leads on
The windings themselves are wedged against the core to prevent
vibration. I have seen wood, bakelite, and plastic wedges used
commercially. What I like to do is to soften up some 30 mil
polyethylene plastic sheet in boiling water, and heat the core in
a warm oven. I wrap dry softened plastic around the core and
gently force the windings down on it. Once cooled, the windings
have some insulation from the core, and they will not vibrate.
The base wire from the HV windings must be grounded to the core.
Use the original grounding point if possible, if not you may
split the core apart slightly with a thin blade and insert the
base wire into the gap before you clamp the core back up. If
required you may also splice on a small piece of wire for added
Neon sign transformers that have been rebuilt may be fired dry.
The asphalt used to pot the cores for neon use does not really
insulate well against the RF and kickback from the Tesla Tank.
The units last longer when they are freed of the asphalt potting.
The only other choice is to sink rebuilt units in mineral or
xfrmr oil (like Shell Diala-X) which is a very good RF insulator.
I choose to fire them "dry"; it works, and there is no mess.
Neons may be run in parallel to deliver the current required to
fire medium sized coils, and I have run up to 4000 watts with
banked neon power supplies. The general practice is to run these
banks off of 240 volt feeds controlled through a variac. Neons
with matched outputs are run in pairs in these banks. The
primaries are paired up in series, and the secondaries are all
paralleled to the HV buss. Phasing is important here, and each
transformer must be checked as it is added to the bank to ensure
it is in phase with the other units. If an xfrmr draws an arc
from a lead wire brought to the HV buss, the primary or secondary
connections must be reversed.
Neons typically have an efficiency of about 50%, in that they
draw twice as much RMS power as they put out. This problem can be
resolved with the use of power factor correction (pfc) capaci-
tance across the line. The pfc capacitors used are the same as
for alternating current motors. The voltage rating should be at
least twice the line current used, and I like a 4x voltage margin
for long life. The formula used to determine ballpark pfc is as
C = Corrected kVA ------ 2
This should read C = Corrected kVA times (10 to the ninth power)
over, (2 pi times f times e squared)
C = required capacitance in microfarads
f = frequency of applied voltage
e = applied voltage
CORRECTED kVA is determined by dividing the corrected power
factor output of the neon sign xfrmr (Volt-Amps below) by 1000
Corrected Power Factor
Secondary Rating Volt-Amps
Using a pair of rebuilt 12 kv, 60 ma neons, with 2 shunting
plates removed from the core next to each HV winding, and power
factor correction capacitance, you can get a nice 1.5 KVA Tesla
power supply with over 90% efficiency. Total cost: $5.00 for the
pfc capacitors, and a few hours of time.
I have unpotted dozens of neon transformers from many different
manufacturers. I have tried to make this as informative as
possible, and have checked it over for mistakes. If I have erred,
or was not clear on something, please let me know. Use common
sense whatever you do, and don't expect the first attempt to work
out. On my first attempt I managed to destroy a HV winding during
the unpotting, as I did not know where the windings were located
on the core. But once you see one core unpotted, with minor
differences, you have seen them all.
> I'm unpotting a neon transformer the way you described. I have
> basically all the tar off except for some dirt that I didn't
> clean off yet. I have a few questions. 1) what are the shunts
> and what are they made of? 2) Is the core the heavy metal band
> that surrounds the windings?
I am going to use your ASCII art as a start for the following
series of diagrams. The simplest, bare (no windings) core is
depicted below. It is made of stamped sections of flat iron
sheet that are coated with a thin layer of enamel. This laminated
core is made to come apart into two "U" or "L" shaped sections
which is how the pre-wound primary and secondary windings are
fitted on to it. Usually where will be a couple of long screws
w/nuts or a couple of rivets that may be drilled out to separate
the iron core into two open sections. The diagram below shows an
assembled neon core in it's simplest form, that is, without
windings or magnetic leakage shunts.
| ----------------------------------- |
| | | |
| | OPEN RECTANGLE IN CORE | |
| | | |
| ----------------------------------- |
| STAMPED IRON SHEET |
The diagram below shows the simplest core above, but with
the addition of the primary and secondary windings. The
primary winding is in the center, the two secondary windings
are towards the outside. There are two wires leading from the
120 volt primary winding, and one wire each from the outer layer
of the high voltage secondary windings. This gives a total of
four lead wires exiting the transformer. The base wires of the
two secondary windings are grounded to the core laminate.
high voltage v v high voltage
__I__ I I __I__
| S | __I___I__ | S |
| E | | P | | E |
_______| C |____| R |____| C |______
| | O | | I | | O | |
| | N | | M | | N | |
| -| D |----| A |----| D |- |
| || A | | R | | A || |
| || R | | Y | | R || |
| ||__Y__| --------- |__Y__|| |
| ----------------------------------- |
| GROUNDED IRON CORE |
Now, in order to limit the amount of energy, in the form of
magnetic field flux, that is exchanged between the primary
winding and the two secondary windings: magnetic leakage shunts
are placed in the core on either side of the primary winding.
These shunts are nothing more than small blocks of the same
core laminate that makes up the rest core. The diagram below
shows the simplest neon core from the first diagram, without
windings, but now showing the correct placement for the mag-
netic leakage shunts:
| ----------------------------------- |
| | | | | | | |
| | | | < SHUNTS > | | | |
| | | | | | | |
| ----------------------------------- |
> 3) You say to take softened polethylene sheet and press it
> against the core and then take the windings and press them down
> on it. How would the windings pressed down on the sheet when
> the core is around the windings?
The windings are around the core. When the two pinch bolts (or
rivets) holding the core together are removed, the core may be
disassembled into two sections. Once the core is apart, the
windings may be slipped on and off. To prevent vibration, the
windings are usually wedged into place. Before reassembly of the
core it may be warmed; polyethylene carefully wrapped around the
laminate; and the windings gently but firmly pressed into place.
Quoting ed-at-alumni.caltech.edu (Edward V. Phillips):
> 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
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 little bit of key 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,
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 at 40 - 50 degrees (min) in a
garage or outside porch (it is winter now, summer is faster). The
unpotted core is then split, the windings removed, and the clean
up can be finished 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.
Ä Area: UUCPE-Mail
Msg#: 1218 Pvt Date: 12-12-95
From: Marcusy-at-ozemail-dot-com.au Read: Yes
To: Richard Quick Mark:
Subj: Reconditioning Neons
From: Marcus Young <marcusy-at-OZEMAIL-dot-com.AU>
To: Multiple recipients of list USA-TESLA <USA-TESLA-at-USA-dot-net>
Date: Tue, 12 Dec 1995 01:32:10 -0600
Subject: Reconditioning Neons
I just thought Ide add comment about reconditioning neons.
When unpotting a neon and re-wiring, oil immersing ect, I
think a good idea might be to have an additional HV terminal
for the centre tap. This way, the transformer can be set up
for several different modes of operation. You can earth the
centre tap as in a standard neon transformer,remove the centre
tap for a floating 'pig' style setup or use the cntre tap and
one side of the winding for half voltage (if ever desired )!
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