Dennis,
A friend of mine is always disassembling things and reverse engineering
them so I sent your message to him. He replied and that me that one of the
guys that he used with work with at Litton Laser told him that he used to
removing potting compound by dipping the item into liquid Nitrogen and
leaving it there until the nitrogen quit boiling and then took it out and
tapped it with a hammer and that the coating would shatter and fail off. I
have to wonder what that treatment would do to the plastic bodies of
transistors and other items but it might be worth a try. IMO you might need
to leave the item in the liquid Nitrogen for a bit less time to prevent the
plastic items from getting too cold and too hard and brittle. But the good
thing about this method is that there are no dangerous chemicals to deal
with and even the liquid Nitrogen just boils away so there's nothing to
dispose of except the potting compound itself.
FWIW
On Sun, Feb 21, 2021 at 3:37 PM Dennis Tillman W7pF <dennis@...>
wrote:
Thank you one and all for your suggestions on removing the potting. Your
suggestions fell into two general categories:
1) Nasty chemicals which I decided had to be avoided at all costs based on
your advice.
2) Heat. This seemed like a slightly better approach than the chemical one.
But before I get to what happened when I applied heat I learned many new
things along the way thanks to a few of our members and especially, Ed
Breya. Ed's comments explained in detail how these HV potted power supplies
worked. That led me to an amazing web site that has a staggering amount of
information on them about every aspect of lasers of all kinds including
their HV Power Supplies. If anyone is interested the site is
I have to caution you that I spent hours roaming around in all Sam had to
say. He is an incredible resource for anyone who owns a laser. I found many
different power supply circuits and details about laser HV modules which is
what I apparently blew out that prompted me to ask about removing the
potting in the first place.
With suggestions from Ed Breya I tried using the other more powerful potted
HV laser supply I had. It was going to try and force too much current
through the spectrum tubes so I added additional ballast resistors and
connected it to a Variac so I had some control over the High Voltage and
the
constant current With this arrangement I was able to test every one of the
spectrum tubes (I have over 20 different ones each containing a different
gas) to determine their breakdown voltage and their steady state current
requirements. That told me what I should be looking for to replace the
Laser
HV supply that I blew out.
In Sam's web site I found the design and schematic for a power supply
designed by the legendary Jim Williams of Linear Technology. It was a
universal design capable of powering every one of the spectrum tubes I had.
I ordered the parts to build it.
I still wanted to see what I might learn from removing the potting of my
failed HV Laser supply so for $15 I bought a toaster oven at a thrift shop.
I set it up outside, set the temp to 200F (93C) for 30 minutes and poked it
with a sharp tool. Nothing happened. At 250F (120C) it got a little softer
and a few cracks appeared. At 300F (150C) I was able to chip off the bottom
and see the solder side of the PCB. A few of the sides also chipped off.
This was starting to work.
Since the oven was on the ground it was hard to see the temperature knob
from the angle I was standing but I raised the temperature once again by
the
same amount.
30 minutes later when I checked on it, it was smoking. I would rather not
say what the reading on the thermocouple was but I will tell you it was way
over 260C (500F). When I touched the potting the PC board fell away from
the
rest of the potting. I knew immediately that was to be expected since
solder
melts at a lot less than 260C. The parts fell away from the epoxy, by
barely touching them but of course there was no way to know what they used
to be connected to anymore. The entire thing was a mess.
It did teach me some things. Heat is definitely safer than chemicals to
remove potting but realistically even with carefully controlled heat
potting
is not going to give up its secrets easily or cleanly. I now have an "oven"
I can use for soldering surface mount parts with whenever I find something
I
can't build with a through hole parts.
In the end TekScopes members led me to a solution for an excellent HV Power
Supply for my Spectrum Tubes in the form of the Jim William's Linear
Technology Application Note 49, August 1992, titled "Illumination Circuitry
for Liquid Crystal Displays". His design is shown in Appendix D.
"Figure D1: Laser Power Supply is Essentially a 10KV Compliance Variable
Current Source".
Thank you Ed and many others who offered their suggestions
Dennis Tillman W7pF
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Dennis
Tillman W7pF
Sent: Saturday, January 02, 2021 6:19 PM
To: [email protected]
Subject: [TekScopes] Slightly OT: How can I dissolve Potting Compound?
I need to dissolve the black HV potting compound of a 12VDC powered
Helium-Neon laser inverter power supply I have that stopped working. I
would
appreciate any suggestions on what works to do this. I¡¯m guessing it may be
epoxy. I stuck the tip of a hot soldering iron in it for a few seconds
without much effect.
When it was working it turned out to be perfect for powering gas filled
Spectrum Tubes. These spectrum tubes (smaller versions of neon signs)
filled
with a variety of gasses are an excellent source of spectral lines for the
7J20 / J20 Rapid Scan (Optical) Spectrometer to measure.
Spectrum tubes require an initial high voltage (1,000V to 1,500V for
example) to break down the gas and start it conducting. Once the gas in the
spectrum tube conducts the voltage across the gas drops (250V to 450V for
example) and unless you limit the current (to a few mA) it will destroy the
tube. Can anyone can point me to a source of information on how to
determine
the proper voltage and current I need to power these spectrum tubes? Is
there a web site or group devoted to Spectrum Tubes?
Something happened to the inverter and it stopped working. The input is now
open. The inverter is a black potted brick 3¡± x 1?¡± x 1¡±. The ballast
resistor has continuity so that is not the problem. The original label on
the inverter is partially destroyed so I can¡¯t tell what its initial high
voltage output was or what it current limits at. All I do know is that it
was made by Laser Drive Inc.
5465 Wm. Flynn Hwy. Gibsonia, PA 15044
Model: 1150-6330, S/N: 610574
The input was +12VDC at 0.35A.
I wrote to the company that took over the company that took over Laser
Drive
Inc. asking if they could tell me the output voltage and limiting current
but I didn¡¯t receive a reply.
At this point I am hoping if I can remove the potting compound I can figure
out what went wrong with it.
I have a different, bigger Laser Drive Inc. potted inverter which is
powered
by 115VAC. It puts out 2350VDC at 6.5mA. This causes the Spectrum Tubes to
flicker. They do not run continuously. I am guessing that this because
6.5mA
is more current than the tube can conduct. The amount of current the tube
draws increases in proportion to the inside diameter of the tube but I
»å´Ç²Ô¡¯³Ù
know much about this matching the power supply to the tube. All I do know
is
the one that went bad seems to be an ideal match for the spectrum tubes I
have.
Battery operated (DC input) inverters are much more desirable than AC input
inverters for this application because the AC rectification and poor
filtering shows up on the output DC as significant ripple causing the
amplitude of each spectral line to be blurred.
Dennis Tillman W7pF
--
Dennis Tillman W7pF
TekScopes Moderator
