Re: Slightly OT: How can I dissolve Potting Compound?


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I talked to a friend of mine that used to do Failure Analysis for Litton
Laser. He has some PSUs made by Laser Drive, Inc. and he's going to look
and see if he can find something with a similar part number. He said that
most of those lasers need 800 to 2000 volts at about 2mA to run and a 5,000
to 10,000 volt pulse on top of that to start them. If you can measure the
tube and get an idea of the coherence length and internal tube diameter
then he might be able to give you a better idea. He has also de-potted a
lot of electrical devices and his advice is to start with the mildest
solvent that will work. He said that anything that will soften the potting
compound will likely break down the plastics on the internal devices
(including removing the insulating varnish on the coils) and will ruin
them. He said to start with Acetone and use Methylene Chloride (ZIP Strip)
as a last resort.

Other things, if the tube is flickering then the current is too low
not too high. I'm sure that you're already aware that the tubes have a
negative resistance just like a neon lamp. If the current is too high then
the lamp can over heat and be damaged but you can montor the temperature in
an effort to prevent that. And since the tube has a negative resistance
then you MUST have a ballast resister in series with it. He said that in
most lasers the ballast resister is built into the connector. Also
polarity is important but if you're using the usual Aldin connector then it
should be properly polarized.

On Sat, Jan 2, 2021 at 9:18 PM Dennis Tillman W7pF <dennis@ridesoft.com>
wrote:

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 1½” 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
don’t
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





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