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A poorly serviced dewar full of LN2, riding in the back seat
of your car, will quite easily snuff your life out, without
you ever even knowing there is a problem...

Your welding gas cylinders won't do that.

-Chuck Harris

- wrote:

I have welding equipment including Oxy-Acetylene and Helium, CO2 and
25/75 gas for MIG and TIG and the same companies sell all of that also

sell

liquified gasses so I doubt that I'd have any trouble getting liquid N2.
Liquid O2, possibly, but not liquid N2 which in realtively inert and
doesn't support combustion.

I have welding equipment including Oxy-Acetylene and Helium, CO2 and
25/75 gas for MIG and TIG and the same companies sell all of that also sell
liquified gasses so I doubt that I'd have any trouble getting liquid N2.
Liquid O2, possibly, but not liquid N2 which in realtively inert and
doesn't support combustion.

Jim Williams found an even more shameless example, where there were two dead transistors soldered to ... nothing. But they were counted in the AM radio advertisement.

Tom

Worst I ever saw was at Powerwave Technologies back in the early 2000's.? With the many acquisitions, they ended up with about a half dozen schemes, XXX-XXXXX, XXXX-XXXX, XXX-XXXXX-XXX, etc. And the worst of all, XXXXXXXXXXXX; yep, that really was 12 digits in a row with no dashes!? At least XXXX-XXXX-XXXX would have been tolerable.? C'mon, man!

Keithley 616 has the MOSFET protection scheme Ed described. I checked my Keithley 196, it uses JFETs for protection diodes.

I know I am going off topic but Heathkit IO-4235 oscilloscope has a "feature" that blows up B-E junction of power supply BJT Q101(and releasing the magic smoke of couple of zeners with it) if not loaded.
Ozan

The large dewars are nice, but they have to have a very hard vacuum
inside if you want them to hold LN2 for any amount of time... We're
talking oil diffusion levels.

Then, there is the problem of getting your local supplier to let you
have some. They consider it rather dangerous, and most won't sell it
to individuals. So, become a company.

-Chuck Harris

- wrote:

I passed up a good size dewar at a garage sale for $5 about a year and

a

half ago! :-/ At the time I just thought that I'd never use it and I'm
getting to the age where I need to be getting rid of stuff instead of
acquiring more; so, for once, I listened to the logical half of my brain
instead of my imagination.

I had a mole removed a few years ago and the Dr used liquid Nitrogen

and

he keeps his in one of the old green metal Aladdin thermos bottles like

the

one that used to be sold with their metal lunch boxes. He said that that
thermos would keep some of the liquid N2 all day long. They only need a
tiny drop of it to freeze moles and warts. The thermos are small but they
might be large enough for a small assembly.

On Mon, Feb 22, 2021 at 1:46 PM Paul Amaranth <paul@...>

wrote:

LN2 is apparently pretty cheap if you have a supplier to sell it to you.
I knew I should have picked up a dewar from the university surplus store
in the old times. If they ever open it up again ...

Paul

On Mon, Feb 22, 2021 at 10:40:26AM -0800, Dennis Tillman W7pF wrote:

Very clever! I never thought of that. There was a time I could have

done

this and I know exactly how it works.

Yes, there is a risk, but as you point out it involves no dangerous

chemicals or careful temperature regulation and messy chipping away of

very

hot epoxy.

Thanks for the suggestion even if it is too late in my case.
Dennis Tilman W7pF

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of -
Sent: Monday, February 22, 2021 3:49 AM
To: [email protected]
Subject: Re: [TekScopes] Slightly OT: How can I dissolve Potting

Compound? FOLLOWUP

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 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







--
Dennis Tillman W7pF
TekScopes Moderator

--
Dennis Tillman W7pF
TekScopes Moderator







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Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix/Linux - We don't do windows