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I have a nice looking Tek 2247A scope here that has been sitting on the shelf for a while, due to a bad Channel i input. the signal is a horribly attenuated and differentiated replica of the actual input. Everything else is perfect. In a moment of covid-induced boredom, I felt I had to get to the bottom of this. I had a hunch that this was the input resistor being broken or damaged (there is always a flying resistor from the input BNC to the vertical input on 2200 series scopes). It was the only thing I could think of that could give this result.

It takes some effort to de-can the scope, and remove the complex shield over the vertical inputs, but sure enough, when opened, the barely visible input resistor (62 ohm carbon film) was BURNED. I replaced it, and ta-da, everything is working. What the heck?

Here's what I just can't figure out, this scope has no 50 ohm input function, so no low impedance path exists to help burn this resistor due to over-driving, SO HOW DID it get burned and leave the scope undamaged?

I am totally mystified by this, and keen to hear any opinions. I just can't see how this fault happened without vaporizing the vertical input channel.

all the best for the new year,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.

Larry is there a chance that there might be a purchase of the Rotary switch as a group purchase from DigiKey or all parts sourced up to the individuals.
Craig...

Hi Larry,
I might well be interested in a board set, so long as the cost and shipping to London, UK aren't too crazy. I have been intending to build one of these testers for a while and have a few of the components necessary, plus a main board that would certainly do the job, but Jared's looks like it could make the job fairly straightforward. I especially like the front panel - very nice-looking!
Colin.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Larry McDavid
Sent: 31 December 2020 23:13
To: [email protected]
Subject: Re: [TekScopes] I built a TM500 mainframe tester, and updated the design. Someone might find this useful?
Anyone interested in buying sets of boards to share?
Larry
On 12/31/2020 11:27 AM, Jared Cabot via groups.io wrote:

Hi all,

I have been accumulating TM500 modules and mainframes for a little while, so I recently finished building an 067-1201-99 TM500 mainframe tester from the Tek construction notes floating around.
I updated the design somewhat and designed it for simplified construction (Not a single wire to strip and solder!) and I thought someone here might be interested in taking a look or even building one themselves.
Here's a link to the youtube video:


And here's a link to the google drive with all the gerbers and schematics and stuff. I went so far as to rewrite the manual with the alterations and corrections included.


Direct link to the new manual:


Let me know what you think, and let me know if you see any errors that need correcting. :)

Regards
Jared.

I have a nice looking Tek 2247A scope here that has been sitting on the shelf
for a while, due to a bad Channel i input. the signal is a horribly attenuated
and differentiated replica of the actual input. Everything else is perfect. In
a moment of covid-induced boredom, I felt I had to get to the bottom of this.
I had a hunch that this was the input resistor being broken or damaged (there
is always a flying resistor from the input BNC to the vertical input on 2200
series scopes). It was the only thing I could think of that could give this
result.

It takes some effort to de-can the scope, and remove the complex shield over
the vertical inputs, but sure enough, when opened, the barely visible input
resistor (62 ohm carbon film) was BURNED. I replaced it, and ta-da, everything
is working. What the heck?

Here's what I just can't figure out, this scope has no 50 ohm input function,
so no low impedance path exists to help burn this resistor due to
over-driving, SO HOW DID it get burned and leave the scope undamaged?

I am totally mystified by this, and keen to hear any opinions. I just can't
see how this fault happened without vaporizing the vertical input channel.

all the best for the new year,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.

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

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

The Mainframe tester will allow for a full functional test without risking damage to a potentially expensive module