Of Magnets and Oscilloscopes....

Thread Starter

Reloadron

Joined Jan 15, 2015
5,646
Got myself going on a small project using an Arduino Uno for a PWM signal currently just driving some LEDs. Fade In and Fade Out working just fine on three channels. So I decided to get a look at my PWM signal using an old Tektronix 2225 50 MHz scope. The scope is piled on mt computer table with all sorts of junk and I used it a few weeks ago. I could not get a trace. Triggering lights working and tried moving the trace vertically but no trace. Push beam finder and still no trace. What the? I finally give the scope a gentle smack and momentarily I see a trace flicker past and again no trace.

A week ago I dug out a little magnetic parts tray (Harbor Freight) and without thinking set the thing on top of the scope. For a week I have been meaning to take the parts tray and put it with another like it on the Motorcycle Lift but always forgot to move it. I picked up the magnetic tray (little buggers have a strong magnet) and like magic there was my trace. Can't believe I did that! :) Anyway I thought I would share my moment of sheer stupidity with the class. Just got back from placing the magnetic tray on the motorcycle lift as I have been planning to do for a week. Guess I was motivated. :)

Ron
 

Kjeldgaard

Joined Apr 7, 2016
411
Back around 1980, I was building some test equipment for current transformers, a device with up to 1000 Amps through the primary of the current transformer.

It was really quite simple, a 1KW ring core transformer with a single winding of some 90mm^2 welding cable and a corresponding lockable welding plug, all of which was a "short-circuited" loop not more than 1m in length.

During the work I also had an analogue oscilloscope standing on the desk, no signal on the inputs and the time base standing at about 2 mS. And at one point I saw that the picture on the oscilloscope made a little bump. Of course, I would have to explore a little more, and with some other settings on the oscilloscope I could actually get a nice loop on the screen, almost like on the big roller coasters and still without signal at the oscilloscope inputs.
 

shortbus

Joined Sep 30, 2009
7,965
Hi Ron. Does the scope have a CRT screen? Ever put a magnet or even a large speaker bear an CRT TV? It will cause the picture to distort where the magnetic field comes in contact with the screens signal. Isn't that what was happening on the scope?
 

Thread Starter

Reloadron

Joined Jan 15, 2015
5,646
Hi Ron. Does the scope have a CRT screen?
Yeah, I managed to suck the electron beam to the top so hard that even the beam finder could not pull it down. :)

Really it wasn't till I smacked the scope and saw a trace jitter momentarily did I get a clue what I had done. I have seen CRT Gun TVs screwed by a magnet. I spent a good 20 to 30 minutes wondering why my scope wasn't working.

Harbor Freight Magnetic Screw Holder Tray. Damn trays do have one hell of a magnet and really work great. Whenever I stop or have a coupon they are giving the things away with just about any purchase. Just do not place on top of a CRT type oscilloscope. :) Actually the motorcycle lift from Harbor Freight is a really good deal on sale for their mid priced model. The original plan called for putting that tray with a few others on the lift. Can't believe I was that stupid and it too so long to find my scope trace. :)

Ron

Ron
 

RichardO

Joined May 4, 2013
2,271
I had a similar ah ha moment may years ago. I had a circuit that needed two test patterns aligned on the CRT of a video monitor. I started by aligning the first image. It was easier align the second image by rotating the monitor 180 degrees. I did this.

When I turned the monitor back to the original position, the alignment was off. Grumble. Repeat procedure being more careful. It was off again. Grrrr.

Much to my surprise, the earth's magnetic field was enough to move the image on the CRT. I had to do the calibration without rotating the monitor.
 

Thread Starter

Reloadron

Joined Jan 15, 2015
5,646
Much to my surprise, the earth's magnetic field was enough to move the image on the CRT. I had to do the calibration without rotating the monitor.
Go figure huh? As simple as it is in your case something like the natural magnetic field of the planet caught you. :)

Ron
 
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