Author Topic: Tektronix T922 Mysterious Triggering Issue  (Read 447 times)

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Offline SmauelTopic starter

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Tektronix T922 Mysterious Triggering Issue
« on: April 12, 2021, 03:51:52 pm »
I need a little bit of troubleshooting help.

I recently acquired an old Tektronix T922 analog scope. It seemed to be in fine working condition (besides some external damage in shipping, which I was able to get a refund for, and then repair to a certain degree).

The scope seemed to trigger normally, or at least well enough, on most signals (I don't have a signal generator, but there is a test signal built into the scope, and I built a small circuit with a 556 timer to help test when I realized something was acting up.)

However, when I made a second order active low pass filter to turn the square wave from the 556 into a sine wave, the triggering started really acting up, on that signal source especially, and even more strangely, the more the potentiometer for the filter is turned (adding more series resistance into the input of the filter, filtering more high frequency out), the more it acts strange.

I've linked some videos of the weirdness. Firstly, the region in which I can turn the trigger level and still get a stable trigger seems narrower than it should be. Not only that, but when I turn the trigger level, it is clearly changing something about the triggering, but I don't think its the trigger level, as the display does not move with the trigger level. This is reinforced by the fact that pressing the slope switch does not seem to change anything.

https://photos.app.goo.gl/u8sVKzDo2Eq8g7mn7
https://photos.app.goo.gl/F8mwiigAFQ8rjpoC8

Changing the trigger settings have the result that it either triggers "normally" (a stable trigger, but the level where it triggers at is very binary, at either the very top or the very bottom, no in-between), or it triggers twice roughly 180 degrees out of phase with itself making two signals that slide around, or it triggers automatically at preset time intervals, making a chaotic display.

I can't make much sense of what I'm observing. One observation that is halfway a guess is that it seems to only trigger at or near the extrema of the signal, if at all, but I don't know what that means.

Please tell me if you have ever witnessed anything like this, so I can see about figuring out what is causing it to try and fix it.

Thanks in advance for any help you can offer!
 

Offline StillTrying

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Re: Tektronix T922 Mysterious Triggering Issue
« Reply #1 on: April 12, 2021, 05:09:20 pm »
Have you got the X variable speed on the Cal position, - not an expanded X speed.
And then move the X position to the right so that you can always see the start of the trace and the actual first edge it's triggering on.
If there is enough noise about it's quite common for edge trigger to trigger on the wrong edge randomly.
.  That took much longer than I thought it would.
 

Offline SmauelTopic starter

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Re: Tektronix T922 Mysterious Triggering Issue
« Reply #2 on: April 12, 2021, 07:04:52 pm »
Yes, it is in the x1 position, not expanded. I don't think it's random noise either, because despite not functioning as expected, it is still relatively consistent. Also the voltage I'm inputting should be plenty to overcome random noise.
 


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