Tektronix 5A21N Diff Amp Signal Present With Input Grounded

Thread Starter

m2circuits

Joined Jul 17, 2016
64
Hi all,

I picked up this 5A21N Differential Amplifier plugin for my Tek 5441, but have a strange problem with it.

With the inputs grounded there are these pulses present on the trace.

Visual inspection doesn't show any obvious problems, and the plugin does work as it should if I feed in a differential signal to the + and - inputs. The attenuator has an effect on the amplitude of the pulses, so it must be coming from before it.

Here is the service manual, and attached is a photo of my problem.

Any help is much appreciated!

M.
 

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MrChips

Joined Oct 2, 2009
19,379
The oscilloscope trace looks like capacitive pickup from a rectangular wave.
Measure the spacing between the positive and negative going glitches to determine the high pulse duration.
Measure the spacing between two positive going glitches to determine the frequency.
Now hunt around for a rectangular wave that would be the source for those glitches.
 

danadak

Joined Mar 10, 2018
3,607
Whats is the screenshot timebase setting ?


Could be bulk bypass caps shot in the plugin picking up backplane switching
noise. There old plugins are 30+ years old so caps suspect.

Look at mainframe manual, see what signals are present in the backplane as clues.


Regards, Dana.
 

Thread Starter

m2circuits

Joined Jul 17, 2016
64
Ok, just had a look in the service manual of the 5441. There is a 200kHz or 100Khz (changes for alt and chop) switching signal applied to both vertical plugins. I will have a look at the frequency of the pulses, and see if they change with the alt or chop mode engaged. That will give a better idea to the source of the pulses.

I'll get back to you all as soon as I get a chance to look at the scope.

M.
 

Thread Starter

m2circuits

Joined Jul 17, 2016
64
Welp, it seems like I was chasing a red herring, because the problem was a PEBKAC. I had taken the side shield off of the plugin to calibrate the gain of the diff amp, and it must have been picking up noise from somewhere in the scope. Since I don't have the card extender, I just take the left side panel off of the scope and install the plugin I want to calibrate in the left vertical plugin slot with the shield off. I put the shield back on on the plugin and the noise is gone.

Thank you all for your responses, I appreciate your time in trying to help me solve this issue.

Cheers!

M.
 
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