Power supply field messing with CRT oscilloscope?

• Power supply field messing with CRT oscilloscope?
  Posted by Jon86 on 04 Oct, 2019 20:00
• I bought a Tenma dual supply from one of the kind folks on this forum, and to save space on my desk I put the power supply on top of my CRT (but digital storage) oscilloscope.
  
  Turning it on I noticed the display jump and rotate towards the upper right, as the supply made a kind of metallic 'pop' noise when I turned it on.
  
  Doing it again seemed to move the display back closer to the center, and again further still, but it still sits kind of wonky.
  
  I've moved the power supply away, at least 20cm or so, and no longer in the same orientation, but the problem still persists. Not every time I turn it on, but I can see the display jump on occasion.
  
  Clearly it must be the H-field from the supply transformer but I never really gave it a thought, since I didn't expect test gear to have that much of a problem sitting near other test gear (magnetic shielding etc where necessary). But now, I'm starting to worry about damaging my poor 9400A from doing this too often.
  
  Is there something strange going on here? Or has this pitfall of CRTs just escaped me all this time?
  
  Also, is there any easy way to degauss this thing? What kind of field strength would be required?
• #1 Reply
  Posted by AVGresponding on 04 Oct, 2019 20:17
• CRT/CRO displays are quite sensitive to electromagnetic fields.
  
  If you lived near a radar station for example (ie on an Air Force base) back when TVs were all CRT, you'd become quite familiar with this effect, even at a range of several hundred metres, making it hard to keep track of snooker programs   
  
  The effect can be permanent if it's strong enough and long enough in duration 
  
  It's not limited to the display either, I've discovered I have to reorganise my TE setup as my desktop pc is interfering with the triggering on my scope (old school Tek CRO DSO) 
• #2 Reply
  Posted by wasyoungonce on 05 Oct, 2019 04:54
• Most TVs has degaussing coil run from a PTC resistor on power up.  We used to re-orientate TV screens N/S, E/W and re-do this power on degaussing as this took out earth field line effects.   
  
  CRT scopes use electrostatic deflection ang have shields to protect from fields. Maybe the shield is missing or magnetised???  Try a degaussing coil wand.  keep the PSU away from the scope.