Member Avatar for Shauna3213

A few months ago, my screen began shaking slightly, and has been worsening every since, to the point that it hurts my eyes now to use the computer. Here are the facts:

* My computer is an iMac about five years old. Other than the screen, it runs pretty smoothly.

* The screen began to shake when I had the computer in my dorm room at school. It continues to shake now that I have it at home.

* I read that screen-shaking is sometimes caused by magnetic fields. I've tried turning off electrical devices like fans and lamps, and removing them from the room, but I'm not exactly sure what things have magnetic fields, though, and how big those fields are, so I could be doing it wrong. I also tried putting a metal cookie sheet around the computer to protect it, but I only have one sheet and it didn't seem to work no matter where I put it.

* A couple of times over the past few months the computer screen has suddenly and completely stopped shaking. This lasts for about an hour or so. Then, rapidly, the screen begins shaking again, until it reaches the same, eye-wrenching level it was at before.

Anyway, I hope you can help. I'm happy to try any suggestions, short of "Buy a new computer."

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Dani AI

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This is a textbook electromagnetic‑interference case with an old CRT iMac — exactly the sort of problem ’s AM‑radio/compass idea helps find, and the solution found (unplugging the broadband box) fits the usual pattern. CRTs are mechanically easy to disturb because their deflection system and video cables pick up nearby magnetic/RF noise; the picture can shimmer or “wobble” when an external device radiates or induces a signal into those circuits. ()

How it happens: many consumer set‑top boxes and modern power supplies use high‑frequency switching that produces broadband emissions. Those emissions can couple into a monitor either radiatively (into the tube/yoke) or conductively (into shields and cable grounds). Ground loops or long cable runs make the problem worse because the cable system acts like an antenna or a loop antenna. That explains why moving or unplugging the offending box stops the effect. (scribd.com)

Practical, safe fixes to try (in rough order):

  • Increase distance and keep the set‑top box and its cables several feet away from the iMac and video cable.
  • Put clip‑on ferrite chokes on the video, coax and power cables (one or more turns through the clamp improves effectiveness).
  • Plug both the iMac and the box into the same outlet/power strip (breaks ground‑loop antenna paths) or use a coax/video ground‑loop isolator if the cable feed has its own ground.
  • If interference continues, ask the ISP for a replacement box or a shielded model; a noisy unit can be defective.
    References for ferrites/ground‑loop fixes and why they work are below. (allaboutcircuits.com)

A few safety notes and limits: do not place metal directly against a CRT face or try to “shield” it with improvised metal enclosures (sharp edges and proximity to the tube can be hazardous). Don’t open the CRT — high voltages are present even when unplugged. Degaussing can fix color purity issues but won’t always stop a radiated/frequency‑coupled wobble; if the problem persists, contact the ISP or a qualified technician. (sector.sunthar.com)

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Member Avatar for kc0arf

Hello,

I am inclined to believe that this is something particular to your iMac. Moving it around, or putting it in the center of a room should have eliminated a lot of the fields in the walls.

Incidentially, moving electrical currents cause magnetic fields. They are 90 degrees out of phase with the electrical current, meaning if the electricity is travelling up a wall, the magnetic field is travelling 90 degrees out of the wall, such as if you were to put a nail into the wall to hang your coat on. Because current is always flowing (alternating current changes direction 60 times a second, 60 Hz) the magnetic field is also pulsing back and forth, thus the "wobble" you see on a monitor.

To test for magnetic fields, you can use a compass. You can also use an AM radio, tuned to "static" and hover that around a device, and listen to the warbling and distortion the fields make.

Christian

Member Avatar for Shauna3213


To test for magnetic fields, you can use a compass. You can also use an AM radio, tuned to "static" and hover that around a device, and listen to the warbling and distortion the fields make.

I couldn't find a compass, but when I went around the room with an AM radio I discovered the disturbance was coming from my digital broadband box. Decreasing proximity to the box had no effect on the computer, but unplugging it did. My screen is now shake-free! Thank you for such a prompt, helpful, and even educational reply!

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