In a bit of a pickle - EMI interference from fan?

raddiux · Post by **raddiux** » Thu Nov 25, 2004 4:44 pm

Well, I finally decided to go through with the procedure that I asked about here: http://forums.silentpcreview.com/viewtopic.php?t=16946. I purchased a nexus 120mm fan since it seemed like the quietest option available.

I followed the instructions, cut out the fan from its chassis and replaced it into the eMac. Everything went great. I turned on the machine - bliss! I couldn't hear a thing! But then I look over at the screen, and to my dismay, the entire image is shaking! This never happened before. If I go and stop the fan blades from spinning, the shaking stops. Now I know that the fan is somewhat close to the CRT neck, but the old fan never exhibited this behavior. The only thing I can think of is that the fan is emitting magnetic interference or something which is causing the screen to shake. I went ahead and tried wrapping the loose fan wires in electrical tape, but that didn't help. Of interesting note is that the original fan is VERY heavy, like 3x the weight of the nexus. Could this mean its shielded better internally?

I'm sure many of you have never had to worry about something like this (its not often the monitor is built into the computer), but i'm wondering if any of you guys have some advice. Right now i'm left with a shaky monitor which hurts my eyes, and there is no [easy] way to replace the old fan since I had to cut it out.

Trip · Post by **Trip** » Thu Nov 25, 2004 4:53 pm

Get a 12cm fan guard.

burcakb · Post by **burcakb** » Fri Nov 26, 2004 4:19 am

ah, trip, I seriously doubt that'll be a solution in an iMac!!

It may be the larger current draw of the Nexus fan is causing the trouble. The Nexus 120mm fan has the highest current rating I've seen at 0.3A. Can you compare that to the fan you took out?

Trip · Post by **Trip** » Fri Nov 26, 2004 6:38 am

Oh yeah, the computer's in the monitor...

Did you connect the 3 pin plug or the 4 pin?

ferdb · Post by **ferdb** » Fri Nov 26, 2004 6:53 am

If the fan is fairly close to the screen the original fan probably did have magnetic shielding. You may have to find an alternate fan or undervolt the original. The Nexus does have a very high current draw compared to other fans of it's type, probably giving up some efficiency to quiet motor noise. It may generate a disproportionally large magnetic field than other fans so try some different fans. I don't think you will be able to magnetically shield the Nexus fan effectively. I would put other fans near the CRT and see if you can find one that runs without generating so much magnetic interference.

Trip · Post by **Trip** » Fri Nov 26, 2004 7:13 am

What about aluminum foil? Could that be put somewhere?

raddiux · Post by **raddiux** » Fri Nov 26, 2004 7:45 am

The fan is connected using the 3 pin plug. Trying out other fans would be nice but its a ton of work to cut a fan out of its chassis and then perfectly glue it into the old chassis. I suppose i'll have to put the old fan back in, but i'm not even sure if it'll go back in. I did undervolt the old fan using a zalman fanmate, but it was still fairly loud, and exceptionally clicky.

As for aluminum foil, i'd rather not go placing bare conductors anywhere in there, considering the fan is like an inch from the CRT neck where there are like thousands of volts running through.

Now to try to find an Apple repair center that will sell me a fan.

Trip · Post by **Trip** » Fri Nov 26, 2004 8:21 am

Some metal even near the fan ought to absorb some of the EMI. Whereever you feel safe putting it.

On the three pin plug, could you connect the fan into a 4-pin molex instead?

Could you paste the fan on the outside of the case rather than the inside? Then glue a fan grill to the inside part of the fan.

Anyway, good luck!

efcoins · Post by **efcoins** » Fri Nov 26, 2004 8:55 am

Might be electrical noise rather than EMI. Try soldering a capacitor between the 0 and 12V pins of the fan.

ferdb · Post by **ferdb** » Fri Nov 26, 2004 7:08 pm

If it's making the image on the CRT waver it's the rotating magnetic field in the motor that's causing the problems. You can't shield against that with just metal as if it was EMI, you need something that will shield it magnetically like mu-metal. But I don't think that's going to be practical in your case. Your best bet is to try other fans in the vicinity of the CRT and see if they produce less interference.

bomba · Post by **bomba** » Mon Nov 29, 2004 1:56 pm

ferdb wrote:If it's making the image on the CRT waver it's the rotating magnetic field in the motor that's causing the problems. You can't shield against that with just metal as if it was EMI, you need something that will shield it magnetically like mu-metal. But I don't think that's going to be practical in your case. Your best bet is to try other fans in the vicinity of the CRT and see if they produce less interference.

I agree 100%. BTW, electrical noise which causes interference equals EMI. Aluminum foil, if placed appropriately would be expected to provide shielding from electrical fields, but it will do next to nothing for the low frequency H-field (magnetic) interference that you are likely having an issue with. If the stock fan is still serviceable, I'd first try putting it back in and undervolting it, as was previously suggested.

DanceMan · Post by **DanceMan** » Mon Nov 29, 2004 5:47 pm

Also in agreement with ferdb and bomba. Electrons fired at the screen are deflected by the yoke to make the image. The magnetic pulses from the fan motor are now doing that also. Only magnetic shielding will help. Perhaps a steel (has to be magnetizable material to shield) shroud around the neck might work. But I think bomba has the simplest solution.