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Joined: Jul 2004
Posts: 19
Local Offline OP
Recently our shop purchased a building and built our first new warehouse and office. I noticed the other day I can not receive AM radio stations while my computer is on. It doesn't matter what circuit I plug the radio into. No matter where I move the radio I get nothing but noise. When I shut down the computer reception is crystal clear. This only affects AM reception, FM comes in fine with or without the computer on.

What are common causes of noise aside from crappy PC power supplies? I'm wondering if this may indicate some problem other then a noisy power supply.

Any cheep easy suggestions to clear this up? Service is 3-0 120/208V and the noise is on all phases all circuits not just the circuit the computer is plugged into. I am not very familiar with things like noise, harmonics, etc. So I am not sure how to go about trouble shooting or correcting the problem.

I know the boss wont care about radio reception when he gets back but I can't sit in a dead silent office and work. It drives me crazy. I can't back a trailer with the radio on, but I can't do bids without a radio. Strange isn't it?

Joined: Nov 2005
Posts: 827
I think a good first step is to take a battery powered radio in abd check various orientations relative to the computer. I listen to AM talk at lunch time with my little Walkman part way into my top desk drawer. All the way out and I get the noise from the desktop. All the way in and I block reception. Moving the radio around the case can give you an idea of where the leakage is coming from. You could try to improve the RF shielding and grounding in the case. Any exterior plastic that doesn't have a metallic coating inside would be suspect. But using the little radio to find a sweet spot should help. Some buildings pretty severely attenuate radio signals and the station received signal strength relative to the noise amplitude has a great effect too.

Joined: Nov 2002
Posts: 174
Using the battery powered radio will also tell you if the interference is coming through the AC or over the airwaves.

I listen to shortwave at home and the interference is something else. Somewhere in my neighborhood is a source of static that is being carried over the AC lines.

It is probably from a bad transformer or insulator, or maybe a neighbor has a bad ballast. I can walk along the street with a portable radio and hear the static as long as I follow the overhead powerlines.

Last edited by kale; 07/03/08 06:45 PM.
Joined: Jul 2004
Posts: 9,788
Likes: 14
A lot depends on the number of layers on the system board and if it has a ground plane. I know I never had a problem with AM radio on my desk at work and I had 2 IBM PS/2s running in my office 24/7 one in a wood case.
Some lesser "clones" would affect my radio whenever they were on in the shop around the corner from me (covers on helped a little).

Greg Fretwell
Joined: Jul 2002
Posts: 8,432
Likes: 3
I didn't think anyone still listened to AM radio apart from Radio Hams and those of the religious persuasion.

What I would try is an in-line Mains filter, that your radio plugs into and then the filter plugs into the wall.

AM radio suffers with computers, because of the internal ferrite aerial rod and the fact that computers have so many harmonics that occur in the AM Broadcast band.

One other thing you shouldn't forget is the fact that radios are built to a price these days and have been for some years, any sort of RFI rejection circuitry, is usually the first thing to be tossed out of the formula, in the name of "affordability".

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
I have not run across this particular problem as yet ...

AM radio in the USA is largely extinct, with the exception of 'talk radio.' My local dial also - if you look hard - carries a sports station, a religious station, and two Spanish-language stations. That's it - a far cry from the full dial of music you could find in the 60's.

In the past year, some legal issues were resolved, with the result that you can get "streaming audio" of most radio stations on your computer.

Joined: Mar 2004
Posts: 251
I run into this problem a lot. First, you can listen to most radio stations online, a good solution. But, as far as the computer goes, most noise is RF, although the power line will help radiate it. Make sure its not the monitor. Flat screens are noisier than CRTs just cut off the monitor, see if the problem goes away. Also the monitor cable, many are poorly shielded. Most RF noise I find comes from 3 sources. One the monitor &/or its power supply. Two the video card. and three the microprocessor or its power supply. Try some clip on ferrite chokes on the video cables (both ends) and power cords on both the monitor and computer. If the computer is plugged into a UPS check that. They will also radiate. Robert

Joined: Jul 2002
Posts: 717
As far as AM radio being dead in the USA, Rush Limbaugh just signed a contract extension for $400 million with a $100 million signing bonus. I guess from that amount of money, somebody either is listening, or that is one hell of a good way to launder money.........

Joined: Jul 2004
Posts: 787
What you are most likely suffering from is radiated EMI (ElectroMagnetic Interference).

PC issues.
1) Check the PC for missing card slot covers.
2) The screws for the card slot covers are screwed in.
3) The PC case screws are fully screwed on.
4) Using a grounded power cord.
5) Verify noise source is PC and not monitor.
6) Perhaps the noise source is not the PC but something else that is plugged into the same outlet strip. What else is turned on when PC is turned on? Things like fans, speakers, desk lights, network switches and hubs, etc.
7) Serial, Parallel, and Ethernet ports are often sources of interference.
8) Do you use any wireless connections like mouse, keyboard or network?

Power issues.
1) Ground pin of outlet actually connects to ground.
2) Radio power cord is fully uncoiled and not running parallel to PC cables.

Good Luck in isolating the source of the interference. Once the the source is found, then we can provide better means of reducing or eliminating it.

Larry C

Joined: Jul 2004
Posts: 9,788
Likes: 14
Rush makes so much money because his advertisers know he has assembled a group of people who will believe anything.

Greg Fretwell
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