RFI Noise and Inexpensive Computer Power Supplies
Created by Larry Kirkland, W4LK on 2024-08-20
"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 09/24/2001
RFI Noise and Inexpensive Computer Power Supplies
I decided I was going to upgrade my old hamshack computer to 21st century standards. It had served me well, was reasonably noise-free, and it had the essentials for an easy upgrade project. I decided to re-use everything but the motherboard, memory, and video card.
So with good intentions and George's check, I attended a regional hamfest that had several computer dealers competing against each other. After a couple of hours of fighting my way up to the counters to see what was being offered, I settled on a whiz bang deal with all the stuff I needed to make the upgrade. They also included a new case with power supply to package it all in.
The next day was spent building up the new computer using some of the stuff salvaged from my old clunker. I took advantage of the new case/power supply because it was rated at a higher wattage than the old box. The good news is that everything started up OK and functioned great. The speed was unreal (1.4 gigahertz). The audio and video were awe-inspiring. I had joined the 21st century!
It was time for our state MARS net just above the top end of 75 meters, so I turned on the rig to get ready. At first I thought we were in the middle of a solar storm! The noise on frequency was over S-9. Signals were way down under the noise. I tried different bands - 80 and 40 were wiped out, but 20 and 15 were better. Then it hit me - the new/upgraded computer was turned on and could be generating the hash! Sure nuff, when I shut the computer down, the noise went away.
I fired it back up and began unplugging cables from the back of the computers to see if I could determine what was radiating the noise. To make a long story short, even with everything unplugged except the power cord, the noise was still there. Then I tried one of those snap-on torroids on the power cable and the noise went down a few db's. AHA! That means the noise was leaking out into the power lines through the power cord and being radiated. Problem diagnosed!
I should have had a clue that all this was likely because the computer vendor was selling this case/power supply jewel for a little more than $30. I cleverly removed 4 screws holding the power supply to the case, removed the power supply and took out the 4 screws holding the little cover on. As I suspected, there was absolutely no filtering or bypassing of the 110V line. Zilch. Another good example of you get what you pay for. Fortunately there was plenty of room at the back of the power plug socket in the power supply to add a brute force line filter. These are described in the ARRL handbook and consist of four .01 f disc ceramic capacitors and a pair of RF chokes. The chokes must be capable of handling the current requirements of the computer, and the voltage rating of the capacitors should be 800V DC or greater. The parts are probably in your junk box or are available at Radio Shack. It took about 30 minutes to solder the parts in and the mod completely removed any trace of noise.
So, if you are experiencing computer noise and your computer has one of those el-cheapo cases/power supplies, you might want to consider adding the brute force line filter mod. It works for me.
