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Author Topic: Computer monitor RF noise  (Read 2291 times)
EI6IZ
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Posts: 33




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« on: August 17, 2000, 06:15:22 PM »

I have a 15" computer monitor that makes a real RF racket, I have done all the basics like using ferrite chokes on both the power lead and the VGA lead, I have tried grounding the internal chassis to shack ground, nothing seems to help, the computer is silent on any band I operate but the monitor generates Wide band RF from top band to 2m. Using a black bacground on my windows desktop helps, changing refresh rates shifts the noise around a bit but this monitor is an RF pig, I wonder if I fit a key and a good antenna what DX I can work with it ;-)

All My coax feeders are choked before they leave the shack to prevent the monitor (and other stray RF noises )leaving the shack along the shield

I have no RF problems with the computer (which stays on 24hrs a day 7 days a week to recive Wefax transmissions from the NOAA sattelites)

Has anyone got any suggestions for a fix and if I am going to have to buy another monitor can anyone suggest a good make of 15" or possibly 17" to buy, I would really hate to spend money replacing it with another RF pig!
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WA4CNG
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« Reply #1 on: August 17, 2000, 10:16:36 PM »

I've got a 17" monitor on my HP Pavillion w/500Mhz Clock, that if it is on, it will trash most everything from 3-30mhz.  On all modes except PSK31 I put it in standby.  On 14mhz. PSK31 I use an ICE-COMM Model 417 Bandpass filter (1-800-ICE-COMM) to reduce the "HASH/COMPUTER TRASH" in front of my YAESU FT-100, so I can operate PSK31.  I am moving into the "Terrace Level" of my house for a Ham Shack/Shop in the next 30 days.  At that point I will have a 200mhz simple Windows 95 (Y2K Compliant) computer to do logging/PSK31/ect minus the HASH/TRASH.  It seems the higher the clock frequency, the more HASH/TRASH....
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K4DPK
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« Reply #2 on: August 17, 2000, 11:24:51 PM »

Some monitors have hash filters as an integral part of the male power plug on the back skirt of the main PC board.  Usually, the filter housing is metal, but if the line is unfiltered, the housing will be plastic.

An effective hash filter consists of 100 uH chokes in each side of the line, bypassed to ground at either end of each choke with .05 mfd discs.  

What brand monitor is it, and does changing screen resolution have a marked effect on the character of the noise?

Phil Chambley, Sr.
K4DPK
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EI6IZ
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« Reply #3 on: August 18, 2000, 08:23:57 AM »

Firstly to WA4CNG,
 I am not having problems with the computer, it's the monitor, PC's can actually be very quiet, here in Europe we have very tough sheilding requirements (CE rules) and all new ce approved cases incorperate very good shielding indeed. If the computer is the source of the noise you probably have a very good chance of fixing it, My monitor, however has me completely stumped.

-------Quote-----
An effective hash filter consists of 100 uH chokes in each side of the line, bypassed to ground at either end of each choke with .05 mfd discs.

What brand monitor is it, and does changing screen resolution have a marked effect on the character of the noise?

Phil Chambley, Sr.
K4DPK  
 
-------- end quote ----

I'll try the trick with the chokes, Changing resolution makes little diffreence but turning down the brightness makes a big difference, I guess that this points to the Switch mode PSU in the monitor as the source of the noise?

I don't know what brand it is.

Being a HAM I am far too much of a cheap skate to buy a new CE rated monitor unless I absolutley have to!

BTW: the CE rated cases that are the norm now in Europe have some really nice Rf features like finger stock on all of the case openings and all drive bays are fully blanked until you need to use the bay. I haven't had any problems with the actual compuer generating noise ever since I had a 286 about 10 years ago.
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K4DPK
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« Reply #4 on: August 18, 2000, 09:19:33 AM »

To EI6IZ:

It would seem so, since varying brightness would be changing the load on the SMPS.  However, if your RFI regulations require shielding to that degree, I can't imagine they would allow a monitor to be produced without a hash filter on the line.  If the hash filter doesn't kill the noise, I would scope the output of the pulse width modulator and the switching FET, then see if the spurs can be bypassed or choked.

Phil Chambley, Sr.
K4DPK
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EI6IZ
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« Reply #5 on: August 18, 2000, 10:05:52 AM »

----- Quote ----

It would seem so, since varying brightness would be changing the load on the SMPS. However, if your RFI regulations require shielding to that degree, I can't imagine they would allow a monitor to be produced without a hash filter on the line. If the hash filter doesn't kill the noise, I would scope the output of the pulse width modulator and the switching FET, then see if the spurs can be bypassed or choked.

Phil Chambley, Sr.
K4DPK

----End Quote--

The problem with the CE regs is that they are relatavley recent (1998?) so there is a lot of older stuff that has really crappy shielding and filtering. Most of the big manufaturers (E.g the likes of Gateway and Dell who manufacture here in Ireland) have been meeting CE regs long before they were required to because some EU markets (Eg Germany) have had strict rules about RF compatibility for quite a bit longer than the CE regs have been in force.

CE RF compatiblity regulation have turned out to be a very good thing for the European ham as not only are Our (and our Neighbour's) High tech toys Quieter but they also tend to be much more immune to stray RF as well.
The CE rules are much tougher than the equivilent FCC rules, Some ham equipment is sold here with addtional filtering or other modifications.(E.g The icom 706Mk2 G comes in Europe with a power lead with an RF filter built into it and there is a note in the manual about having to use the filtered DC lead to retain CE compatibility.)

Anyway back to my Cheap 'n' Nasty monitor, I am going to have a go at filtering the mains socket at the back, i will probably just buy the metal sheilded socket with the filtering built in, all parts suppliers here seem to carry them and there is room to fit one. If this doesn't help much I'll probably treat myself to a new monitor (although this goes against the grain somewhat!)

73's
Brendan Minish EI6IZ
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K4DPK
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« Reply #6 on: August 18, 2000, 11:43:03 AM »

Brendan,

BTW, one thing you could try is disconnecting the B+ to the flyback transformer.....It might be an internal arc and not the SMPS at all--although I would think that would be evident in the video.  Something to try before you junk it out.

Phil Chambley, Sr.
K4DPK
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VK3SL
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Posts: 3




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« Reply #7 on: August 26, 2000, 10:48:52 AM »

I don't like to be part of the problem, but I am about to buy a new PC and plan to use it for digital modes on HF. Obviously I need to buy the quietest (17") monitor I can. The curative suggestions are all helpful, but can anyone lead me to where I can find which one(s) are good and which are not? Does anyone have one that gives no noise problems? I guess the LCD types would be good, but they may be a bit expensive.
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K4DPK
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« Reply #8 on: August 28, 2000, 12:19:36 AM »

My opinion is that NEC has best filtering and shielding. This is part of the reason they cost so much more than the thrown-together types.

I haven't tried this, but it would be interesting to take a sensitive field strength meter into a computer store and just probe around the monitors with it.

Phil Chambley, Sr.
K4DPK
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VK3SL
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« Reply #9 on: August 28, 2000, 01:59:04 AM »

Thanks Phil, it is funny you should suggest that. I asked a friend who works in a local electronics outlet to run his multiband handy over the monitors in his store. He set it to "spectrum display" mode on 14Mhz and, of the limited range in-store, he found a flat LCD type to be the best (obviously) followed by IBM G54 and Compaq MV520 as equal second. It is obviously not the best way to test them, but it may give some relative indication. Your idea of a field-strength meter would probably be better as it has broader bandwidth, the spectrum display would only show a few hundred Khz of spectrum.
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