Strange CRT monitor flicker

An expanded description of what you mean by flicker would help.

It is possible that you have been moved to some lines carrying interference caused by heavy plant. Do you also get interference on broadcast transmissions (eg am radio) at the same times? Does it get worse at the same times each day and is it better on no working days?

A radio can be a simple but effective tool for tracking down interference. eg, whether it gets better or worse when you bring it near the power line.

My experience with this problem involved neutrals and grounds tied together in places that were not to code. I AM NOT saying that this is your problem. My situation you could pull out the monitor farther from the plug and the problem would lessen all the way to going away completely. Do you have an VOM meter that can measure HZ? Place the meter in HZ and plug it into the socket, ( no it does not matter which probe goes where as long as you use the hot and neutral connections) If you read 60 or close to it "forget about it". If you read some odd multiple of 60 HZ i.e. 420 HZ then you need to investigate further. Check the grounding at your main service for tightness, and for that matter all of the connections. If your not comfortable with it hot shut off the main. Safety first..... Pay special attention to any new wiring connections. Snug is good, you do not need to twist each one a half a turn. The correct torque settings for most residential panels is inch pounds not foot pounds.

About 12 years ago, I kept complaining to the manager in our department about the flicker on my computer CRT driving me nuts. He was also our self-appointed computer guru. He'd come by, take a look, and say "You are already nuts. There's nothing wrong with it." After a few complaints, he called some company computer people (I don't think IT had been invented yet) to look at it. A lady, who was in charge, and a young male trainee came. The young man looked and said, "There's nothing wrong with it." The lady, however, said, "How can you stand to look at this thing." I replied, "I can't. That's why you're here." She changed a setting, I guess the refresh rate, and it was no longer a problem for me. Different people must "see" at different frequencies. :-)

Hi

Thanks for your replies, I'll answer each of them in this post.

The best way to describe the flickering would be a horizontal shimmer of the content on the screen. Everything seems to move left and right at a high frequency (It's slower if the refresh rate is changed). At good times, the flick is small enough to be manageable, but at lower frequencies or worse times, it gets much worse and makes the print hard to read. The top-left corner seems the most stable, while the bottom left is the the worst. There is no obvious vertical flicker. I would say there is no pattern to the flickering. At times, it seems bad at times in which electricity use should be at it's lowest (early hours for example). Other times you would expect high usage, it appears most stable. Screen resolution seems not to affect it.

I tested the house with a radio, thanks for this suggestion. The computers themselves seem to give off more interference than anything else. I moved a monitor away from it's normal location and run it off a laptop. The flicker was still present. So I'm not sure what might be causing it. I'm thinking of buying an EMF meter. Are you worth buying and can anybody suggest or recommended a good one if they are?

Pulling the monitor away from the plug seemed to have no effect. By a VOM, I assume you mean a multi-meter. I do have one of those. I'm not sure if it does Hz, but it probably does. Not sure how happy I am sticking it in the mains though even though it's safe in theory. I'll check all the outside fittings when I get a chance tomorrow.

I think I should say that the problem is very severe. I do have sensitive eyes for flicking monitors and I have oddly enough also had a dilated eye examination for eye floaters. But this problem is visible to all at any time. On a 17" monitor with a refresh rate of 85hz, the flicking (or shimmering it could be called) ranges from slightly visible, to a point where it becomes difficult to read. Drop it to 60hz, and the problem becomes much worse. At this level, the sides of the screen seem to move in and out constantly. On an average day, it is possible to see letters move to what seems to be two different positions. It's very odd.

One of the monitors I mentioned previously has actually gone today, I switched the mode from 85hz to 60hz and it just went off and started burning. I put an old spare monitor in it's place and now that's shimmering too. I think that proves it's not the monitors.

Regards,

Pan

If there were "magnetic interference" strong enough to cause harm, you wouldn't be able to see your monitor. The electrons wouldn't be hitting the screen. ;-) OTOH, you could be sensitive to flicker. Simply increase your monitor's refresh rate. In Win go to the Control Panel->DIsplay function. Select the settings tab and then click on advanced. The display refresh settings should be in there somewhere, depending on the WinVersion.

Don't worry about that. The flicker is more harmful than any "magnetic interference". Flicker will cause eye fatigue and possibly headaches. Magnets will cure the common cold. ;-)

That's perfectly normal. My manager runs his monitor at 60Hz. I can't stand looking at it, to the point of getting physically ill, when I'm meeting in his office and have to look away. He thinks it's just peachy. I can barely stand 75Hz. I can see the flicker, but it's not terrible. I generally try (depending on how good the monitor/video card is) for 85 to 100Hz refresh rates so that flicker isn't apparent. The eye's rods are more sensitive to flicker, so diverted vision (peripheral) will show flicker worse than central vision. AIUI, this is a part of the fight-or-flight response of the central nervous system. If you don't see flicker looking directly at the screen, look a little to the side and you may see it get worse.

Thanks for the added info. well, the good news is that you don't appear to be in the primary lobe of the local gigawatt radar jammer, or have someone down the road using his electrical supply to evil ends.

The bad news is that it sounds like you are particularly sensitive to refreshed displays. It happens. Often, suprisingly perhaps, with good quality displays used on low refresh rates - dead cheap monitors are often so crap that the effect is not noticed.

What to do? Well, turn up the refresh rates to as high as the monitor and grahics cards will manage. Stick a polarising filter in front of the monitor. OK, I can't think of any theory why that should work and it may just be a placebo, but I have had "customers" quite happy with that as a solution. Swop any fluorescent lighting that falls on the screen with filament lighting. Make the lighting indirect, rather than direct. Adjust the brightness and contrast. All of these can help.

Finally, invest in new non-crt displays. This will surely fix the problem. They also use less power, can be carried without causing permanent back damage and look sexy. Don't let your kids use them as targets for their rubber-sucker dart gun though.

Hi everybody,

Thanks for your advice. It seems all of you agree that the problem is not the equipment, but the sensitivity of my eyesight. I must admit that I'm very suspicious of that conclusion. This issue doesn't strike me as a standard flicker issue. I'll list my reasoning for this in point form below, thereby giving you all a chance to shoot them down one by one :)

1) Although I may myself be sensitive to flicker, I doubt that my entire family has the same problem, it just seems too unlikely to me. The flicker is very noticeable to anybody I've asked. You don't have to look hard to see it.

2) I have been using CRT monitors for years and years. I've never had this problem before and nobody else has experienced it either. It just seems highly improbable that this thing would just appear.

3) I always use my monitor at a standard rate of 85hz. That's a pretty high refresh rate, I believe the highest my monitor will do in this mode. It's a Iiyama Vision Master Pro 411, not really a cheap monitor (at least when I brought it, it wasn't). I've owned it for over three years. I can see the flicker below about 70hz. But 85hz used to be solid. If my eyes were the problem, then surely I would see the flicker consistency across the whole screen, not worse at different points?

4) I'm aware of the increased flicker when you don't look directly at a flicking screen. I get that with TV's :) At the moment, I'd say the flicking is quite low on my monitor. But when I look away from it, I can't see anything at all, only when I look directly at it. Of course, if I reset the screen to 60hz, it might not be the same. But then I would see the flicker anyway

What drew you to that conclusion? Is there anything I said that rules out electrical issues?

Once again, thanks for the help you are providing me. I appreciate the effort expended by all of you.

Regards,

Pan

Move the monitor to the center of the room. Does it still flicker? Move it to a room on the opposite side of the house. Does it flicker?

This will eliminate fields from power lines as a culprit.

Charles Perry P.E.

Hence my points about ambient lighting. That can make a little flicker a lot worse. Eyes can run in families, though (especially when Lassie gets run over...)

So stick the complete system in the car, drive to a friend's place, set it up and see. That will localise the problem to the system or the environment.

Anything is possible with eyes - just look at the web sites of optical illusions. But trying the system in a different location will at least let you know whether it is your system or your environment. Of course, if it does turn out to be your system, you are part of that system, after all.

Monitors invariably use switch mode power supplies these days. If you read the back panel of the monitor and see the range of voltages and frequency of supply that it can manage with, you will see that it is unlikely to be that your supply line voltage and frequency is the problem. Frequency anyhow could only be a problem if you were on a genny

- even then it is unlikely.

So it could be crap superimposed on the power lines. But, to create a visible effect on your monitors, it would be driving an AM radio crazy, anywhere near a power line.

So it could be RF energy through the air. But, again, your AM radio would go bananas.

It could be strong magnetic/electromagnetic/rf fields. Say, if you had a faulty microwave oven going the other side of the partition wall. But you would notice it only went wonky when the TV dinner was on. Further away than that, the strength of field would be doing other wacky things, like lighting fluorescent tubes even though the power was off.

All in all, because a monitor converts the power supply to DC, then switches that DC on and off very quickly, then filters out all the switching transients - the chance of anything coming in from the supply is a bit unlikely. Just being bad enough to affect one synch and not the other is asking a bit much too..

No, it is an interesting one. Shame that I can't come and look... Your Homeland Security people would never let the likes of me in though...:)

Do try the system in a different location.

The radio is a pretty good test - the nearest thing to what you describe that I have come across before was a ham radio wizz who had just put up this super duper rig and was seeing if he could get sparks off the aerial insulators....

Pan,

I work with magnetics and have had opportunities to view monitors around a few different sources; mostly DC, 60 Hz, and ~350 Hz transients. If the source is external magnetic interference, I would expect it to be line frequency. Then with a 60 Hz monitor, interference is likely to be slow waves, even seconds if the monitor and line frequencies are only slightly off. At 85 Hz monitor rate, the flicker will be 25 Hz, or the frequency difference.

The inverse square law is a powerful thing for radiated interference. If the flicker is similar at two locations, a radiated source must be nearly exactly between them, in the plane perpendicular to the line between them, or very far from both so they see similar effect, and then of enormous strength at the source, like the transmitter suggestion. Stray effects in electronics can demodulate radio, say into TV refresh frequencies.

Conducted interference on power lines would much more likely have similar effect at different locales. Has anyone considered interference from phone or cable lines? Maybe a ground loop in the cable to a modem?

This is one of your best first tests to narrow down many of the possibilities.

I will mention that simple wiring errors (like a neutral-ground connection that someone referred to) can result in high magnetic fields. I also see magnetic field problems in close proximity to single conductor cables carrying moderate amperages. In both cases, the underlying cause is that there is less cancellation of power frequency magnetic fields arising from the ac currents. Magnetic fields are a very common source of monitor flicker. There are products like mu-metal for magnetic shielding and 'jitter-boxes' for shielding monitors from magnetic fields to prevent flicker. Maybe something is wired not-quite-right in your expansion. A gaussmeter would be helpful to check magnetic field levels and track down sources, but they are normally a couple (or few) hundred bucks. It is common to have ambient residential magnetic field strengths, at power frequencies, of about 1mG (0.1uT), while fields of 10-20mG start to effect crt monitors. I would prefer to get rid of the magnetic field rather than shield the monitor and sit in the magnetic field. As far as I know, "prudent avoidance" is still indicated for magnetic fields.

j

[sorry if this post appears twice ... j]

This is one of your best first tests to narrow down many of the possibilities.

I will mention that simple wiring errors (like a neutral-ground connection that someone referred to) can result in high magnetic fields. I also see magnetic field problems in close proximity to single conductor cables carrying moderate amperages. In both cases, the underlying cause is that there is less cancellation of power frequency magnetic fields arising from the ac currents. Magnetic fields are a very common source of monitor flicker. There are products like mu-metal for magnetic shielding and 'jitter-boxes' for shielding monitors from magnetic fields to prevent flicker. Maybe something is wired not-quite-right in your expansion. A gaussmeter would be helpful to check magnetic field levels and track down sources, but they are normally a couple (or few) hundred bucks. It is common to have ambient residential magnetic field strengths, at power frequencies, of about 1mG (0.1uT), while fields of 10-20mG start to effect crt monitors. I would prefer to get rid of the magnetic field rather than shield the monitor and sit in the magnetic field. As far as I know, "prudent avoidance" is still indicated for magnetic fields.

j

Pan,

Please take that multimeter and measure the line voltage where you plug in the monitor.

Regards,

Ken

Hi,

This post is getting too difficult to manage :) Thanks for all your comments. I have read each of them. If you could all reply to this new post, it would make management easier :)

I should say that I'm 100% sure this is not my eyesight, or anybody else's. This is not normal flicker you would get from a monitor, there is something decidedly strange going on here. It's not only flicking, there's shimmering there too. I just know it's not my eyesight.

I plugged the monitor in at the other side of the house. The flicking was unchanged, so I'm starting to think it's not magnetic fields. It seems the next most likely thing is a voltage/frequency problem. As somebody said, monitors have a huge voltage input level, so it's more likely to be frequency.

The electric board is sending around an engineer with a voltage/frequency meter to measure it over the next week. I think that's the next stage so I'll wait until then. If that's the problem, I expect it will be picked up pretty quickly, although he said it might take a week to fully catalogue the issue. In addition, I have seen some gauss meters on eBay for sale from £30 for the better looking ones. Is it worth getting one?

Thanks for all your help,

Regards,

Pan

How about a flat panel display? These are not affected by "stuff" that affects CRTs. I sometimes work in an industrial facility where there are large magnetic fields. The CRTs in the control room had to be replaced with flat panel displays to get rid of color distortion and other maladys caused by the fields.

That would work, but I'd prefer to get to grips with the source problem before I consider any preventative means. In addition, it would cost me to purchase a new flat screen when I shouldn't really need to.

Regards,

Pan

Hi to all,

Found out one more piece of useful information. Seems a neighbour a few doors down has been experiencing similar problems.....

Pan