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#6329 - 12/03/09 10:37 AM 27" iMac i7 graphics card problem?
JoBoy Offline


Registered: 08/05/09
We've enjoyed very much the BEAUTIFUL 27" iMac i7 for the past week, but there's one persistent problem: There is an irregularly intermittent flash of light across the entire width of the screen. It is mostly horizontal and roughly one inch wide maximum, but there is also a narrow, vertical component usually near the center of the screen that is roughly four inches or less in amplitude.

Some days, it occurs only once per hour, but it is now increasing in frequency, sometimes every minute. This machine has the new ATI Radeon HD 4850 512MB graphics card. What do these symptoms suggest to you?

Perhaps I should mention that I installed an additional 4GB RAM from OWC to bring the total RAM to 8GB. Any suggestions would be gratefully received.
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Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5

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#6338 - 12/03/09 03:39 PM Re: 27" iMac i7 graphics card problem? [Re: JoBoy]
joemikeb Online
Moderator

Registered: 08/04/09
Loc: Fort Worth, Texas
The flash you describe is unlikely to be the result of the OWC RAM, but you can easily eliminate it as the cause by simple removing it and seeing if the problem persists. If it does, it is time to take your beautiful 27" iMac back to the Apple Store before you have a complete failure. If removing the RAM resolves the problem, my experience with OWC has been they stand behind their no questions asked lifetime warranty.
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#6340 - 12/03/09 05:12 PM Re: 27" iMac i7 graphics card problem? [Re: joemikeb]
JoBoy Offline


Registered: 08/05/09
This is weird. I disconnected the iMac from the GbEthernet network and also from the FireWire 800 connection to the OWC external hard drive and then restarted. Presto! No flashes. Then, I reconnected them and still no flashes. I have no real reason for doing that except to rule out all possibilities before digging into the innards of the machine. At this point, we'll watch and wait. My wife is using it and has had no issues since the reconnection. That problem was present since the day we got it. The network and external drive were connected before it was started up for the first time. Pulling the RAM was next on the list, but, so far, there's no need. The extra RAM also was installed before the first startup. confused

Off topic: There is a stock Apple desktop image of a satellite view of the world and the rest of the screen is black. On the 27"iMac, it is stunning. It matches the black frame of the display and the desktop icons are dramatically clear with white names. I've also started using it on my 30" Apple HD display because I don't lose sight of the mouse with its white outline over black background and the file icons are easy to scan visually. It's a fortunate combination of beauty and functional usefulness.


Edited by JoBoy (12/03/09 05:21 PM)
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Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5

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#6349 - 12/04/09 01:30 AM Re: 27" iMac i7 graphics card problem? [Re: JoBoy]
artie505 Online


Registered: 08/04/09
> This is weird. I disconnected the iMac from the GbEthernet network and also from the FireWire 800 connection to the OWC external hard drive and then restarted. Presto! No flashes. Then, I reconnected them and still no flashes.

That suggests that you maybe ought to check, i.e. swap-out, your cables.
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The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire

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#6353 - 12/04/09 08:36 AM Re: 27" iMac i7 graphics card problem? [Re: artie505]
JoBoy Offline


Registered: 08/05/09
Originally Posted By: artie505
...That suggests that you maybe ought to check, i.e. swap-out, your cables.


I should have thought of that. I used the FireWire 800 cable provided by OWC to connect to the new external drive. The rest of my hardware uses Monster cables with which I've never had a moment's problem. Thanks for the heads up.
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#6360 - 12/04/09 12:11 PM Re: 27" iMac i7 graphics card problem? [Re: JoBoy]
artie505 Online


Registered: 08/04/09
> I used the FireWire 800 cable provided by OWC to connect to the new external drive.

I hated the first external HD I bought and was able to return it for a full cash refund because the USB cable with which it came with was defective.

> Thanks for the heads up.

I'm happy to have been able to offer the suggestion.

And in closing... Don't forget that you may have an issue with one of your iMac's ports.

I suggest that you give them a workout with cables you know to be good and see if you can recreate your issue.
_________________________
The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire

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#6367 - 12/04/09 02:25 PM Re: 27" iMac i7 graphics card problem? [Re: JoBoy]
Virtual1 Offline


Registered: 08/04/09
Loc: Iowa
backlight flickering, or video?
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#6372 - 12/04/09 02:40 PM Re: 27" iMac i7 graphics card problem? [Re: Virtual1]
JoBoy Offline


Registered: 08/05/09
Originally Posted By: Virtual1
backlight flickering, or video?


I believe it's the video. How does backlight flickering look on the display?
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Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5

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#6373 - 12/04/09 02:41 PM Re: 27" iMac i7 graphics card problem? [Re: Virtual1]
artie505 Online


Registered: 08/04/09
Could a backlight actually display the symptoms JoBoy has described?
_________________________
The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire

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#6377 - 12/04/09 04:42 PM Re: 27" iMac i7 graphics card problem? [Re: artie505]
JoBoy Offline


Registered: 08/05/09
Originally Posted By: artie505
...I suggest that you give them a workout with cables you know to be good and see if you can recreate your issue.
I have no proven FireWire 800/800 cables so I just ordered two from Apple. Yes, I looked elsewhere for a better bargain, but couldn't find one that was substantially better. I want to save the remaining USB2 slots for future stuff. The new iMac doesn't have a FireWire 400 port.
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#6379 - 12/04/09 06:53 PM Re: 27" iMac i7 graphics card problem? [Re: artie505]
Virtual1 Offline


Registered: 08/04/09
Loc: Iowa
Could a backlight actually display the symptoms JoBoy has described?

depends on what's flickering... the lighting or the video.

I've only seen a video card cause flicker a couple times, and it was the entire screen. Usually if the video card is having problems, you lose all video (and have backlight) or you get frying pixels or whacked color, all over the display. I've never seen a video card cause issues on a specific region on the screen.

LCD panel problems are well known to cause horizontal and vertical problems, both in single lines and in swaths (typically of one scan group, around 1-1.5" wide, that cross the entire screen, horizontally or vertically, or affect entire areas of the display such as bottom 1/3) LCD problems can also cause issues that look like video card, that affect the entire display, but don't cause frying pixels. (color can be off)

You can also have a loose LVDS cable in theory, but it's a digital connection so I'd be surprised to see such a connection do anything but completely kill the video on the entire screen.

In most cases a vertical/horizontal issue is LCD. But if it's lighting and not video, you may need to look at the inverter. (the backlight itself is in the LCD panel too, so the inverter is really the only thing outside the LCD panel that can cause h/v issues)
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#6384 - 12/04/09 10:33 PM Re: 27" iMac i7 graphics card problem? [Re: Virtual1]
artie505 Online


Registered: 08/04/09
I've always thought the backlight surrounded the screen, which is why I was wondering how it could cause the horizontal/vertical flickering that JoBoy reported seeing in its middle.
_________________________
The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire

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#6397 - 12/05/09 12:13 PM Re: 27" iMac i7 graphics card problem? [Re: artie505]
JoBoy Offline


Registered: 08/05/09
Originally Posted By: artie505
...That suggests that you maybe ought to check, i.e. swap-out, your cables.
FWIW, there still has been no more screen flickering since disconnecting the Ethernet and FW800 cables from the iMac and restarting. Network and external drive continue to function normally. Go figure. I spent $96.00 on Apple FW800/800 cables for the new OWC external drives located on the iMac and Mac Pro. They have not yet arrived. When they are installed, I hope I feel $96.00 worth of increased reassurance that a potential vulnerability in the system has been eliminated.<grin> Truthfully, I've become something of a fan of higher-quality cables. I was disappointed that I couldn't find a Monster cable that was FW800/800.
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Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5

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#6402 - 12/05/09 05:15 PM Re: 27" iMac i7 graphics card problem? [Re: artie505]
Virtual1 Offline


Registered: 08/04/09
Loc: Iowa
I've always thought the backlight surrounded the screen,

On almost all laptop screens, and on the smaller desktop screens, the backlight runs along the bottom of the display. There's a fresnel lens ("light spreader") on the whole back of the LCD behind the LCD glass that diffuses the light at a gradient up from the display so the lighting is evenly bright from top to bottom. For them, backlight issues evenly affect the entire display's brightness.

Larger desktop LCDs are lit both from the top and the bottom due to their size because the light will get too hot if it has to make light for the entire display. With them, if one light or one arm of the inverter power supply is having problems, the top or bottom will be noticeably darker. Some of these displays have two bulbs per half, and so in theory one bulb or one of four arms could go bad and cause a quarter of the screen to be dimmer.

LED backlit displays simply replace the CCT bulb with many LEDs. They still use the fresnel lens. (they don't have a grid of LEDs all over the back) It would be very difficult to smoothly light from all sides, because the center would tend to be dim, and I don't know how you'd deal with the corners being overly bright.
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#6411 - 12/05/09 09:24 PM Re: 27" iMac i7 graphics card problem? [Re: Virtual1]
artie505 Online


Registered: 08/04/09
A bit more knowledge; thanks for the tutorial. smile
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The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire

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