iMac G4 17" White/Gray screen problem

Question

Level 1

14 points

iMac G4 17" White/Gray screen problem

Hi All,

I am collecting and repairing old Macs and I just got an iMac G4 17" from eBay few days ago. It was marked as non-working machine with a power supply problem. I have fixed the issue with the power supply (few capacitors had to be changed) and now I'm facing to the next problem, and there I need your experiences:

The iMac boots nicely but the 17 inch flat screen just gets the electric power - no graphic signal. It displays just a white screen.

If I connect an external monitor I can see everything working fine. I can go the the System Preferences and there I can change the different settings of BOTH(!) screens. If I change the brightness of the "iMac Display" then it goes darker or brighter. If I change the resolution from 1440x900 to something else then it goes "off" and then comes back but still with the white screen. If I change something again then it will goes "off" and then get the new resolution but still displays the white screen. Looks everything fine but no graphic display just a white screen.

What could be the problem?

I think the GPU is OK because it would not have picture on the external monitor.

I think the LCD is ok as well, because I can change the brightness and if I change between the resolutions then it does something...

When it boots it displays white screen on the LCD and on the external monitor too. After the boot it loads the Mac OS X 10.5 and then the external monitor displays the "real" screen.. but I can not see the gray Apple logo on grey background on any display before my "desktop" comes on the external monitor.

Few more information about this poor iMac:

- I think it was already opened few times because the bottom disk plate says it sould be an iMac G4 17" 1,25Ghz with a serial number W8******PJH but if I'm going to check this in the About this Mac window it tells me it is an iMac G4 17" 1Ghz model with the SN: W8******SD3.

- I am not sure the LCD with the neck belongs to the "logic board" or to the "bottom disk plate"

I have already checked many posibilities to find out what could be the problem and now I'd like to ask you too.

I have no problem with soldering or replacing components...

Thank you in advance,

Attila

iMac, Mac OS X (10.5), iMac G4 17" 1Ghz, 1GB RAM

Posted on Aug 18, 2013 4:06 AM

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Answer 1

Allan Jones

Level 9

78,266 points

Aug 18, 2013 10:36 AM in response to fehervaria

Welcome, Attila!

You are correct in assuming that an image on the external indicates the GPu is working. I'm suspecting the fault is in the wiring between the logic board and the display. If you manipulate the display arm, do you get an internittent image?

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Answer 2

fehervaria Author

Level 1

14 points

Aug 18, 2013 1:04 PM in response to Allan Jones

Hello Allan Jones,

thank you for your fast answer. I checked that too. Moved the LCD panel from each maximum position to the other maximum positions already. Nothing changes. The screen keeps the constant white everywhere. No interference nor any changes on the screen.

I already tried to disconnect the video signal cable from the logic board: also no changes on the LCD.

How can I test the cable? Can I test the logic board connector somehow? Is it possible the connection problem is not "down" at the logic board then "up" in the LCD housing? Or in the neck?

EDIT:

Is the 15" LCD arm cabeling compatible with the 17"? or any 17" cable is compatible with each other?

Thank in advance,

Attila

Message was edited by: fehervaria

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Answer 3

fehervaria Author

Level 1

14 points

Oct 29, 2013 2:38 AM in response to fehervaria

Hi,

Just to reply to my own post and let other to know what was the problem:

Well, I bought a new neck what is for an iMac G4 1Ghz and find also an LCD what is for the same model.

After spent hours and hours with multi-meter and signal measuring with oscilloscope I find the LCD is wrong.

I did test from the GPU till the end of the cables up in the "head" and till this point everything was okay..

Actually the original screen of my iMac is one of the bad series from the iMacs, serial numbers starting with W8... This series has a production failure on the logic board of the LCD! Wrong (too thin) soldering on few of its components.

Knowing this issue, I decided to replace the LCD itself instead of repaire the bad LCD electronic board. Could be very complex to resolder each connections.

If anybody has same kind of problem with an iMac G4 and the serial number starts with W8... then follow this troubleshooting steps:

1.) access an external monitor to be sure the GPU works properly (rarely goes wrong, if yes, Logic Board failure: replace it. tip: eBay)

2.) open your iMac, disconnect the neck cables from the Logic Board and open the head, remove the LCD and test the neck cables and connectors with multi-meter. Many times during the years the neck cables breaks inside the neck and then has mis-connections or totaly "no connection", if this is your problem: replace the neck or if you're hardcore electric hobbiest replace the cables only. tip: eBay

3.) if both of the previous checks did not point to any failure then probably your LCD has the problem. It is very difficult to repaire, maybe impossible. Replace it. Be carefull: iMac 15" and early 17" LCD are different than late 17" and 20" LCDs. Be sure what kind of you have and buy the correct replacement component. If you have mixed them up, then you'll have lot of trouble with the neck cables/connections and Logic Board GPU connector pinout...

Hope it helps,

happy repair,

cheers

Attila

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Answer 4

Vrodion

Level 1

4 points

Mar 16, 2014 6:30 PM in response to fehervaria

Hi Attila,

I do have the same issue with my 20 inch iMac G4. And serial starts from W83.....

Is it comon issue with LCD in this machines?

I bougth a used LCD of Ebay, but still no luck.

Thanks

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Answer 5

fehervaria Author

Level 1

14 points

Mar 18, 2014 2:22 AM in response to Vrodion

Hi Vrodion,

as I find over my resources that models are affected which have the W8 at the beginning of their serial number. Not each, but the risk is high.

Did you check the GPU and the neck connections of your iMac 20"? Over your question is my summary of the troubleshuting. Please follow it and then you will know where is the problem exactly. Maybe your neck cables are damaged and then you need another neck (for the 20" model). What is your exact syndromes? Does your iMac works properly with external monitor?

/Attila

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Answer 6

Vrodion

Level 1

4 points

Mar 18, 2014 9:30 AM in response to fehervaria

Hi Attila,

My iMac has a blank screen but works perfect when connect to the external monitor. LCD back light is okay, not a inverter problem. I bought a used LCD from ebay, but still not working at all. Checked TDMS cable with multimeter- looks

okay. Replaced neck , stiil nothing. Pulled logic board and inspected connector on the board-okay.

I did a LCD swap from my G5 , looks like a different sygnal, display blinking all colors( it is differednt model LG

instead of my IDTECH m201z2). And now looks like I have a TWO defective LCD`s. 🙂

This iMac for my collection, not a primry machine.

Please let me know

Thanks

Vrodion

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Answer 7

Allan Jones

Level 9

78,266 points

Mar 18, 2014 10:35 AM in response to Vrodion

I do have the same issue with my 20 inch iMac G4. And serial starts from W83.....

Is it comon issue with LCD in this machines?

The risk is historically highest with G5 iMacs and G4 PowerBooks. Reports of the same issue are less common with teh G4 series but, obviously, it does happen.

Allan

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Answer 8

fehervaria Author

Level 1

14 points

Mar 20, 2014 1:31 PM in response to Vrodion

And now looks like I have a TWO defective LCDs

Hi Vrodion,

It seems to be right, looks like you had no luck with the second LCD either.

Here, in eBay, I find one more LCD, what could fit to your iMac. It is in Boise, Idaho:

http://www.ebay.de/itm/Apple-LCD-Display-Panel-for-iMac-G4-20-661-2991-20-WSXGA- /161241995286?pt=LH_DefaultDomain_0&hash=item258ac59816

If you've checked already the mainboard, the neck, then only the LCD is the last part.

I've study a lot from this guy's guides:

http://www.dremeljunkie.com/2011/11/all-in-one-20-imac-g4-genuine-tmds-to.html

Good luck,

Don't give up, it is a very good feeling when something going to work what you had a fight monthes long. 😉

/Attila

PS: I've a PowerBook G4 12". I had to repair the mainboard. It had a damaged (break off from the PCB) power switch connector. I spent more than 40 hours to read after, collect PCB schematics and try 100 different way to fix the problem till I find THE Sollution. Now, it has an SSD drive and this is my livingroom internet surfer "veteran". 😎

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