iMac 24 inch early 2008 no backlight at all (ccfl)

Discussion in 'iMac' started by donmoron, Nov 21, 2012.

  1. donmoron macrumors newbie

    Nov 21, 2012
    hello guys!
    I have an iMac 24inch early 2008 (model A1225) with no backlight (no one of 7 ccfl tubes are not shining). It started like that - flickered for a short time and then went out forever. Since i have some electronic experience I disassembled it and went tracing the problem.

    My first thought was that it would probably be an ccfl inverter, and since they have an input fuse and no ccfl is shining, my first bet was that it would be the fuse - bad luck dude, fuse was okay. I measured the inverter cable for some open circuit - cable was okay and inverter power supply voltage (24V DC) was okay too.
    So next thing to check was the inverter itself. I went check high voltage transformers secondary coils - they sometimes got burnt or open circuit - no luck too, they were okay too.
    Third thing was measuring the mosfet transistors driving the primary coils of HV transformers - okay too.
    Next suspect was logically the inverter driver IC (BD9892K), so I found datasheet (which is not very informative tbh) and found out that there is an standby pin (pin no.7) - it should be high(5V) in order to get the inverter to work, but the pin was low(0.02V) when the MAC was running and 0V when the computer is sleeping or shut down. So I went trace the source of this "enable" signal and I actually found it is going to the logic board.

    Because of the LOW value on the enable pin, next thing to try was to try firing up the inverter manually by supplying the HIGH level on the enable pin of the IC (since the enable signal is going back to logic board, I first disconnected the cable enable pin going to logic board in order not to fry the logic board by this test). Then I supplied the IC VCC voltage to its enable (standby) pin and voila, the inverter wen up and backlight was shining, so the inverter is good.

    So we now know that the problem lies somewhere on the logic board in the source of the backlight enable voltage. However since the logic board has about 16541648994 components, It would be extremely cool to get the logic board schematic (I have seen schematics for macbooks so I believe they also exists for imacs). Does anyone know if they exists and where to get it? It would make tracing the problem so much easier.
    Or has anyone faced the same problem as me and got it solved?

    Thanks in advance :)
  2. donmoron thread starter macrumors newbie

    Nov 21, 2012
    in case that anybody will read this: i found the service manual and looked in it for source of the backlight-enable signal - it was the graphic card that supplies it. i removed the graphic card and tried to trace the problem, but the signal got lost in one of the vias on pcb (probably continued in some middle layer of the pcb), thus no chance to repair that.

    since the backlight starts at computer power on and switches off at power off/sleep i took the signal from the third diagnostic led (GPU present) on the main logic board and used it also as backlight-enable signal by connecting wire between this led and the inverter backlight-enable signal wire (note that i disconnected the backlight-enable wire from the main logic board to not to fry the GPU and left it connected only on the inverter side). working perfectly!

    It works and the main thing is, that the repair cost me only one 1meter long wire, thats all :)
  3. rhoydotp macrumors 6502


    Sep 28, 2006
    thanks for the tip! this might come handy on a rainy day :D
  4. cyclotron451 macrumors regular


    Mar 16, 2005
    an excellent repair job!
    glad it was just a logical low in the wrong place and time.
    (I think all of my repairs at home in the last few years have been due to the chinese 'fake' electrolytic capacitor epidemic of '99-2010. I luckily haven't noticed this plague so much in my Apple gear - but all home satellite devices have needed a handful replacing) Could that be the root failure of your graphics card?
  5. donmoron thread starter macrumors newbie

    Nov 21, 2012
    thanks man :)
    It is not surely related to the electrolytic capacitors since that GPU card did not have any of them, there were just tantalum capacitors, transistors and some ICs. But yeah, chinese capacitors causes a lot of trouble in all electronic, I fixed a lot of stuff just by changing these :)
  6. robjones macrumors newbie

    Jan 8, 2013
    Hi there,

    I am having the exact same problem with my 24" iMac. It's killing me because I'm self employed and can't anything done!

    Is your fix something I could do myself? I'm a pretty handy DIY-er and own a couple of soldering irons. Would it be possible to get a photo of where the two ends of the wire attach? Is it that simple?

    I'll add you to my Christmas card list if you can help me out! :D
  7. zimbeldek macrumors newbie

    Mar 17, 2013
    My 24'' iMac has a dead backlight. I can see the images on the screen but with external source of light. The all four LEDs are on and according to the service manual it should be the inverter or the cable. I checked the cable and replaced the inverter board but no luck so far.
    I have got all the voltages coming out the PSU normal (25V and 3V between the last two pins),3V does not present when the LCD is missing or not detected, unfortunately it is tricky to measure the voltages on the inverter board itself.
    I was wondering which is the "enable" cable coming out the graphics card to try this or any other suggestions?

    Thank you!
  8. zimbeldek macrumors newbie

    Mar 17, 2013
    Sorry my bad.
    After double check it appears that I have the 3V VBR for brightness adjustment and have 0V on the on/off pin. Please note that I measure the voltages coming out the power supply.
  9. Steve Cook macrumors newbie

    Jun 15, 2013
    Same issue here, with an iMac 21" late 2009. :mad:

    Could someone little genius explain to me clearly which points to connect and which ones to disconnect before I completely tear my hair off. ;)
  10. titaniumjones macrumors newbie


    Oct 29, 2011
    My poor old 24" iMac

    Hi Don,
    I have what you described. I don't suppose you know which wire was the one that fed back to the logic board?
    I would like to replicate your repair. Currently I had to buy a miniDVI to VGA adaptor and a VGA cable and plug them into a 19" monitor that I borrowed from a friend.

    Any help appreciated.

    p.s. my email is if you want to take this off the forum.
  11. mbayrock macrumors newbie

    Nov 8, 2013
    Did anyone get an explanation or pics on where the jumper connects to?
  12. Reefer78 macrumors newbie

    Nov 26, 2013
    Hi, I had the same problem, so I fixed my Imac the same way. Here are 2 pictures about the fix. You need to solder one end of the cable to the resistor next to the third diagnostic led, and the other end to the inverter board cable. I just unplugged carefully the second cable( this is the on/off) from the psu side of the inverter board cable, and attached it to my cable with a pin from an old connector. I hope that helps.
    The picture about the connector is the wrong end of the cable, but the other end is similar, just there is no gap between the 2. and 3. cable.

    Attached Files:

  13. bosscharles macrumors newbie

    Mar 3, 2013
    Hi Reefer78

    Thanks for the pics they are very helpful but I still have some questions. I understand that we need a wire from the resistor below the 3rd led. Any side of the resistor in particular this needs to be soldered from? Also once I've done this, can you confirm what wire on the inverter cable is the backlight enable wire? As I will need to feed the wire from the 3rd led resistor into this cable (also is it on the inverter side or psu side?) Also, from there what else needs to be disconnected from the logic board so that the GPU isn't fried? (donmoron's 2nd comment on the thread). Sorry for all the questions. I think it would be really great if you or donmoron could write up a full step by step or ifixit piece? Many Thanks.
  14. spsk007 macrumors newbie

    Aug 13, 2014
    Thunderbolt backlight out (image visible with flashlight)

    I'm curious if this can help me with my thunderbolt display as well. I am hoping someone can help me identify if I may have this same issue with my Thunderbolt display.

    I took parts from a working T-bolt display (2011ish model) and swapped them 1 by 1 into a non-working T-bolt (image visible with flashlight leading me to think it’s the backlight not being on somehow/burnt out). Swapping the entire LCD display was the only way I was able to get it to work. This makes me think there is something internal in the LCD display that is broken (fuse / inverter / ?).

    The pictures in this repair are for an iMac but I wonder if they use similar logic board connections so it would still help me repeat the repair on my T-bolt display?

    I’m computer savvy but don’t have much experience measuring voltages so I’m really hoping for some help with how I might be able to troubleshoot and fix my display. Also, any advice on how to test voltages of the LCD internals while having it connected to the T-bolt case would be lovely. I’m thinking that most of the connections I want to test on the LCD are on the back of it and I won’t be able to get at them if things are plugged in.

    My tentative plan is:
    1. Take LCD display out of T-bolt
    2. Open up the LCD display to get at its internals
    3. Look for blown fuses / charring
    4. Figure out how to have the LCD display plugged in and powered on so I can do voltage testing around (although I really have no idea what voltages to test as the apple tech repair guide does not go into any detail inside the LCD, just the logic / PSU and such).
  15. ryguy92000 macrumors regular


    Sep 20, 2014
    Salt Lake City, UT
    So just a heads up, this worked PERFECTLY. My client was really concerned about the enhanced cost with a new graphics card (the apple fix). I actually used the built-in harness which I pinned across from aforementioned replies. Ill post pictures below of the pin needed. Its actually a DIRECT line with no branching in the harness. I now have a functional display!

    1) the made connection to LED #3; The 4th pin puts out 12 volts & #3 is just shy of 5 for me, was plenty to enable the backlight.

    2) The section of harness (black lead) and the wire (red lead) I had an old sensor lying around from a previous job which worked a treat. Dont use anything much larger than say a number 24 as its just unwieldy to get in place.

    3) This pin on the Power supply is the pin that directly goes to the LCD backlight board. a direct solder here from the previous LED would also work just make sure to disconnect the other wire first!!!

    @spsk007: The Thunderbolt displays main fault stems from the control board failure and backlight dimming. Unfortunately the Tbolt display uses an entirely different hardware principle.

    IMG_2085.jpg IMG_2086.jpg IMG_2087.jpg
  16. highmate macrumors newbie


    Jan 18, 2018
    Hey... old thread i know. But after Graphic Card replacement the imac has noch backlight. I can see slightly the booting screen and can type in the password... im not able to do any soldering work here. What i have to replace? Power Supply, or inverter board on the panel or the whole lcd panel? Thank you!

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