MikkieRSA

Q: 24" Cinema Display Backlight problem

Hi, I have a 24" Apple Cinema Display model A1267. A couple of weeks ago it started flashing black and then eventually went completely black.

 

It still charges the Macbook Pro, audio works and screen is detected by the Macbook.

 

After reading quite a bit on the internet, I found that it seems to be a generic backlight issue with these displays.

 

One of the debugging steps was to disconnect all power consumers from the display, i.e. any devices plugged into the usb port. the Macbook pro charger, and also the USB that goes into the Mac. Then plugging the mini display port plug in, flashed the expected screen image on the display for a split second, possible two flashes.

 

This tells me that the backlight LED's are sort of working, but that the backlight controller and/or the CPU on the logic board thinks there is a problem. Now from what I could gather out of the data sheet for the HV9982, the fault conditions could be either:

1. Over voltage protection

2. Short circuit protection

3. Open circuit protection (basically OVP)

 

I checked the power output signals of the backlight on a oscilloscope and found that the voltage rises to about 74V (after 24V level for short time), then decays down, jumps again to 74V and then decays down to 24V. At 24V the LEDS are not visible. I can post the images of the scope if needed.

 

The voltage ties in with the panel (LG240) data sheet that indicates a voltage of 73.2V (off the top of my head) for the 3 LED strings.

I also monitored the TP81 with the scope to see if there is a dip, but it remains constant at +24.5V

 

All components on the board were visually checked and they look fine, no burnouts etc.

 

So my questions are:

1. Where can I obtain a schematic diagram for the logic board

2. Could this be a CPU firmware issue

 

I have all the equipment required to test and repair this logic board. I have my own electronic design house designing many different products, including LCD backlights.

 

There seems to be quite a few "dead" cinema displays out there that could benefit from a repair guideline that does not involve swapping out expensive logic and PSU boards, only to have them die a few months down the line with a similar problem.

 

I am offering to find the root cause of this problem, which seems to have something to do with the tolerance on some components, causing the problem to increase with age.

 

Best regards

Mikkie

MacBook Pro, OS X Yosemite (10.10), Cinema Display

Posted on Nov 9, 2014 11:56 PM

Close

Q: 24" Cinema Display Backlight problem

  • All replies
  • Helpful answers

  • by Hell Pie,

    **** Pie **** Pie Dec 10, 2014 11:18 PM in response to MikkieRSA
    Level 1 (0 points)
    Dec 10, 2014 11:18 PM in response to MikkieRSA

    I am having the same problem with my 24" LED cinema display. The screen is completely black, but audio, USB, iSight, and magsafe charging all still work. I've tried plugging the display into a Macbook, and Mac Mini, and a Mac Pro, all with the same results.

     

    I found a service manual online and used a multimeter to check voltages at the various test points on the logic board. The voltages all came up fine. Based on the step by step guide in the service manual, everything seems to point to the LCD panel. I started disassembling the LCD panel and noticed a chip and possibly a diode on the T-con board were fried.

     

    I looked online to see if it would be possible to buy just the T-con board, but it doesn't seem available, and I am not sure how it would even be possible to remove it from the LCD panel. I might just have to buy the whole LCD panel assembly, which is not cheap.photo 2.JPG

  • by anirvan_m,

    anirvan_m anirvan_m Jan 24, 2015 9:56 AM in response to MikkieRSA
    Level 1 (0 points)
    Jan 24, 2015 9:56 AM in response to MikkieRSA

    Hi Mikkie,

     

    Yours seems to be the first of such posts related to this problem that I found to have a very thorough approach. My config too is similar to what you have – a 13" MBP (Macbook late 2011), running Yosemite (10.10.1), plugged to a 24" Cinema Display.

     

    I first encountered my blackouts around the same time Apple pushed out the Thunderbolt Display Firmware update v.1.2. I cannot be absolutely sure that there's a correlation here. Just to be sure, I tried my display with a friend's MBP (without the update) and the display had the same problems.

     

    So far what I've been able to determine by trying various things:

    1. Turning off the power supply to the display and replugging it after 10-15 seconds brings back the display. But only momentarily. Display goes dark in a minute or two.

    2. I stumbled upon another scenario: that of brightness. If I kept the cinema display's brightness set to around the 35% mark, it would work just fine. No issues whatsoever. However, whenever I plug in the display, the default config puts the display on auto-brightness. So in the window of a minute or so i have, i need to set the brightness to the lower mark. Thereafter, the display works just fine endlessly.

     

    I haven't taken to any service centre just yet. I fear they'll employ the take-the-sledgehammer-to-a-nail approach. So, I want to have some understanding of this problem before taking that step. I unfortunately, have no knowledge or possession of the electronic tools.

     

    I'd love to know more about what you've tried, and if there's anything I could try as well..

     

    After reading your post, I was wondering, is there is any chance that employing a "voltage stabiliser" instead of feeding the power directly to the Cinema Display might help?

     

    Thanks,

    Anirban

  • by SiliconSurfer13,

    SiliconSurfer13 SiliconSurfer13 Feb 1, 2015 10:22 AM in response to MikkieRSA
    Level 1 (0 points)
    Feb 1, 2015 10:22 AM in response to MikkieRSA

    MikkieRSA,

     

    I've researched this issue a little since I have the exact same problem. Charging, iSight, Speakers all work. In the System Preferences you can even see the display being recognized, just no image. Initially I could get it to come on for a few seconds, then it progressed to just flashing briefly when plugged in, now nothing.

     

    It appears most people just opt to replace both the power supply and logic board for the unit. Since they seem to think it's a combined issue. I'm considering just purchasing the logic board first (both parts are roughly the same $130 - $160) so it's a 50/50, if one doesn't work I'll purchase the other and if both don't work I'll just return them for a refund. If I get the logic board to work and if it looks like the logic board has damage like Pie's would you benefit from having a board to play around with and attempt to repair? I know you probably need a good working board as well to compare. So I've provided a link for the parts. They don't specify if it's refurbished or new.

     

    http://www.macpalace.com/661-4823-logic-board-apple-led-cinema-display-24-mb382l l-a1267.html

    Screen Shot 2015-02-01 at 1.17.11 PM.png

  • by rdjh,

    rdjh rdjh Nov 19, 2015 8:55 AM in response to anirvan_m
    Level 1 (0 points)
    Nov 19, 2015 8:55 AM in response to anirvan_m

    Hello... I realize that this is an older post, but I thought I'd add my experience to it in case it can help someone down the line.

     

    My symptoms were exactly the same as described in the original post. I followed the suggestion of anirvan_m to power the display off for 10-15 seconds, and I was able to get the monitor back on. Although, now it's slightly dimmer than it was originally. I'm still trying to figure out why, or if it's just slowly burning out, but it's been working fine for about 30 minutes now, other than that.

  • by playingwithwords@me.com,

    playingwithwords@me.com playingwithwords@me.com Dec 2, 2015 9:29 PM in response to rdjh
    Level 1 (10 points)
    Dec 2, 2015 9:29 PM in response to rdjh

    I'm having the exact same problems as many of you here... 24" Apple LED monitor (but mine is connected to a 2009 27" iMac). For a while the screen would get really light. Unplugging it and plugging it back in solved that for a few days at a time. Then it went completely black. Still has power to the USB ports in the back though.

    I found something interesting though - now when I unplug the unit and plug it back in, it stays black.... UNTIL.... I reboot my iMac that the 24" LED is plugged into (with mini displayport). Then it powers up fine and runs fine.... until the iMac goes to sleep and the 24" LED goes black and stays that way.

    Does this indicate anything? Power supply? Or logic board? Mine is in excellent cosmetic condition, and I've had it for probably 5 years. I love it and don't want to part with it. Suggestions would be appreciated. Thanks!

  • by malcolmhall,

    malcolmhall malcolmhall Jan 17, 2016 5:34 AM in response to playingwithwords@me.com
    Level 1 (4 points)
    Jan 17, 2016 5:34 AM in response to playingwithwords@me.com

    Sounds like I've got the same problem. Starting with the non-working black screen, un-plug the mini display port from the laptop and display's power from the wall. Plug in the power then upon plugging back in the laptop the backlight flickers rapidly on and off about 5 times in roughly half a second and then it stays off. By repeating this several times sometimes it is possible for it to stay on. Then it can be fine for weeks (even going off via energy saving) before the issue arrises again.

     

    The other symptom is the capacitors have been squealing loudly for months now.

  • by JSS540,

    JSS540 JSS540 Jan 24, 2016 9:55 AM in response to MikkieRSA
    Level 1 (0 points)
    Jan 24, 2016 9:55 AM in response to MikkieRSA

    I have a 27" apple cinema display and having the same backlight problems.

    Most of the times that I plug it in my macbook, the screen stays dark with sometimes a flicker. What I found that worked, was to remove the charger and usb cable from my macbook and just plug in the Mini Displayport connector into the macbook. After pulling it in and out for 1 to 10 times, the screen will flicker and turn on and I can use it until the next time that I unplug my macbook.

     

    Sometimes the brightness doesnt seem to be a 100%, and sometimes its very bright, but as long as this screen is working, thats good enough for me. Although I'd like to fix it too