Low light or no backlight on LCD

Hello All,

I did as "LikeARabbit " suggested above, and it worked for me.

I was having an issue of a very dim to complete shutdown of my Apple LED Cinema Display 27".

I opened the cinema display as per the instruction from the "Apple Thunderbolt Display Teardown".

The inside are almost the same as I have both and can attest to this.

I tested the power on the logic board that "LikeARabbit" suggested and mine was running a consistent 24.3 V on my digital multimeter. As to where to test this, please see attached photos. sorry for the bad quality photos, but I had very little time.

Since my power was within in range I made sure all the cable were seated properly by unplugging and replugging them one at a time. Do this with the power cord unplugged so you do not fry the boards, LCD, or yourself.

One plug I noticed right away was the "Function" plug which was very loose. I believe this is the LCD background lighting system as it istantly increased my brightness after reseating this cable.

After powering the LCD up, I moved this one cable around a bit, and sure enough, it was the one causing my issues. It is now properly seated and I have not had any problems as before. The LCD is working as it did when new.

I did create new monitor profile after this as I noticed the colors were off at this point. I am a photographer and need these LCD to match. with my iMac 27.

Below are the photos of the cable that was loose inside my Apple LED Cinema Display 27 and the two connections where I tested my power to the LCD backlight system.

I hope this helps.

Kind regards,

Johnny

I had the same problems. Backlight was gone or very low. The short story: I found out that the micro controller does not send the PWM signal to the LED driver. Very mysterious! I checked the LED driver components (Mosfett, diode), I read the data sheet to understand the function of the HV9982 (LED driver). In the end there was no hardware problem there. If I connected the computer the micro controller enabled the LED driver but "forget" so send the PWM signal, so the LEDs stayed off. Mmmm...maybe this is a software error or Apple wants to tell me to buy a new one? So I bridged the PWM input pins with a little wire connected to a 3.3 V pad (which I found about 1 cm around). Nice that die PWM output pins of the micro controller were bridged by 0 ohm resistors, so I could remove same and cut the connection from the micro controller. And then I got full backlight again. Of course I am not able to dim the backlight anymore.

I had this same problem and managed to fix it by frankenstein adding inverter from iMac 27 and routing 12v LCD Power on signal to enable backlight for screen.

Hello all… I’d like to add some information for those who are still struggling with this. Just a bit more of a clue, as I managed to briefly get my 24” Cinema Display to work today. Also, a cautionary tale… as I immediately fried it right after.

I’ve tried pretty much everything on this and several other threads to no avail. Today, I thought I’d give it one more go before pitching the Cinema Display into the bin. I took it all apart, put it back together while checking for loose connections. I left the glass off and the LCD connected, but not screwed down, in the hopes that I could check some connections while it was powered on. I do NOT recommend doing this. It was simply a last-ditch attempt on my part.

In the image below, you will see the heavy black cable that connects to the Logic board. I’m not entirely certain what this is, but when I unplugged it, and then plugged it back in, the monitor started working! Full brightness, brightness control returned, etc. I went about trying to repeat this, to narrow down the issue, and found that once I unplugged the power, the old problem returned (very dark, flickering if brightness is turned up, blacks out, etc). I wasn’t about to dismantle the monitor every time I wanted to use it and re-seat this plug, so I kept trying different combinations of plugging, unplugging, etc. I was able to get it to work with this method over and over… and then disaster struck. The LCD slipped out of my hand, and shorted the power supply BZZZT! So, on one hand I’m relieved that this effort is over, and on the other, happy to have finally gotten close to a solution, also… RIP 24” Cinema Display :)

I hope this information helps someone else get their display fixed, but be careful if you’re working with live juice. It’s NOT a good idea :)

Does someone know where the 3.3 v pad is on the Thunderbolt Display Board ?

Which pads i have to bridge ?

I have a black screen on my thunderbolt Display after Firmware update.

Is it maybe possible to reset delete the Firmware to downgrade where the

the PWM signal works ???

Thank to Michael very good work

Hey guys,

I have even a stranger issue. I noticed the low light on my monitor so I decide to take of the front glass and realize the problem was gone... it's bright as new.... only issue is when i put back on the front glass it gets dimmed again....

magnet issues? can someone answer this....

Backlight LED strips need to be replaced, ether the LEDs are bad or the connector of the strips is bad because of the heat over the years…

Got a free 27 LED Cinema Display with dark back light (BL) 99% of the time plugging in Display Port. Finally isolated a deterministic sequence that worked for a day. Wife has the same monitor and also not coming on 80% of time when plugging in Display Port. Here is the working sequence.

This sequence got the monitor up and working for a couple of hours but since then seem to have more difficulty and backlight just flickers now on the lowest sitting and shutoff at any higher settings. I guess there are thermal related weak solder points likely in the LED strip in the panel.

Need

1. Total darkness + a flash light to help see
2. External keyboard for the Mac/Macbook*

Preparation

Plug in Cinema Display Power

Plug in Cinema USB to Mac

Plug in Cinema Macbook Magsafe connection (probably don’t need this but might as well get MB ready to close the clamshell later while Cinema Display remains on)

Plugin Cinema mini Display Port (mDP) to Mac

Use external keyboard (pair it in advance if wireless) F1 key. Click it many times to set external monitor brightness to lowest setting (even though you think its not on)

Sequence (A)

Unplug mDP

Unpower Cinema Display (I use a power strip to repeat this sequence easier)

Wait few seconds

Power on Cinema Display

Plugin mDP, you will probably now see back light strobe dimly in total darkness

Click F2 1x (brighter), strobing probably go a little faster, wait 10s and see if it comes on permanently. if not, click F2 1x brighter and repeat until backlight stays on stable.

If backlight goes completely dark, need to restart at top of this Sequence (A)

Different/same problem as the LED strip old solder variable resistance?

This is probably the common LED strip solder resistance problem. The backlight circuit compares the different LED channels for voltage. If difference too large, circuit shuts off. Had this in my 27” iMac (2009 so same LCD panel as the 27 LED Cinema Display) It would not go very bright before self shutting off backlight. I pulled apart the LCD panel and reflowed the LED connections and now works perfect. Tearing apart the panel to get to LED strip require handy skills and cautious approach. Very easy to damage and hard to keep dust+threads out of panel+diffuser layers during reassembly. Below is the full link on that.

https://drive.google.com/file/d/13GqPCJh...

Fixed 2 of these LCDs (2009 iMac 27”, Cinema LED Display 27”) Only need to apply nice leaded solder to the LED strip connectors. On my Cinema LED Display, one of the connectors just fell off the strip during disassembly. Was barely hanging on. Wasn’t just the 6 pin connection. Also the 2 connector anchor solder points. Must be really bad soldering during manufacturing.

Each connector has 6 lines so each line drives 6 LEDs (36 LED/strip). The connector solder joints is the highest thermal stressed solder joint in the circuit.

On the iMac, I also touched up every LED. I’m guessing this wasn’t necessary and the iMac color temperature is also a lot warmer. I’m guessing the heat from soldering iron affected the LEDs.

Can’t find the post now but saw an Australian tech say he fixed a bunch of these by just re-soldering the 2 connectors on the strip.

BTW, as these start fail, lots of weird symptoms. I’ve seen all of the following and varies with different logic boards.

• Faint flickers at lowest brightness setting
• Sometimes don’t come on if mac’s brightness is set too high during connection
• No brightness control (after swapping a failed panel with one with early failure symptoms (still lights up but not all the time unless mac brightness is set to low at connection time)
• Max brightness is really dim (after swapping a failed panel with one with early failure symptoms (still lights up but not all the time unless mac brightness is set to low at connection time)

Fixed 10 of these by now. Always weakened lead free solder on the LED bar connector. A key trick is not remove the LED bar (thermal strip underneath will tear to pieces) and curved plastic reflector (fragile) Just touch up solder while LED bar is in the display's sheet metal tub. This trick is noted in the above pdf file link

Hi there,

On my side I've changed the logic board and no result. The power supply shows 23.9V when light is on and 24.9V when off. Do you think it can be the power supply ?

Or this issue, backlight out, can be caused by the all-in-one cable ? Possible ?

I had this very same issue happen to me.

I have a 2014 5k iMac and 2 x 27" TB displays.

Here is what fixed it for me. Unplug the power cable from the back of the display, wait a few seconds, then plug it back in.

It has been working fine now for about 4 days so far.

Hope this helps someone else.

“johnny “ & “LikeARabbit” you’re absolute F*ckin’ legends! Thanks to you I saved at least 250€. Fixed my issue by inverting the LED cable and blowing hot air on the components around the connector.

Thanks for the great instructions and ideas. I was able to resurrect my 24” LED Cinema Display which was showing a no-backlight picture by opening up the display and reseating the LED driver cable connection to the logic board. This has been sitting in a recycle / donate corner for a year, and thankfully I discovered this thread and bothered because it’s big and bright and beautiful again!

Thank you for all the suggestions throughout this and other threads. I’m still having an issue with my 24” LED Cinema Display. I can get it to work with the brightness all the way down, but once I try to raise the brightness, it begins to blink and eventually goes black. I have a short video of this behavior here: https://youtu.be/4UoOGBID95I

I tried taking the display apart and reseating all the connections. Does anyone else have any other things I should try?

Can anyone tell me where R1135 is located on the Cinema 24” logic board? Someone suggested in another thread that removing it is a “dirty hack” but brings back the monitor to life.