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