23" Apple Cinema Display (A1081) No power

[sdev2004]

Hi guys, firstly i'm steve and i am seeking some advice on repairing my monitor.

I got the monitor from the tip and trying to bring it back to life! Its completely dead, no power, no lights (on front panel or on the logic board) no back lights. I have done numerous tests myself, I'v tested the PSU (firstly) measures 24V DC 2.3A which seems ok (i think  ). I have tried to cut out the grey centre pin that causes most issues on these older monitors but no luck! tested pico fuses and cannot seem to find the issue. Sure its not to serious as there is no light so i can only imagine there's a short somewhere or its loosing power. Any help would be much appriciated! Thanks 

[olewales]

Firstly I would stick to power and look for any local voltage regulation ICs on the logic board inside, find their pinout and check output. If you have problem locating them you may try to check voltages on the input of non-proprietary ICs you can identify. You may even find some marked test points.

[sdev2004]

Hi, thanks for you reply! i have to admit i am a little confused to what i am now looking for  i have attached a picture of what i believe to be the culprit, could you circle what i should be testing? thanks 

[olewales]

Hi, thanks for you reply! i have to admit i am a little confused to what i am now looking for  i have attached a picture of what i believe to be the culprit, could you circle what i should be testing? thanks 

I am not seeing your picture but basically if the monitor has external power supply and you verified that its working properly then this single voltage supply must be converted internally into different voltages needed for monitor operation. There are probably a few power ICs clustered near external power supply connector. I'd google their symbols and check if their output voltages seem reasonable.

[Rasz]

first take a multimeter and measure L1 (dual choke) it might be burned
then the yellow flat F1 (polyfuse?), it should be shorted
then measure in front and after LDO VR3 (1117-3.3), should be clean 3.3v after
then measure smps VR1 and VR2 (easiest on caps), no idea what voltage should be there, probably 12v and 5v

but since its totally dead there is a short or fried fuse/choke/broken cable

[sdev2004]

Rasz i will check them just now  i tried to upload a picture but it won't let me upload i will try again now.

[sdev2004]

http://imagizer.imageshack.us/a/img674/779/1B0C33.jpg

[sdev2004]

L1 is not burned, there is continuity.  I have also removed the top piece of plastic to check the coil and it looks healthy 

F1 Poly fuse measures 24V going in and 24V coming out. There is continuity between the two pins.

Test on VR3 shows,
- left pin on VR3 -0.33 with red (positive) probe, when black (ground) probe is grounded.
- right pin on VR3 3.02V with red (positive) probe, when black (ground) probe is grounded.
- centre pin on VR3 2.10V with red (positive) probe, when black (ground) probe is grounded.
- left pin on VR3 with negative probe, centre pin on VR3 with positive probe 3.30V, centre pin on VR3 positive probe and right pin to negative probe -1.83

The caps measure 22.8V

Also I noticed that when I put the probe to continuity, between the negative (ground) wire that goes from the PSU to the logic board to the ground (frame) pins of the logic board there is no connection at all. is this normal? maybe an earth short? i would assume that there would need to be continuity between the frame ground and that negative wire for the circuit to get proper ground 





i hope i hae not made it to complicated to understand 

[sdev2004]

Imageshack links as still unable to upload pics 

http://imageshack.com/a/img540/2333/TTi3Z9.jpg
http://imageshack.com/a/img661/3307/Id1dYx.jpg

[Rasz]

sorry, i meant smd caps
left pin IS the ground, measure everything against pcb ground. It might not necessary be the same as power supply minus (apple likes to do weird shit like ac power over video cables)

anyway looks like 3.3 is working means 5v is working means monitor status led should light up, and you said its dead?

you can check diodes and switching mosfets (U3 U4 U8), those weird pieces of metal F3-5 are fuses, check them too

[sdev2004]

Hi mate, thanks for your advice i did notice that Q3 witch acording to the data sheet is a diode. i checked for power coming from it when the PSU was powered up and power goes into it but it dont come out, also you see them 4 caps on the image? they are all 24v but the other 2 on the oposite side of the board there is no power going to them whatsoever 

[sdev2004]

I will follow your advice and report back! 

[Ferrix97]

Did you managed to solve the issue? I have a 20" (the 20 and 23 use the same board) with the same exact problem.

The Mac still sees the display, probably because the main IC is powered by the DVI port, but there is nothing on the screen and DS1 remains off

Thanks

[Rasz]

scroll to myt first post in the thread and start from there https://www.eevblog.com/forum/repair/23'-apple-cinema-display-(a1081)-no-power/msg639314/#msg639314

[Ferrix97]

I did check the standby rails and they seem to be present (3.3V is there, and 5V is also present, as 3.3V is obtained from 5V).

All the fuses are good, but I think they are only used for protection on the two USBs and firewire on the back. I also had a look at a few diodes and none of the is shorted. Looking at the main regulator (the one with the big diode and inductor, I don't have a board with me now) I see that the enable pin is always low, even when connected to a Mac (that sees the display)

Also, I'm not sure about the presence of a 12V rail, I did a quick check on a functioning one (that had a bad 3.3V regulator) and it seemed more like 18V, but I could be wrong.

[MacNB2]

scroll to myt first post in the thread and start from there https://www.eevblog.com/forum/repair/23'-apple-cinema-display-(a1081)-no-power/msg639314/#msg639314

Came across this thread in the hope to fix my ACD 20" (A1081). That link above does not exist so hoping some one has some ideas how to debug my issue.

The ACD I received from my old office shut.
When I first powered there was some some flashing LED on the front but did not take note of the sequence as I was not expecting it.
Every touch of the Power switch would light the LED and turn off. Nothin on the display.
But now that front LED never comes on.

Connected to the Mac Pro as a 2nd screen, it shows that the ACD is plugged in and displays the serial number in the System Profiler. But there's no details of any USB Hub (with the ACD's USB cable plugged in). Interestingly, even WITHOUT the ACD's PSU, the Mac sees the ACD...looks like the DVI cable to the ACD is carrying power from the PC enough for the ACD return this info.

I opened it up and measured the voltage around the three Voltage Regulators (VR1, VR2 and VR3).

VR1 (LM2672) Controls 12v (not sure if that's correct or it should be a different value). It shows no outputs on it's Inductor L3 & Caps C16 & C18. Vin = 24.74 (same as ACD's PSU)
VR1 Pin 5 (ON/OFF) = 0.00v which means it is disabled - hence no output.

VR2 (LTC17781) Controls 5v. It shows 5.11v on it's inductor L2 and caps C189 to C192.  Vin = 24.74 (same as ACD's PSU)

VR3 (1117-3.3SJ) Controls the 3.3v. It shows 3.21v on it's Vout Pin.



The LED DS1 on the logic board is OFF when the DVI cabe is NOT plugged into the PC.
The LED starts to flash every 3 seconds as soon as the DVI is plugged into the PC.
When this happens and I look at the VR1 Pin 5 (ON/OFF), it is fluctuating randomly...0v...1.19v...0v..3.1v..etc. Similary the VR1 output fluctuates from 0v up to 12v randomly but never static.

I traced the origin of the VR1 ON/OFF signal and it comes from the microcontroller (6805 based MC68HC908LD64) U1 Pin 10.
If I INJECT 5v onto VR1 Pin 5 and hold it there, the backlight turns ON the LCD displays the incoming content. And if I measure VR1 output it shows 12V and stays on (latches) after I remove the 5V.
If I INJECT 3.2v onto VR1 Pin 5 then the VR1 outputs 12V (and latches) BUT there's no display on the screen or backlight.

So is the microcontroller faulty as it is not providing a steady 3.2v to VR1 Pin 5 ?
Why injecting 5v to turn ON the VR1 enables the display but 3.2v does not ?

VR3 output is just below 3.3v and seems to be within the spec of the microcontroller or does it have to be 3.3v and above ?
What else can I try to debug this ?

BTW, if I plug the ACD USB cable into the PC, the flashing LED (DS1) stops and seems to put the board into an unknown state as I cannot do the trick of injecting 5v into VR1 pin 5. When I check the 5v rails, there no reduction is voltage of this rail when inserting USB cable. So this is weird why the USB is effecting the display.
 
UPDATE:
I managed to fix the ACD.
For anyone finding this post in the future, it may help.

Turns out that the problem was the VR3 linear regulator after all.
When Injecting 5V onto VR1 Pin 5 to turn on the 12V, I measured the voltage generated by VR3 as 4.01V that was over the maximum regulated output of the device.

I did not have any 1117-3.3SJ regulators and eBay source I found was in China with 6-8 weeks delivery via slow boat for a 30p part sold in batches of 20.
To verify that is was the VR3 at fault, I had an old test switching regulator board I built while back for another project based on the KIS-3R33s (http://www.datasheetcafe.com/kis-3r33s-datasheet-converter-module). That board took 5v IN from USB and generated 3.3v which I soldered on to logic board as shown in the attached.


 ld1086.pdf (1076.67 kB - downloaded 125 times.)

Turning on the PSU turned on the DS1 LED and there was display on the LCD. Great. Now I needed a replacement part for the 1117-3.3SJ regulator.
I found the spec sheet attached below. It's a Low Drop Out 3.3v regulated rated at 1A with the Dropout voltage of 800mV. I found an equivalent as close as possible at a local supplier. It was an ST Micro LD1086DT33TR in TO252 (DPAK) packaging to match the existing VR3 packaging.
It's rated at supplying 1.5A and a Dropout voltage of 1.2V which is OK since input voltage is 5V so it can still supply 3.3V at 1.5A. Specsheet attached.

Cost £5 for a batch of 5 and it fixed the issue.
USB ports work too.

So the VR3 seems to be a common problem on these displays. Even if it looks to be working, it's worth replacing.

 AMC1117.pdf (180.02 kB - downloaded 143 times.)

 ld1086.pdf (1076.67 kB - downloaded 125 times.)

[Rasz]

https://www.eevblog.com/forum/repair/23_-apple-cinema-display-(a1081)-no-power/msg639314/#msg639314