Apple Cinema Display 23' A1081

[Saco]

Spent a couple minutes debating wether to ressuscitate this old topic about the exact same issue but decided to create a new one instead since it's more than 3 years old now.

I am also a beginner, trying to fix this for myself before taking the display to a shop, so please cope with me.

So the problem is the display does not turn on. None of the LEDs (the one on the front and the one on the PCB) turn on.

I have already checked the suggestions by Rasz on that old post and:

- Inductors (L1, L3, L4) are good
- Fuses are good (all that I could find).
- C5 (near the dual choke) reads 24.6V

- U3 is outputting 4.8V (according to specs it should be 4.9-5.1). Can I assume it is still acceptable?
- U4 has 0V on the input (and output too, of course).

- C30,C32,C33 (the 3 caps between the 2 big inductors) read 5V.

- C108, C109, C38, C39 (the caps near U4) read 0V.
- C20, C21 read 0V
- the LED reads 0V (duh)

Should I start taking out the caps and testing them, even if they all seem to be in good shape or is there anything more obvious I can check first?

Full picture of the board:

Gallery with close ups: https://imgur.com/gallery/LevHs

[KhronX]

I'd kinda rather turn my attention to the power supply, than the control board. At least at first.

But if you insist on sticking to the logic board, it might be worth checking for voltages on the connector going to the keypad / power button.

[Saco]

I didn't mention it but I did start by checking the Power Supply first and since it is outputting the expected 24.6V I moved on to the logic board.

[KhronX]

In that case, which connector goes where?

For what it's worth, the TI chip is the Firewire transceiver, and the SMSC one is the USB hub.
That other chip with the white label on it (and the yellow-marked flash chip next to it) will be the one handling the logic side of things.

What part number is U4? If you look carefully, right next to it is Q6, and given its position in the circuit and the terminal markings, my hunch is that's a P-channel MOSFET.
When its gate gets pulled to ground, that'll provide input voltage to U4. Looks like it's an adjustable linear regulator, most likely putting out 3.3v

If C20/21 measure 0V, that means U2 is off (or at least not doing anything). What's the voltage on its pin 5?
Couldn't quite make out what it says on it, but i think it says "2672" on the middle row?
http://www.ti.com/lit/ds/symlink/lm2672.pdf

[Saco]

In that case, which connector goes where?

The bottom ones are the Firewire and USB connectors.
On the top, from left to right:
don't know, power, DVI (input), LCD (output).
The first one goes to a secondary board which in turn has 4 outputs, 2 going to the top of the frame, 2 going to the bottom, but I can't tell for sure what they are.

For what it's worth, the TI chip is the Firewire transceiver, and the SMSC one is the USB hub.
That other chip with the white label on it (and the yellow-marked flash chip next to it) will be the one handling the logic side of things.

What part number is U4? If you look carefully, right next to it is Q6, and given its position in the circuit and the terminal markings, my hunch is that's a P-channel MOSFET.
When its gate gets pulled to ground, that'll provide input voltage to U4. Looks like it's an adjustable linear regulator, most likely putting out 3.3v

1117 3.35J

From my limited understanding, I thought it was U4 that was the voltage regulator.

Q6 does have 5V on the source, 4.5V on the gate and 0V on the drain, as per your hunch.

If C20/21 measure 0V, that means U2 is off (or at least not doing anything). What's the voltage on its pin 5?
Couldn't quite make out what it says on it, but i think it says "2672" on the middle row?
http://www.ti.com/lit/ds/symlink/lm2672.pdf

You got the part number are right, and it is OFF, pin 5 reads 0V.


So, am I totally lost or the problem should be upstream from U2?
I checked Q2 (between U2 and F1) and it's Base reads 17V which is a value that caught my attention, but I can't tell for sure if it is expected,