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Late 2011 model, A1278 / 2.4 GHz i5 or 2.8 GHz i7 processor.

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No backlight, ext. display ok, measurement points

Hello

A friend of mine came up with an MBP 13" late 2011 - in general it does not have any backlight (the logo is dark, too), but it works on an external display / monitor.

You can see are very light shine on the internal display and if you disconnect it and run only the external device, everything is ok - the MBP is running perfect.

Now my question ...

Can anybody show me some measurement points where to measure voltages in order to know if the TFT / LCD is broken in a way or even the cable or if the "controller" / fuse is bad?

I would refer to the image shown here https://d3nevzfk7ii3be.cloudfront.net/ig...

Some red arrows would be fine ;-)

I have got a sheet of the display where voltages are mentioned.

Thanks, Stone

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p.s:

sometimes the internal is running normal for a few seconds or minutes and then it turns to dark ... moving the top case does not bring back the light, so I am assuming it is not the cable - the MBP was dropped as I was told

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Update:

I just found a picture here on ifixit which shows the fuse (labeled with "P" on the MacBook I have on the table).

On one side I can measure a voltage of 3.3 volts (3.28 according to my multimeter), on the other side there are 0 volts.

The point of the fuse, where I can measure 3.3 volts is to the inner side of the logicboard (away from the LVDS connector), the point with 0 volts is to the outer side of the logicboard (near to the LVDS connector).

Switching to "diode test" of my multimeter (with beep) there is no beep, but a small resistance of around 58 ohms - not that much.

If I make a "diode test" on both sides of the fuse, I notice that the side with 0 volts has no resistance to ground, so I got a beep ...

All measurements have been taken with one lead / tip "grounded" to the DC-board screws (the silver connection around them).

Without having a scheme of the late 2011 13" MBP I assume that I should measure 3.3 volts on both sides of the fuse - am I right? But this does make no sense, if one side of the fuse is "grounded" - or could someone provide me a scheme?

Any suggestions, tips, experiences are welcome.

Thanks in advance, Stone

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update:

I read some more of the threads here on ifixit and came across some which are dealing with a broken or stuck sleep sensor, but I am a little bit unsure about it.

I assume the sleep LED on the front and the sleep sensor are both located on the small PCB which is located under the outer HDD bracket; if I remove the HDD cable and switch on MBP the display just shows the grey screen for around 3 to 5 seconds and then dims to black.

This looks a bit like going to sleep just for the display.

I located a magnet on the upper right side in the display cover and also there is a magnet located under the outer HDD bracket - do they just "hold" the display cover closed when it is closed or do they belong to the sleep sensor.

Is there a second cable for the sensor - if yes, where? And can I disconnect it? I assume then MBP would never go to sleep mode.

Regards, Stone

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update:

I measured voltage on fuse while turning on the MBP - 3.3 volts every time. The MBP starts, the display does show the grey screen for some seconds, the measurement on the fuse shows me 3.3 volts, the display dims to black, but I still have 3.3 volts.

And I noticed, it is not going to sleep if I pull off the HDD cable.

If there is anyone having an idea, I will exchange the LCD tomorrow.

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update:

the last one for today ;)

I put the MBP to sleep and after opening it the internal display is working for some seconds - this does work every time after awakening the MBP

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Update:

I do not know why, but after finding a schem and a board layout file on the web, I decided to analyze the error in more detail - so I started measuring again, after a close look to the board layout and the schem where to take some measurements.

I found out that two caps right "in front of" the LVDS connector are connected to backlight and should contain some higher voltage - also the fuse should let me read more than 3.3 volts.

So I took out my leads and measured and found out, the caps let me read 19.3 volts and the fuse - don't know why - gives me 11.6 volts.

As the used leads are rather "thick" in comparison to the parts on the board I guess there are some crossover resistances and the measurements are ok. From the data sheet of a Apple 13" display I get 20 volts as typical for the backlight.

I can still measure those values even if the backlight is off, so I assume it is the LVDS cable or the LCD panel - it seems, as if I leave the bottom case removed, the light stays (I need to check this) which points to a broken cable or connector.

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Update:

I exchanged the LCD panel with an old one I got (scratches on it).

It is now working, until I dim the panel ... or it is working because I switched off "automatic brightness adjustment" (don't know how it is called in english exactly).

The same moment I reboot the machine, wether I dimmed it before, the backlight is working again.

This leads me to the question, if it has something to do with the PWM source (not sure, if it is voltage or data word driven, need to look it up) or with the light sensor.

Will continue ... but it is pretty strange, that with the previous installed LCD panel it did not matter, if brightness was adjusted or not.

Maybe it has something to do with current draw of the panels, but then backlight voltage should have been dropped, but it was not.

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