A1278 MD101 2012 Core i5 UMA - 820-3115-B - Chime but No Displays

[anandvsalunke]

MD101 board 820-3115-B boots,[hidden]chimes but no display. Keyboard Caps Lock lights glow when pressed, suggesting that the machine is capable of booting, but no display.

Its usual to see these problems, given the poor engineering of LVDS cable and connector.

No surprise to find the burned cable, which also necessitates replacement of connector, since burned remains stick to the connector as well. The problems are usually due to missing isolator on connector lock at the LCD end or PCB end, other times the cables burns its insulation and creates short circuit across backlight/ EDID power rails.

In this specific case, the EDID power 3.3V (Pin 4) and Backlight power (pin 21 & 22) found burned. Reading the EDID via RT809f confirms the suspicion that the EDID chip on LCD panel has seen better days. I removed the connector and powered the board to ascertain of any other issues that might be present.

No specific issues found except blown backlight fuse F9700 (32V, 3A) and some general corrosion stains. I went ahead and replaced the LVDS connector, fuse and touched up the solder wherever required. It was time to remove front glass and take out the LCD for LVDS cable replacement.

After 1 hour of battle with the LCD panel, finally got the screen and glass removed without any scratches and damage to rubber gaskets around the screen.

I absolutely forgot to put up a new EDID chip and found that after all this rework, there is no display. On this platform, the Vcore_AXG is not powered up until a valid EDID is detected. I connected a scope to DDC_CLK and DDC_DATA to see normal activity, but there's no EDID chip, so no Display.

I now started searching internet forums, but could not get the EDID dump of this screen. Came across the datasheet of LCD panel of AUO B133EW07 and glad to see all EDID defaults specific to APPLE were given, with this data, I was able to recreate the EDID file and managed to flash into new EDID chip 24x02. I have attached this EDID file for those courageous people who dare to repair LCD panel this way.

I happily went ahead and powered on the machine, just to find that there was no display, the Vcore AXG comes alive with chime and goes away forever. I suspected bad BIOS, so flashed clear ME and still the same problem.

At this point, suspect was bad PCH, but i decided to continue troubleshooting. The scope view of EDID_CLK and EDID_DATA did not look proper this time, the high levels were hardly crossing 1.8V, which was strange. It quickly came to my mind that EDID power rail must be going down during the I2C transfer, so i checked the inductor L9008 and voila.!! inductor has been ruined and acting as 28k resistor!!! A replacement was put in and I was welcomes by bright APPLE Logo...

I have attached EDID dump & AUO datasheet, that might prove useful to people. Keep your head cool and dont give up guys,.[/hidden]

[mcplslg123]

Great Job Anand Ji

[samanthban]

Thanks for sharing

[Juandi]

Good job

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[Sukhvinder]

Deat Anand i am having a problem with my 820-3115-b the display hangs on booting and a green patch appears on upper left corner the mouse works . i 've changed ram and hard disk but it seems like a graphics problem. In safe mode it works fine. do you think that PCH is faulty, please advice

[DJAMEL1986]

THANKS