820-3462 A1425 with no internal display only external display works

[djpat1974]

This a1425 Board chimes like a bird but works from external monitor using HDMI you could use the computer normal which is weird but the screen is not recognize not even back light doesn't work or no image when testing voltage for on U9701 two missing voltage BKLT_EN and BKL_FET_CNTL even when you put voltage to BKLT_EN am still missing BKL_FET_CNTL but before i go to far when you turn on the mother board just for about 5 seconds you get on the LCD cable DP_INT_AUX_N 1.3v /DP_INT_AUX_P 1.6v /PP5VR3V3_SW_LCD 5v/ LCD_HPD_CONN 3.3v /LCD_HPD 2.8v/ but i don't get PPVOUT_S0_LCDBKLT 0.v when I jump BKLT_EN i get only 12.v on PPVOUT_S0_LCDBKLT one strange thing is on C9028 /C9029 toward the CPU is short toward each other but still gives voltage when you just turn on the motherboard on a good board there is no short on those caps. Things i replace LCD connector /u9701 tried two different screens / replace resistors around u9701 check diode mode all good reading and even replace R7402/R7403 from u7400 and u7400 and over kill still all the same even did pram reset and smc lid is working any help is needed maybe i miss some thing or i may have left out something i did and forgot to write it on this post thanks.

 

[2informaticos]

"maybe i miss some thing"
Exactly, you've made the wrong assumption considering the backlight comes first.
Backlight MUST stay off, until image appears (and get stable); is the only way to avoid artefacts on the screen.

If LCD_HPD comes high, PCH should recognize LCD.
If this signal disappears in 5s, something went wrong with PCH-display communication and LCD 5V voltage will go off too.

Any history available?
Check C9028/29 and traces.
Be sure to test with known good COMPATIBLE display.

 

[djpat1974]

the mother boots with image on the external screen only and image comes before back light this might be a type on my part i type incorrectly the history on the board it had no PPVCORE_S0_CPU_PH1 and no chime only fan spinning with amber light i change R7402/R7403 now i have PPVCORE_S0_CPU_PH1 with chime but only display on external monitor not internal screen.

 

[2informaticos]

Please, post accurate information.
Do you get image on internal screen, only missing backlight???
If 5V not steady at pins 28-30 of J9000, you cannot have image.
Of course, no reason to search for backlight in such case...

 

[djpat1974]

wait what is going on i said no internal image or backlight only external screen with monitor works please read my first post { This a1425 Board chimes like a bird but works from external monitor using HDMI you could use the computer normal which is weird }

 

[djpat1974]

820-3462 A1425 with no internal display only external display works

 

[2informaticos]

Pay atention to my posts.
I already explained you, no reason to search for backlight, if you do not even get image on internal screen.
You wasted lot of time digging into backlight circuit, for no reason.
Steady 5V at L9000 is a mandatory step, explained too.

 

[djpat1974]

i understand that i was saying i have no image and no back light on the screen and 5v comes for 5 second then disappear the only way for me to use the computer is with and external monitor i want to know what can cause the 5v to stop working so fast on l9000

 

[2informaticos]

"If LCD_HPD comes high, PCH should recognize LCD.
If this signal disappears in 5s, something went wrong with PCH-display communication and LCD 5V voltage will go off too.
-------------------
Check C9028/29 and traces.
Be sure to test with known good COMPATIBLE display."