820-2838 Missing LCD_BKLT_EN & BKLT_PLT_RST_L

Kittu

Member
Hi:
I have a peculiar issue with the 820-2838 board display working but no backlight due to missing LCD_BKLT_EN & BKLT_PLT_RST_L. On investigating I find that PCIE_RESET_L is also low.

Apparently the missing signals is what's causing Q9806 not to turn on although the booster circuit seems to be working fine because shorting 4 & 5 on Q9806 brings back backlight.

Will appreciate any help on the missing signals.
Regards.
 
Last edited:

2informaticos

Administrator
Staff member
"I find that PCIE_RESET_L is also low"
None of PCI-Express devices can work in this case.
Very strange to get image.
You may have corroded/broken trace for PCIE_RESET_L somewhere.
Check presence of AP_RESET_L and PCA9557D_RESET_L.
 

Kittu

Member
AP_RESET_L and PCA9557D_RESET_L both are low but not shorted to ground.
Image on internal LCD as well as on external monitor through Mini DP Port!

Everything seems fine except no backlight due to missing LCD_BKLT_EN & BKLT_PLT_RST_L.

please advise.
 

Kittu

Member
The board is not water damaged and traces seem OK under scope. Problem appeared by flickering and dead backlight and after 5 seconds. LCD good and fuse OK.
 

Kittu

Member
Removing R2891 (for PCA9557D_RESET_L) takes the PCI_RESET_L to 1.4 V.
A. It is normal?
B. LCD_BKLT_EN still low.

R2891 removal leads to another problem of why PCA9557D_RESET_L is 0V and pulling down PCI_RESET_L!
 

Kittu

Member
Sorry for the delayed response!
yes I did remove all one by one with no change in each step. Voltage on PCI_RESET_L remains as 1.4-1.55V (of course this measurement is with LCD panel removed, which shouldn't impact PCI_RESET_L)
Additionally, PP3V3_S0_MCP_PLL_HVDD and PP1V05_S0_MCP_PLL_PEXSATA (both going to PCI Express section of U1400 are good and 3.3V and 1.05V respectively)

Strangely Mini DP port is outputting image on external monitor despite PCI_RESET_L being down.

Help Please.
 

2informaticos

Administrator
Staff member
Something wrong inside MCP.
Check if board works correctly without R1600, resoldering the other resistors.
If not, try replacing with 100-220K instead of 22K
 

Kittu

Member
Removing R1600 and replacing R2891/93/94/95 restore PCIE_RESET_L to 3.2V.

LCD_BKLT_EN is still low. Please advise how to diagnose and get it back to 3.2V...
 

Kittu

Member
I am currently testing the board without SSD and confirm getting the question mark folder on mDP as well as LCD. Replacing R1600 with 200K maintains 3.2V.

Further, removing R8910 (10K) from LCD_BKLT_EN results in 1.5V and triggers the no-load PPVOUT_SW_LCDBKLT to 39V.

Will revert on USB and on-load Backlight once I replace the LCD panel as the backlight connector ir broken as hence the no-load test of PPVOUT_SW_LCDBKLT as 39V.

There seems to be something MYSTERIOUS going on as pull-down resistors to ground on both - the PCIE_RESET_L as well as LCD_BKLT_EN screw the signal which otherwise seems to jump back to normal (3.2V) in case of PCIE_RESET_L and partially (1.5V) in case of LCD_BKLT_EN with panel connected.
 

Kittu

Member
LCD backlight working only with R8910 removed. Changing it from 10K to even 200K doesn?t pull up the backlight enable signal. Will test USB and SD/ SSD and revert.
 
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