A1708 820-00840-A no backlight

Hi,

I have an A1708 820-00840-A that has no backlight. Already trued with a known good lcd, but no success.

I get 13V on F8400 but I get 0V on PPVOUT_S0_LCDBKLT.

On D8410 I get 0V and on diode reading I get on 0,394V and 0.940V on the pads.

Can you help?
 

2informaticos

Administrator
Staff member
Check R8450-53 and their traces.
Post voltage and diode mode at SCL/SDA lines.
U8400 receives backlight adjust from PCH, via T-CON (inside LCD assembly), through I2C protocol.
 
R8450-53 seams to be ok 1.4K ohms and traces seams to be ok

Diode:
BKLT_SCL = 0,549V
BKLT_SDA = 0.549V

Voltage:
BKLT_SCL = 4.5V
BKLT_SDA = 4.5V
 

2informaticos

Administrator
Staff member
Keep in mind, x1-3 means x1, x2 and x3; x1/2 means x1 and x2.
Just in case, x/y1/2 means x1, x2, y1 and y2; example R/C7030/40 (look at 00840 board).
We use many times such kind of short names in the forum.

Try changing U8400.
Any history available?
BKLT_SD stays low?
 
OK, tks for the explanation.

I've replaced U8400 but same result, and BLKT_SDA/SCL still 4.5V.

The client says that it was working and the screen turn black. It has no signs off liquid damage or corrosion.
 
Last edited:
Hi.

R8432 should be 150k ohm (and it is), but when I solder it back, I get 16k resistance to ground on it. Is this value normal or do I have a problem with the backlight feedback?
 

2informaticos

Administrator
Staff member
Is normal to get lower reading for high value resistor measured onboard; other components simulate parallel resistor.

"I only measured R8450 and 8453 (those are 1.4k)"
Look at schematic and check again the four resistors...
 
Hi,
I mesured all resistors around U8400 and all of them are with the right values and their traces seams to be ok.
R8451 and R8452 are 0 ohms.
Can it be Q8401?. On U8400 i get 0v on LCDBKLT_SW
 

2informaticos

Administrator
Staff member
If U8400 is good, there could be I2C communication problem; like LCD assembly, video cable/connector, or bad traces.
 
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