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820-00928 - A1707 - No Backlight

I

iFix

Member
No backlight issue all resistors on circuit are good besides R8432 which should be 150k but it's 24.2k. Also I get diode mode on PPVOUT_S0_LCDBKLT 0.50 which might be a shorted Cap? In the past on the same laptop I had C8471 burned but maybe happened again?

Thanks :D
 
2informaticos

2informaticos

Administrator
Staff member
"I get diode mode on PPVOUT_S0_LCDBKLT 0.50"
Doesn't look like short; post reverse reading too.
You can compare with the readings from similar board.

What about PPVOUT_S0_LCDBKLT voltage (LCD connected)?
 
I

iFix

Member
I only get till pin 3 & 4 on Q8400. I have another board but lots of parts taken out and not showing properly. No voltage on PPVOUT_S0_LCDBKLT with LCD connected and i tried 2 lcds
 
I

iFix

Member
Can you please explain reverse readings?

R8442 3.3v & BKLT_SD is 12v as i said the resistors are good :)
 
2informaticos

2informaticos

Administrator
Staff member
"Did you try known good LCD assembly?"
Backlight adjust comes through I2C bus from T-CON board.

"Test in diode mode:
Connect positive probe of the multimeter (red) to ground and touch the test point with black probe.
Sometime (rarely) we need to test in reverse direction."
Lot more information on that thread...
 
I

iFix

Member
I understand diode mode measurement just didn't know why reverse measuring :) Yes I've tried known good LCD from another laptop. reverse reading on PPVOUT_S0_LCDBKLT is 0.035 and also diode mode same which was not before...
 
2informaticos

2informaticos

Administrator
Staff member
These are low readings; compare with other similar boards.
You should get OL in reverse direction.
Possibly corroded cap...
 
I

iFix

Member
The board is clean i took it out completely ... I replaced another lcd and now it shows reverse 0.419 and diode is 0.373
 
I

iFix

Member
I replaced C8467 now i have diode mode 1.1

The only resistor which is not showing correct value is R8432 and instead of 150k it shows 33k and diode mode on it is 0.700

R9827 is 7.4k

I replaced Q8400 also.

On Q8400 i have 5v on pin 5 and pin 18.

Pin 11 12v
pins 15,16,17 0v

Any suggestions please?

Also if someone knows if the flex cable for backlight on the lcd is broken will it still boost and give voltage on PPVIN_S0SW_LCDBKLT.

Thanks guys
 
2informaticos

2informaticos

Administrator
Staff member
Depends is backlight out is shorted, or interrupted.
Getting 55V denotes no contact with LEDS strip.
0V can be short, or backlight circuit blocked for other reason.

"pins 15,16,17 0v"
So missing BKLT_EN_R; EDP_BKLT_EN too?
Check R8450-53; strange to get 0V at pins 15 & 16...
 
I

iFix

Member
R8452 and R8453 are 1.3k

This morning i measured again and i have 3.3v on pin17, and on pins 15/16 i have 4.6v
 
2informaticos

2informaticos

Administrator
Staff member
R8450-53 refers to 4 resistors.
However, all seems good, if get 4.6V there now.
Still no volt at D8410 output?

Do you have image for sure?
3V3/5V_S0SW_LCD steady voltage?
 
I

iFix

Member
Yes i have checked. On PPVIN_SW_LCDBKLT_SW i have 0.14 only

R8450-53 all 4 resistors checked good
 
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