820-00875-01 Spill no screen

2informaticos

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First check in diode mode; be sure no short w/ LCD connected.
Post voltage on Q8400 pins.
BKLT_SD stays low now?
 
BKLT_SD at about 1 - 2 V

Diod mode doesn't work for me, will have to check why, maybe doing something wrong.

Voltage on Q8400

13V - 3,4 pins

rest pins have about 0.002
 

2informaticos

Administrator
Staff member
Did you remove power when checked on diode mode???

If you get 1V at BKLT_SD, then pin 3/Q8400 should be around 4-6V; depends the value choosed for R8402.
Did you connect the resistor correctly?
Check its traces to U8400 and R8401/Q8400.
 
Something is giving 12V on pin 3 in Q8400, and I am assuming there should not be 12V. I removed the R8402 just to make sure it's not complicating this. Do you know what should be my readings on R8401 or C8400 on ( pins 2 ) since they are connected to pin 3 of Q8400. I am guessing they should not give 12V
 
Not really. I am suspecting weird behaviour from Q8400 now. Since some time ( as mentioned before ) it had 12V on both 3,4 pins. I suspected some short before and removed it but then when soldered back all of the surrounding components it was the same. I am looking at the schematics and trying to remove possible sources of power that might be getting to it. I removed now C8400, C8401, R8401, R8402. When I measure voltage I can see now 12.9V on pin 4 and about 7V on pin 3 ( Q8400 ) - is that "normal" ? I would expect 0V at this point on Q8400[3]. I am suspecting now that voltage on pin 3 is getting from pin 4. Can I test it if it's bad ? Would a picture of a broken pad of R8402 help ?
 

2informaticos

Administrator
Staff member
With 5V Vgs difference, Q8400 should work.
Do not expect to work if you remove R8401/02.
Put back all things and change Q8400.
 

2informaticos

Administrator
Staff member
It should work, but seems not in your case.
In such case, is damaged.

Q8400 must allow 12V voltage at D8410, if has G-S polarized correctly.
 
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