Another 820-2936 Fan spins 2 sec

[Bohmmen]

Hello

This Board has no sign of water damage.
When it's plugged in the fan spins for 1 sec, stops for 3 sec and keeps repeating that process.

PPBUS_G3H is good
PP3V42_G3H is 3.5v

What can be causing this fanspin?

 

[Bohmmen]

meassured at l7550 and there is a short to ground

 

[Bohmmen]

3,4,6,7,8 on q7550 shorted

 

[Bohmmen]

L7550 is about 0,1 ohm
voltage is 0 all the time at both L7510 and L7550

 

[dukefawks]

Check R7402, especially pin 2 for corrosion and measure 90K.
Did you measure over L7550 or L7550 to GND?

 

[Bohmmen]

measured over.
diode mode L7510 and L7550 is 0.015 on both
R7402 is ok but pin 2 on u7400 does look a bit worse than the other pins.

 

[dukefawks]

Why would you measure over a coil? Resistance to GND is the one that matters.
Measure R7402 by measuring from F7040 to pin 2 on U7400, this way you ensure that there are no corroded traces.

 

[Bohmmen]

it measures 91k

 

[dukefawks]

So what is resistance to GND on L7550 and L7510?
Does all_sys_pwrgd come high when the fan spins?

 

[Bohmmen]

all_sys_pwrgd goes from 0v to about 3v when fan spins
resistans to ground on both coils is 0.015

 

[Bohmmen]

yes, it's to low. I wrote that in my second and third post in this thread. Or is it not a short to ground when it measures that low?

 

[dukefawks]

0.015 Ohm to GND is too low. Nothing more I can say. Compare it to some working boards, but the ones I have here are 5 Ohm minimum. It would most likely be a shorted CPU as it is on 2 different busses.

 

[dukefawks]

OK, voltage on both pins of Y2805 when fans spins?

 

[Bohmmen]

it's a bit hard to see but 0,9 volt on both pins.
the "no image" board is 0.97 volt

 

[dukefawks]

That is all ok. Running our of ideas here too. You say R7402 does look corroded, are you able to take a picture? I'm pretty sure it will be corrosion somewhere. Any history on this board?

 

[Bohmmen]

I bought the mbp for 50$ with no other information from The seller than that it wouldn't start.
R7402 doesn't look corroded. It's the pin on U7400 that it's attached to that was corroded.

 

[dukefawks]

Well I would think it has been a liquid spill then. Look closer for corrosion.
Also what voltage are you getting on both sides of R7402?