toshiba equium power problem

[woodford]

hi everyone,

i have been given a toshiba equium a300d.the problem is when i plug the ac adapter in the light on the adapter go's dim but not completely off and very faintly flashes around twice every second, the voltage at the back of the dc jack also drops to around 3 volts.ive detected a short on the multimeter on some of the ceramic and tantalum caps but even after removing them there is still the same problem.if its one of the 8 pin mosfets shorted how do i test them, i understand that they have to be removed from the circuit to do this, is this true.

thanks in advance and thanks for this great forum.

[mattbrad2]

Be sure to remove the cpu and memory before checking caps. Also, usually if its the 19v rail that is shorting out, check the larger ceramic caps and also follow the positive lead to the first (few) mosfets. Sometimes these will go closed or half open both ways (this will cause current drain but not necessarily a short). The best thing to do however if to use a variable power supply and start with 1.5v/1a and check which component heats up. Use a IR temp reader (from the local auto parts store) or some here run a crayon across components until it melts.

[jetadm123]

Originally Posted by mattbrad2

Be sure to remove the cpu and memory before checking caps. Also, usually if its the 19v rail that is shorting out, check the larger ceramic caps and also follow the positive lead to the first (few) mosfets. Sometimes these will go closed or half open both ways (this will cause current drain but not necessarily a short). The best thing to do however if to use a variable power supply and start with 1.5v/1a and check which component heats up. Use a IR temp reader (from the local auto parts store) or some here run a crayon across components until it melts.

First time I've heard of using a crayon as a troubleshooting tool!

[mattbrad2]

Yeah I've never done it personally but so many people recommend it I figure its worth a shot if you don't have the proper equipment around.

[momaka]

Originally Posted by woodford

ive detected a short on the multimeter on some of the ceramic and tantalum caps but even after removing them there is still the same problem.

That's because these caps were most likely all in parallel along with probably quite a few other ceramic caps. If one is shorted, they will all show shorted.
Check the caps that you pulled out - if they show open circuit when they not on the board, then they are good and the real shorted component is still on the board.

mattbrad2 suggested an excellent technique - apply a low voltage source with a decent amount of current and see which component gets hot. Once you do, remove the "faulty" component and test it out of circuit to see if it really is bad or something else down the line is causing the problem.

[woodford]

thanks guys for the help but i'm still not sure how to test if a mosfet is faulty when its off the board,any help on this would be appreciated.

[momaka]

Originally Posted by woodford

thanks guys for the help but i'm still not sure how to test if a mosfet is faulty when its off the board,any help on this would be appreciated.

First find the data sheet for the MOSFET and see which pins are Source, Gate, and Drain.
The first most basic test you can perform is to see if there is a short circuit between Gate and Drain or Gate and Source. If you do get a short for either of these, the MOSFET is bad.
If you don't get any short circuit for the above test, try this test:
http://www.4qdtec.com/mostest.html

And if you can't find a data sheet for your MOSFET, just check resistance between all pins. One of them (the Gate) should not show any resistance between the others.

[mattbrad2]

Originally Posted by woodford

thanks guys for the help but i'm still not sure how to test if a mosfet is faulty when its off the board,any help on this would be appreciated.

You have 4 pins on each side. Find the one pin at the far end of one side that is soldered to its own trace. This is the gate. The other three pins on this same side are the source pins. The 4 on the opposite end are drain. Take a DMM and set it to diode mode. Measure from one of the source pins to one of the drain pins. Then flip the leads around and measure again. One way should read a value (usually .4 -.6) the other way should read infinite (open). Also check that there is no continuity between the gate and source/drain pins. This will work on either P or N channel FETs. I'm also assuming this is a single channel mosfet which most will be if its closest to the DC input.