Compaq CQ-50 Laptop power questions?

[BobMCT]

A friend of mine asked me to repair his laptop. Yes - Ive done quite a few in the past. My diagnosis was that the DC power jack had failed as its LED wouldn't illuminate and hence the laptop would not turn on. So I moved forward to replace this part. Fortunately in these "newer" machines this part is not soldered to the MB. They mount in a clip and have a plug to connect to the MB. I did finally get virtually everything apart and replaced the part. My first test showed that with the power cable plugged in the jack's LED was illuminated and I did get some flickering on the monitor. So I buttoned up the laptop only to find nothing has changed. The LED on the jack still flickers and the screen only flickers. The power button seems to have no effect at all. So did a little re-disassembly and rechecked all the connectors and ribbon cables all which appeared OK.

So, before I write this one off, Im hoping some more skilled in diagnosis can advise as to what might be happening? Might the power adapter not be sending sufficient power to bring the laptop all the way?

Any ideas, suggestions greatly appreciated.

 

[rclarke250]

Possible, if you have a meter, test the power out of the dc plug. But some things I would try. Disconnect the video cable, and the dc-dc converter board. disconnect the dvd/hdd. pull out the wireless modem. In other words we are trying to get to a minimum config. just cpu,memory, and the dc power, remove battery also, as shorted battery can cause all kinds of issues, and the monitoring chips for the smart charging circuit will not allow the laptop to come on if it detects a shorted battery. power it on, what do the LEDs look like? If you have a solid green power led, start putting things back one at a time, testing it as you go. If you don't have a green led, you either have a bad power supply or bad motherboard. IF the LED is blinking with a 50/50 duty cycle it is most likely a bad mosfet that takes the power from 19v down to 12v, it is a small chip right by where the dc jack plugs into the main board, you can not replace this chip, as it needs a reflow station, motherboard needs to be replaced,or repaired, this is a pretty common failure in this unit.

 

[BobMCT]

Thank you for the super quick reply rclark250!
My guess based on what I followed is that the problem is the bad mosfet. But I figured the $3-4 charge for the jack replacement was a worthwhile investment. I'll counsel my friend to keep an eye out for a $300-ish unit to replace this one.

Thanks