Finding shorts - can a 12v 1a PSU (from a modem etc) be used?

[seanc]

Hi all,

I've got a couple of laptop motherboards with shorts, but I don't have a bench power supply yet.

The one I'm working on right now is somewhat disposable.
It's a Compaq CQ58 (AMD based) (lack of actual schematics) - I can get a brand new board for £19.99 from eBay at the moment and even then it might just all go in the bin.

So there was a crispy 4435 mosfet (damage seemed mainly on the source and gate side rather than drain) right next to the input for the power DC jack. I pulled another from a dead board - tested OK and installed it. The replacement part starts to get warm rather quickly. There seems to be a lack of protection in the circuit or charger? The output from the charger is about 19v 3a.

I have a short in the circuit somewhere, evident by quite a few of the surface mount capacitors showing dead shorted (including some on what I believe to be the chipset or GPU). My current method of pulling off each shorted cap from the board and testing to see if the short disappears (or the removed cap is shorted) is tedious to say the least and I'm not particularly skilled at it...

If I improvised with a modem/hard drive power supply rated at 12v 1-2amps and connected it to the usual 19v input, would this work for helping me to find a short without doing damage (especially to the mosfet which already fried) or would it be the wrong approach?

Thanks for any advice!

[piernov]

Re: Finding shorts - can a 12v 1a PSU (from a modem etc) be used?

Buy a lab PSU. Or at the very least a DC-to-DC converter with adjustable voltage and constant current mode (DPS3005 at a minimum), if you already have an acceptable AC-to-DC PSU.
Also low resistance to ground is normal on CPU/GPU/PCH/northbridge/southbridge core rails.

[dj_ricoh]

Re: Finding shorts - can a 12v 1a PSU (from a modem etc) be used?

No.
If you apply 12v to CPU rail or gpu rail That requires 0.8 and 1 to 1.2v you will instantly explode them. No matter if they are bad or good.
You need adjustable voltage and apply that amount that is maxim on that rail.
Dc line 19v , 3v and 5v always, CPU,you, 0.8v 1.2v but if you go over 1v on gpu you kill it. On 1v the GPU needs to get hot.

Nevertheless you need adjustable psu. You can't use a normal unadjustable psu.

[seanc]

Re: Finding shorts - can a 12v 1a PSU (from a modem etc) be used?

Thanks for the responses guys.
I will look to invest in an adjustable lab power supply.

In my usage of the 12v 1a PSU, I was intending to use it where there is usually 19v 3a, in order to provide less power throughout the board, to perhaps locate the short without cooking things, or at least as quickly.

Once I get a power supply, I then need to work out where to apply the power at a specific voltage as there isn't a definitive schematic for this board. Hopefully I can work it out from similar boards which use similar components/layout.

Also need a new soldering station because the button to decrease the temperature on my Duratool (Maplin branded) ZD-916 is broken. It's not the button or the cable, so it looks like it might be the micro controller on the board.