Dell Studio 1558 very hot PQ41

[coltron]

Hope someone can help with diagnosing/fixing a dell 1558 that I've been working on, with the aid of Quanta FM9C schematic and boardview, (downloaded from this site...Thanks

Original symptom was shorted Mainboard, cutting out the psu.

PQ41 (high side Mosfet on 1.5v Rail) got warm when 19v rail was injected with 5V-2a.

The Mosfet was changed out from a donor board, and the short disappeared, but the laptop wouldn't start and would only charge the battery.

On inspection the voltage regulator chip Tps51116 showed signs of a previous repair .... a blown trace next to pin 23 (VBST).

I've re-bridged the trace today and the laptop now starts, as soon as 19v is plugged in. Fan spins at full chat immediately, but no display. PQ41 gets hot, very quickly. I get a low ohm reading from 1.5v coil to ground 3ohm..?

I took off a couple of caps, but then i notice quite a few on Dram that I,m reluctant to lift, as the problem may lay elswhere. (When coil is lifted, the low ohm short apears on 1.5v sus... Any advice? (other than the bin)

[mcplslg123]

Re: Dell Studio 1558 very hot PQ41

You need to change both fets-HIgh as well as Low side along with PWM regulator associated with them. Thats the basic rule whenever a highside fet is shorted. Btw, it doesnt shorted without any reason-so better change all the components first and then report back.

[coltron]

Re: Dell Studio 1558 very hot PQ41

Thanks for reply and your advice, I have 2 of these dell 1558 motherboards, both with the very similar 1.5v supply issues. I,m new to this forum and have posted 2 threads (by accident) with the same problem.

Board 1 is described above,

I tried voltage injection on 1.5 sus and found 1 shorted cap (big cap c646). (The board then booted for 5 seconds and shut down again) original pq4o got hot, PQ41 is now cool.
all need changing along with pu5 ? (as you suggest)

I was going to use board 2 as donar , but found the same/similar issue (shorted PC175) next to L7, so would like to try and fix both. This means buying high and lowside fets, and PU5 regulators. is ebay the best place for buying components?

Could you recommend alternatives to fets FDMS 8692 (PQ41) and FDMS 7670 (PQ40)?

[coltron]

Re: Dell Studio 1558 very hot PQ41

You need to change both fets-HIgh as well as Low side along with PWM regulator associated with them. Thats the basic rule whenever a highside fet is shorted. Btw, it doesnt shorted without any reason-so better change all the components first and then report back.

PU5 has been renewed with pq41 and PQ40. 330uf Cap replaced


On applying 19v (SRC)

a) laptop charge light remains lit, (portable battery is not installed).
b)No fan spin and no power on with button.

On investigation

a)The graphics chip and PQ36 begin to get warm without pressing the start button, (cmos battery is installed)

found 1.1v on PL6.

Nothing showing on 1.5v rail (pq11),?

I,m thinking check the gate on pq36 and broken maxim chip?

[coltron]

Re: Dell Studio 1558 very hot PQ41

Update:

Today, the 1.1v measured pl6 has gone?, however gfx chip still warms up and on the flipside of the board PQ10 warms up to.

4.2v on the gate, 3.3v always present and 1.8v on source?

gate voltage normally 3.3v,? (should read zero since power button hasn,t been pressed yet?)

Has anyone experienced this type of behavior?

(low 29ohm on PL6, time to pop it off)

[coltron]

Re: Dell Studio 1558 ATI broke?

29ohm reading?
is this normal for ati GFX power supply, not short?
However, flip the board and pci + is short on (see pdf)

Normal for ati chip?