Toshiba L450 Completely dead - hot MOSFETs

[spleenharvester]

Got this one this morning, board's clearly had a rough life. CMOS battery removed, lots of sticky residue (looks like some is from a reflow attempt and some is from the removal of mylar tape but I need to get it scrubbed off eitherway)

Full schematic is here:

Component PF2 (on battery connector) appears to be missing. According to this picture it's some kind of fuse, I should be able to grab it off another board. (Bottom left by batt connector)

Voltage at the adapter oscillates from 1.4v to 1.6v. The adapter works fine (I am using my L300 from it now). A high pitched oscillating squeal comes from the board a few seconds after plugging adapter in.

Components PQ6 and PQ7 (shown below) get very hot. PQ7 appears to be short. Nothing nearby seems to get hot.



To do:
Replace PQ7 - AO4407 MOSFET
Replace PF2 - 15A 65v Fuse
Replace CMOS battery

Any other suggestions?

Cheers

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Update, Northbridge is also getting quite hot.

[ben7]

Re: Toshiba L450 Completely dead - hot MOSFETs

Update, Northbridge is also getting quite hot.

Is this after you replaced the mosfets?

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Nah, haven't replaced the MOSFET yet. Can I replace the AO4407A with an AO4407?

I was quite surprised since there is almost no voltage across the rest of the board. Also, the sticky residue appears to be nonconductive (probably flux).

[fddizon]

Re: Toshiba L450 Completely dead - hot MOSFETs

i think you can

[spidey]

Re: Toshiba L450 Completely dead - hot MOSFETs

yes you can. any 30v P channel mosfet would fit.

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Sweet, I'll get right on it.

Also, found another two, these are both scalding hot, and 0v through both PQ24 and PQ26. PQ26 is shorted (single digits on diode test, same as before). neighbouring PQ23 and PQ25 are fine so it doesn't appear to be related to TPS51124.



It's also worth noting that PQ24 and the adjacent ceramic cap are sort of misplaced, like they've slid during a reflow.

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Also, is it normal for the non-shorted MOSFET to be hotter in the pair? It's the same in both cases.

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Ignore

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Replaced PQ6 and PQ7, ACIN is now 4.2~4.7 volts. PQ7 nearly burnt my skin off when touched - should I replace it again?

PQ24 and PQ26 also heat up.

There's now a stable 3.1v near the BIOS chip (as opposed to 0.45 before), though. That irritating 0.8~1.1v oscillation is still there.

[whaldsz]

Re: Toshiba L450 Completely dead - hot MOSFETs

What's the resistance of B+ to ground?

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Seems to be infinity. There is a brief flicker on the DMM display when changing to 200ohm or to 200kohm settings but nothing reliable.

Also, PQ26 changed, no change whatsoever. I think this low ACIN is key though, what could be dragging it down from 19v to 4~5v?

Only thing I can think of doing is replacing PQ7 again, the AO4407 that practically burns your skin off when touching it.

[whaldsz]

Re: Toshiba L450 Completely dead - hot MOSFETs

Resistance at VIN, PL3? Check PQ8, PQ9.

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

VIN resistance to ground appears to be about 56kohm.
PL3 resistance to ground appears to be infinity, with only very brief displays on the DMM when changing settings.
Resistance across PL3 is 1.2ohm.

PQ8, the diode test is working both ways? Is this normal?
PQ9 not present, this seems to be normal looking at images of other identical boards.

[whaldsz]

Re: Toshiba L450 Completely dead - hot MOSFETs

PL3 is not normal. There's a short somewhere there. You got to desolder one side of PL3 and check the resistance on the pads on both side to locate where the short is coming from.

[whaldsz]

Re: Toshiba L450 Completely dead - hot MOSFETs

Ignore my previous post. I misunderstood your message.

Try to test the resistance between gate to drain, gate to source of the mosfets.

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

I think you may be onto something with PL3. I didn't notice before but it gets very hot. Should I change it?

I think I measured them earlier today and they were okay, I'll check again in a sec

[whaldsz]

Re: Toshiba L450 Completely dead - hot MOSFETs

I think you may be onto something with PL3. I didn't notice before but it gets very hot. Should I change it?

I think I measured them earlier today and they were okay, I'll check again in a sec

Not yet. Do a resistance test on all the mosfets first. There could be a short in gate-source pins. Most likely, you will have to change those mosfets including PU5.

But make sure all mosfets are ok and shorts dont exist before powering the system.

[spleenharvester]

Re: Toshiba L450 Completely dead - hot MOSFETs

Checked every FET on the board, only ones I found shorted are PQ26 (which has been replaced as it showed as shorted before) and PQ25.

EDIT: Also, the ticking sound I mentioned earlier - it's not just coming from the above fets, it's coming from loads of different areas on the board.

[whaldsz]

Re: Toshiba L450 Completely dead - hot MOSFETs

Checked every FET on the board, only ones I found shorted are PQ26 (which has been replaced as it showed as shorted before) and PQ25.

EDIT: Also, the ticking sound I mentioned earlier - it's not just coming from the above fets, it's coming from loads of different areas on the board.

PQ8, gate-source and gate-drain resistance?