Toshiba A665 blown MOSFET

[Rolin]

Re: Toshiba A665 blown MOSFET

OK, I removed the Q30 chip. Tested the source Pin1 to ground and got 8.39 KOhms.

Tested Q45 (took a while to find that little guy), and as I understand it: The top single pin is Drain, the bottom two pins are Gate and Source (in that order)

---D---
G-----S (Q45)

I tested D to motherboard ground and got a different number every time, but somewhere around 4mOhms. Then I tested D to S, same thing - varies around 4mOhms (also tested in reverse order). Don't appear to be shorted. Although I should mention that the first time I placed my probes on each of these Q45 measurements I got a reading around 100KOhms, but the second time and from there on it was around 4mOhms. I assume that's normal...

[budm]

Re: Toshiba A665 blown MOSFET

You mean 4 Mega Ohm. So far it looks good, I would get new Q30 and install it.

[Rolin]

Re: Toshiba A665 blown MOSFET

I did get a 4800B from a scrap donor board and installed. The board won't power up all the way - fans spins very slowly and whines - no post. I found another one off the donor board - same thing. I think there are 2 more on that donor board I could try as well, but didn't think it was worth it since the first two didnt' work. But when I install the blown 4800B, the board will POST. I reverified by swapping the chips again - same outcome - blown chip POSTs, replacement chip has slow fan/whining no POST. Very strange.

So if I purchase a new 4800B, do you think that will have a different outcome from the ones I tried from the donor board?

What do you think?

[budm]

Re: Toshiba A665 blown MOSFET

You need to look at the Source pin and see what Voltage you are getting, it should be 5V (0V when computer is OFF, pin4 is LO) same as the Drain pin. Pin 4 should be above 3V (HI). I hope it is installed correctly also.
Please also check the VSB Voltage as shown on page34, is it 5V or 3.3V? It feeds Q11 through R410.

[Rolin]

Re: Toshiba A665 blown MOSFET

Thanks... It's after 1AM - so I'll do this tomorrow night.

I'll resinstall the replacement 4800B and test voltages when ON and OFF. I can't see how I would have installed incorrectly the previous times - I see the mark for pin 1 and installed more than once. Every time I resolder in the burned 4800B it POSTS. But, I'll check this again as well though. Overall this certainly seems like a strange problem...

[budm]

Re: Toshiba A665 blown MOSFET

OK, more fact finding tomorrow. I have a feeling that the output at the Source pin is not 5V.

[b700029]

Re: Toshiba A665 blown MOSFET

VSB is the system main bus voltage and should be close to 20V (or whatever the adapter provides) when the adapter is plugged in, and battery voltage otherwise. Q30 is a high-side N-channel switch so it needs a voltage greater than 5V to turn on the gate fully. The 3.166V measured before is not sufficient.

Because the burned Q30 is shorted, it doesn't matter that the voltage is insufficient, it always passes current. I'd also check the gates of Q29 and Q31 to see what voltages they're getting. Should be >10V.

Also there is a method to test MOSFETs via multimeter to see if they're switching correctly, Google for it (or it might be posted before in this forum somewhere.) It involves charging the gate using the meter and measuring resistance, then discharging the gate and measuring again.

[budm]

Re: Toshiba A665 blown MOSFET

B700029, you are correct, the +VSB is generated from the main B+, and it should be higher since Q30 is used as high side switch, I ifnally found where +VSB is generated from, thanks for pointing that out. I can see that Q11 is still switching on and off but when it is off, it still has bad Source to Drain resistance leakage which keep the Gate of Q30 going closer to the B+ voltage. So Q11 should be check out or just repalce it.
By the way where is Q29 and Q31 (so many pages to look through)? Thanks.

[Duncanbest]

Re: Toshiba A665 blown MOSFET

Q30 +5VALW TO +5VS

+5VS NG , VTTPWROK NG(Page39) ,system will not boot.

refer Page 43: PU15 switching +5VS (source) to voltage +GFX_COREP & voltage +GFX_B+

VGA chip as known need more current then other chip.
-----------------------------------------------------
The capacitor next to heat sink works at high temp.

Assume there are a capacitor leakage on +GFX_COREP(+GFX_CORE) like PC161 C271 or other ceramic capacitor etc. cause +5VS need provide more current then Q30's spec. and burned Q30 4800B .

[budm]

Re: Toshiba A665 blown MOSFET

+5VS is for VCC PU15 to drive the Gate of PQ46, PQ47 only, +5VS is not for driing the load, the main current for the load is from B+ not from +5VS, the Ipeak from this swithcing power supply is 22A and about 15Max, +5VS feed PU15 through 10 Ohms resistor PR28. The B+ fuse will blow if the output filter cap PC161 is shorted or shorted loads and will take out PQ46, PQ47 and B+ fuse. Q30 is for low current (IN MILLI-AMP range) VCC of the PU15 only.

[Duncanbest]

Re: Toshiba A665 blown MOSFET

+5VS is for VCC PU15 to drive the Gate of PQ46, PQ47 only, +5VS is not for driing the load, the main current for the load is from B+ not from +5VS, the Ipeak from this swithcing power supply is 22A and about 15Max, +5VS feed PU15 through 10 Ohms resistor PR28. The B+ fuse will blow if the output filter cap PC161 is shorted or shorted loads and will take out PQ46, PQ47 and B+ fuse. Q30 is for low current (IN MILLI-AMP range) VCC of the PU15 only.

Yes,you are right. I mistake the power path. +5V Pin20 PVCC just only provide the mosfet gate driving current.

[Rolin]

Re: Toshiba A665 blown MOSFET

I installed the replacement 4800B (Q30) with one from a donor board again. Of course same issue - won't fully power up, no POST, and fan spins slowly and whines.

Here are the pins voltages on the replacement Q30

OFF**** PIN1-3 7.9mv, PIN4 1.1mv, PIN5-8 5.22V
ON **** PIN1-3 1.76V , PIN4 3.13V, PIN5-8 5.22V

It appears the PIN1-3 source is pretty low as you expected.

So should I be checking Q29, Q31 to see if the voltages are >10V? And also check Q11? Can I check them with the motherboard plugged in but OFF? I really don't like powering up the board when it acts like it does, and it turns off after about 20 seconds.

[Rolin]

Re: Toshiba A665 blown MOSFET

Q11 checked with it plugged in, but OFF

Pin 1-6
0, 5.14V, 0, 0, 5.14V, 0

[budm]

Re: Toshiba A665 blown MOSFET

I believe Q11A (PIN6) is not fully off to allow the pin 4 of Q30 to be fully turn on as suggested by b700029. I would replace that Q11 or you can verify that by removing Q11 and see if pin 4 of Q30 will be on with 5V at the source pins. What voltage are you getting at VSB in post 28?
I am still trying to locate Q29, 31 in the diagram, cannot find what page they are on yet.
OK, I see them now:
Q29 PIN1~3 should have +3V
Q31 Pin 1~3 should have +1.5V.

[Rolin]

Re: Toshiba A665 blown MOSFET

Q29 plugged in but OFF (another 4800B)

Pin1-3 19.2mv, PIN4 1.1mv, PIN5-8 3.337V

Q31 plugged in but OFF

All pins are 0v (except pin4 is actually .1mv)

I'll read your last post to answer your next questions...

[Rolin]

Re: Toshiba A665 blown MOSFET

Is VSB at Jumper PJ7 PIN1? If so, I'll look for it on the board and check it (again plugged in but OFF)

[budm]

Re: Toshiba A665 blown MOSFET

I expect to see when the system is on there should be 1.5V at pin 5~8 of Q31 (see page 40) which is generated by PU10)

[budm]

Re: Toshiba A665 blown MOSFET

VSB is connected to resistor R410 PIN2, PIN1 OF R410 is connected to pin 6 of Q11A.

[Rolin]

Re: Toshiba A665 blown MOSFET

Checking Q31 when ON.

PIN1-3 (1.5V), PIN4 (14.85V), PIN5-8 (1.5V)

I can't find that dam PJ7 - I'll keep looking... any hints??

Ah saw your next post. I noticed you're mentioning Q11A, but I checked Q11 - Should I be looking for Q11A? I'll check R410 Pin2

[budm]

Re: Toshiba A665 blown MOSFET

Q11 is a multi MOSFET packed into one case.
Q31 is OK, it is putting out 1.5V per diagram.
How about Q29, it does have 3.3V at 5~8, what does it have at 1~3 when computer is ON.
For VSB, look for R410.
OK VSB is OK since you are getting 14.85V at pin 4 of Q31.
So you need to lift Q11A PIN6 to see if Q30 will be fully on just like Q31, Q30 PIN 4 should also has about 14vdc on it.