repairing PC motherboard ASRock Z68 Extreme 4

[sender jones]

I am trying to fix this board (3 years old) which cannot boot. While measuring voltage at various points, I encountered another problem.

Original problem first. If I leave it untouched for a few days, it boot for 3-4 sec, the 2-digit LED lighted up and changing values, then it went off and only the CPU fan and chasis fan turning, nothing else works. If then I shut it down and restart the power again, only the fans turn and LED never light up.

After studying for months, I now can figure out the VRMs and most of the large components on the board. So I check the connectivity without the power. For components that I can identify their pinouts, they look good. Then I power it up and measure the actual voltage of the points I know. Chips Vcc seems ok. Power MOSFET has their reasonable difference between D & S. Vtt is 1.05V. VRM for CPU display (i7 2600K) is only 0.91V which I don't know if it's ok or not. When I measure Vcore I got 0V, input to VRM all 12V. But then only when I saw the psu fan stopped I realize there was no power to the board.

Here comes the second problem. I then tried to power it up again, but it never would. Even the fans didn't turn any more. Pressing the power button just trigger nothing. Checked the +5VSB on ATX power plug is 5.01V. The SIO chip (NCT6776F) also has its 3.3V SB.

I don't know how to proceed further. But I know I have to solve the second problem first before resuming the frist problem. Can anyone help me?

 

[throbscottle]

First thing to attempt would be reseating memory, then a memory swap for known good ram. Also try one memory stick in either slot, and no memory to see if you get a no memory error (some kind of beeps usually). Also try re-seating the cpu, and disconnecting all drives and cards (including graphics card) and also try a different PSU.

I guess you've tried all that... So, if it still doesn't work...

It's most likely that soldered joints on the board have become weak or fractured, especially if has been used for long periods of time. I see this with shop tills, which are on 24/7, when I have to shut them down to do work they sometimes won't reboot, especially if they've been crammed in with crap under the counter and getting hot due to lack of ventilation.

So it may be worth attempting to reflow the board, especially any parts which get hot, including power components. I have successfully used this method on 2 PS3's and my laptop, so it's definitely worth a shot. If you can get access to a reflow oven, that would be ideal. I use a heatgun, which isn't a very good method. If you do it this way you MUST practice on some scrap boards first. You want to melt the solder, but not scorch anything or have parts falling off. First remove the battery. Then flood all the areas you want to re-flow with no-clean flux, shield any plastic parts such as connectors and the bios battery holder using some foil (ideally something more substantial), ensuring it can't blow off. Warm the board first before concentrating on specific areas. Ideally you want about 350 deg C from your heat gun - mine has 2 settings high and low, so I use low. It's still a bit too hot.

 

[sender jones]

Thanks for the reply. For the second problem, it was due to a demaged upper mosfet with its D-S shorted inside. The +12V went straight to the Vcore and demaged the CPU. I managed to find a replacement mosfet and now it has been resolved. So back to my original problem.

My original problem: If I store the board for a few weeks, it boots normally but only last 2-3 mins and then reboot itself but failed. If I try it again tomorrow, it always fail to boot. When it fails to boot, the on-board diagnostic LED doesn't light but the CPU fan, psu fan and chasis fans turning. At this status, Vtt and Vcpu_display seems to be normal but Vcore is 0V.

I also have some other finding which may or may not related as I don't have the basic knowledge to interprete them:
1. After boot failed, I turn off the power and measure the resistance between S-G of upper mosfet. There are 8 phases, 5 seems normal (open, at 200Ohm selector), 3 has exceptionally low at 3 to 7 Ohm. But the odd thing is they continue to rise slowly. It takes >10 hours for them to reach open. This seems to be consistent with the syptom of my problem: when this resistance is at low value, it can't reboot; when resistance rises to open, it can reboot.
2. While the PWM controller has 3.36V at all its PWM output and can reach the drivers, the upper and lower mosfet always has 0V at their G. Sounds like the driver has problem. But chances for all 8 drivers have problem should be low.
3. I draw up the schematic of the VRM here. All are DMM measurement, may not be component values. Resistance on AB is 95.9 ohm (yes, the cap) and on A-Gnd is 94.8 ohm, which is larger than AB!! Is this normal, or just my ignorance.

Plesae help.

 

[throbscottle]

Your AB measurement looks like it's reading through the PWM. I think you made a typo though so I may be wrong if you didn't.

Still looks like a thermal fault. Try warming/cooling the components whilst you test them. Still don't rule out faulty soldered joints either.