Electronics > Repair

Asus mainboard repair

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sunfire:
Hello guys,

I received a non-working mainboard Asus Crosshair III Extreme which doesn't start and I'm trying to repair it for myself. The mainboard seems decent and I've got it for free. When connected to power the onboard RGB leds and the OLED display do their "dance" but the mainboard doesn't react to the power button. I traced the power button and the PSU start pin to the Nuvoton NCT6798D super io chip. Unfortunately I couldn't find any datasheet for this super io. One big question which I have is the voltage across the crystal oscillator: when measured (while the mainboard is connected to power) with my Fluke 189 the crystal voltage reads ~12mv AC in one direction and ~38mv AC in the other direction. Doesn't this seem low ? I would have expected a few hundreds of mv at least. Unfortunately I don't have an oscilloscope (it would have sped up the investigation significantly). The super io chip has correct Vcc (3.3v) but other than that I have no idea how to test it. If the crystal is indeed bad what frequency does it have ? It's marked "15G61", I would guess 15MHz ? The crystal oscillator size is ~ 3mm x 2mm. I'm attaching pics taken with my phone of the super io chip and the crystal oscillator.

Help please ?

Big thank you !

PS: Electronics is my hobby, it's not my main profession. I have enough tools to repair most electronic things but unfortunately no oscilloscope. I know my way around electronic things but I'm far away from EE level. Also, English is not my native language but I think you guys understand me.

fzabkar:
You won't see the crystal's AC voltage on your DMM. That's because your DMM's frequency response is much too low.

Measure the DC voltage between each end of the crystal and ground. If the crystal is oscillating, the DC bias should be about half the supply rail.

sunfire:

--- Quote from: fzabkar on October 06, 2022, 11:12:12 pm ---You won't see the crystal's AC voltage on your DMM. That's because your DMM's frequency response is much too low.

Measure the DC voltage between each end of the crystal and ground. If the crystal is oscillating, the DC bias should be about half the supply rail.

--- End quote ---

I just did, at one end the DC voltage is ~1.8v, at the other end the DC voltage is ~2.15v. So the crystal seems to be oscillating then. I will investigate the auxiliary components around the super io chip, if those are fine then I need to see where I can source a super io chip for this board. As far as I know these super io chips require programming (with a programmer which I don't have).

Thank you fzabkar !

Le_Bassiste:
whoah! bottom pin row in upper picture has the most shitty solder job i've ever seen on any mobo.  :palm:
you sure that's an Asus and not an A-sus?

edit: might easily be reparable by just resoldering, using a ton of flux.

sunfire:
It must have been bad lighting when I took the picture with my phone. it was close to midnight and the angle of the photo does indeed hint at bad soldering of the bottom row pins.
But fear not, all the pins are soldered ok, I already tested each one of them before I posted. The soldering job is also Asus quality, it must be my crappy camera of the phone, sorry.

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