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Asrock z77 motherboard rebooting, but no debug codes showing

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MathWizard:
I have a broken asrock z77 motherboard, condition/history unknown. I'm testing it with known good ram/PSU/cpu.

It used to power up and spin the fan, but freeze forever. I found an op-amp trying to turn on a mosfet power switch, but it had a shorted cap in it's feedback network. So I fixed that, and now the mobo at least power cycles.

To my great surprise, I found the datasheet for the motherboard SuperI/O /MCU. And someone on here even had the boardview for a similar mobo with that chip, so that helps a lot.
https://media.digikey.com/pdf/Data%20Sheets/Nuvoton%20PDFs/NCT6776F,D.pdf
https://www.eevblog.com/forum/repair/mainboard-suicide-after-sleep/25/

So far I haven't checked all the voltages in operation, most seem to be working, and I already went around looking for shorts in the main places. I'm just getting used to the general layout and soldering on wires for the scope.

But looking at the SuperI/O, I can see  CPU_PWR_GOOD never goes high, then Vcore enable turns back off, and for sure S3# goes back high, and it reboots.

It's hard to solder onto 0.5mm pitch with a loupe, but it's fun trying. Just getting the wires to fit on nearby traces/pads, turns the area into a forest.


The SuperI/O tries to talk to the BIOS, IDK if it answers back or if it's all just the superI/O, but a fair bit happens before the SuperI/O tells the PSU to reboot. I might try and check the checksum of the BIOS.

But another thing tho is, this mobo had a 2x 7-seg display for debug codes, and that never lights at all. I never scoped it yet. It's wired to the SuperI/O basically. When should the debug codes normally start up tho, before or after the BIOS gets working  ? Are the codes stored in the BIOS chip ?  So maybe I'm saying it's not posting, if PowerOnSeq.Test ? is everything before the BIOS starts

PKTKS:
nobody....
well  these  ultra crammed SuperIO chipsets are a headache mess....

If you are willing to check them.. start by dead sure they are ok
ACPI is usually builtin ...

Typical readings... below..

Paul

MathWizard:
Yeah I have wires on most of those nodes, and scattered around the board. There's a lot of computer slang, nicknames, abbreviations, that I'm just guessing at.

On my chip, pin 62 SLP5# is shorted to ground either by a jumper, or a dead resistor. I never found if anything else goes to that pin, to make a voltage divider, if's it's supposed to be a resistor. But then there is pin 84, also SLP5#. I think I soldered a wire onto p84, and a bunch more, but I haven't powered it up since. If that thing was a non-zero resistor, what's the chances it would burn out short ??

I know Vcore is switching, but now I have test leads on the outputs of all the VRMs for the CPU, and ram, and the other ones on the PCB.

I did put 1 on LRESET, pretty sure it was always low.

I'd like to get some chip clamps, thing's that clamp on top of a chip, with needles and breakout wires. I can't remember the name.

I de-stranded some 18 gauge wire, and it fits perfect on the pins, but I better find some insulated stuff in the box. I haven't tried soldering those strands, close to each other, I need to get some taped or glued down first, then 'i might have a chance of getting side by side pins

There's a lot of empty pads for parts around the 6776 chip, nothing looks dirty tho, like someone worked on it. It's definitely not wired the same as the 1 in that boardview.


Most of the big chips, probably have multiple Vcc, so yeah there's a lot to check, but on the whole, most of it seems to get power where it should be, or isn't shorted anyways. I downloaded as many datasheets as I could find so far, most are out there.


I need to attach some wires around the control chips of 1 of my ATX PSU's so that I can make a circuit to see the current it's delivering. My bench PSU would probably struggle with the LV high current loads.

PKTKS:
there is no magic wand for that...

I ve invested a small fortune over the years in :
- Optical assist tools
- bench fixtures
- probes and all sort of things like..

Usually .. non _do_that_for_living.. folks..  get a decent USB scope
these with a small LCD attach...   handy to inspect...  tragic for soldering...

Usually  a large number of folks just solder a NEEDLE on the tip of the DMM probe..
isolated with heat shrink tube...  works marvelous for free...

Your chip will have voltages even if the bios is rebooting...

the bench fixture is not trivial and require a tinkering test

Paul

PS> got yourself a needle probe kit or DIY cheapo (attach)

Nominal Animal:
Exactly which version of Asrock Z77 series motherboard do you have?  (Should be silkscreened somewhere on the board, with revision ("rev.") nearby.)

Asrock is one of the "good" manufacturers, in the sense that they participate in motherboard and chipset development (that they then license/offer to other MB manufacturers too), and do have Repair Guides and even board schematics flying around on the web (among motherboard repair folks).  Indeed, with a bit of Google-Fu, I found and downloaded a board view (.fz) file for Asrock Z77 Pro3 rev 1.04.

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