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Thread: ASUS B-85M-G mobo repair manual / schematics

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    Senior Member BCNZ's Avatar
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    Default ASUS B-85M-G mobo repair manual / schematics

    Can anyone assist with the repair documentation for this mobo?
    I have found various references to it online, but the usual mousetrap websites or ones which require you to sign up and jump through hoops just to get one document.
    If anyone has this I would be most grateful.



Look Here ->
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    I actually wonder if these exist ~ and if they do exist, I would wonder if they're in English or not. A lot of those sites you mention, actually prey on the fact that ppl are looking for a manual the site owners know isn't 'out there', and list it anyway behind a thousand pay-per-click links. Click-baiters.

    May I ask, why exactly are you seeking a mobo repair/service/schematics for the B-85M-G?

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    LSemmens
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    Is this any good?
    I'm out of my mind, but feel free to leave a message...

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    Quote Originally Posted by lsemmens View Post


    Is this any good?
    Unfortunately not. I have already checked out what is readily available online. I need indepth technical documents, the sort of thing they never make available to
    anyone other than their "authorized service centres".

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    Quote Originally Posted by wotnot View Post
    I actually wonder if these exist ~ and if they do exist, I would wonder if they're in English or not. A lot of those sites you mention, actually prey on the fact that ppl are looking for a manual the site owners know isn't 'out there', and list it anyway behind a thousand pay-per-click links. Click-baiters.

    May I ask, why exactly are you seeking a mobo repair/service/schematics for the B-85M-G?
    Thanks Wotnot. Yup, it's definitely possible that the sites I found are nothing more than click-baits.

    I accidentally nuked the mobo when I sent 12 volts up a 5 volt line on a piece of equipment I had connected to it via USB.
    The group of USB ports no longer have power and the sound has also died which suggests a 5 volt rail has been taken down most likely
    due to a fuse (or fuses) going open. I have been around with the meter and checked what I can find that looks like an SMD fuse and all
    of them test Ok.
    If I had a schematic I could do some further testing to see what's gone open or why there's no +5 V on the USB sockets etc.

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    Quote Originally Posted by BCNZ View Post
    Thanks Wotnot. Yup, it's definitely possible that the sites I found are nothing more than click-baits.

    I accidentally nuked the mobo when I sent 12 volts up a 5 volt line on a piece of equipment I had connected to it via USB.
    The group of USB ports no longer have power and the sound has also died which suggests a 5 volt rail has been taken down most likely
    due to a fuse (or fuses) going open. I have been around with the meter and checked what I can find that looks like an SMD fuse and all
    of them test Ok.
    If I had a schematic I could do some further testing to see what's gone open or why there's no +5 V on the USB sockets etc.
    I've got a good enough image of the mobo top layer ; I'm seeing what I'll suspect as TVS diodes, the realtek audio chip looks like it's protected by a 0 Ohm/.25w resistor, and I see the USB power rail fuses, From what I can see, I'm thinking there's stuff on the bottom layer of the mobo that may be responsible for some of these 5VDC rails....I might be able to point you in the right direction ....but I need a good image of the bottom layer of the mobo, and to know which USB port got spiked by 12vdc?

    If you can provide an image and info in these regards, I might be able to spot the area worth poking at =)

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    The group of USB ports no longer have power and the sound has also died which suggests a 5 volt rail has been taken down most likely
    This doesn't bode well ~ the mobo uses a Realtek ALC 887 sound chip ; this is a 3v3 device that does not connect to the 5vdc power rails =^/

    edit: correction ~ the ADC and DAC sections of the ALC887 do indeed get 5vdc....
    Last edited by wotnot; 04-07-22 at 11:55 PM.

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    I'll just tap out a brief example of how I would go about diagnosing something like this mobo, when one can't source any schematics. It helps quite a lot if you understand how most of these mobo's are designed/made, and what goes on during SUS* (start up sequence)...ie; since the advent of 'superI/O' chips, this all happens very different compared to older ix86 mobos...and do note, I wouldn't bother ; I'd just buy another mobo ...these things are typically beyond economic repair IRL....

    For reference, I'm drawing on this image to try divine things ->

    So cause of fault as described, is 12vdc was accidentally applied to a USB 5vdc power supply pin. One doesn't know for how long that condition was maintained, however one can construe for the fault report that the mobo is still booting, and functional enough for the user to discover that USB ports are missing 5vdc power supply rail(s), and that the audio section of the mobo is now apparently dead as well. One does not know if the USB-C ports were involved, which is something one would like know because those ports are on a separate controller IC.

    The user has been able to identify/check the various fuses adjacent the USB port headers/sockets, and found these to be good (not OC)




    That's just one section, the fuse marked 'P 260' (the 'P' is likely for 'polyfuse' self-healing fuses), the no-stuff buffer cap to it's top right the OEM decided to cheap out on and not include, and it's unclear whether or not that fuse is connected to just one or both headers (5vdc disappears down 3 vias on USB1314), and if both you'd have to wonder how they're implementing per-port-current limits....but anyhow...

    The audio section dying is likely the clue ~ the speculation being that both it and the USB ports missing 5vdc is more than coincidence.

    As a rule of thumb, mobo OEMs typically follow the datasheet application notes, when they utilize 3rd-party IC designs -- ergo, you download the audio chip datasheet for the device used, Realtek ALC887 ...(and make sure you download the right datasheet for the specific IC used, so you don't get tricked like muggins here did initially =), and what you're looking for is this;





    To figure out 5vdc is going to pins 25 & 38..and you take note each of these has an attendant buffer MLC cap and max voltage spec might only be 6vdc for those...(10vdc if you're lucky)...then get the package footprint, rotate 90 degrees...




    Then look at the mobo implementation/topology...




    ...and you can see the 5vdc rail clearly by the OEM cheaping out again with another no-stuff...and yon '0' Ohm resistor likely being used as a fusible link. I imagine the MLC cap just below the '37' pin marking is the buffer cap for pin38 5vdc in. To test the theory of whether or not the 5vdc rail here is common to the USB 5vdc rails, just meter for continuity...

    At this moment, you're perhaps looking at a situation where the ALC887 saw 12vdc on it ADC/DAC power supply rail..this probably ends badly =) You'd immediately suspect the 5vdc rail on the mobo, may actually be shorted to ground, so you'd have to establish that position right now.

    Then you think....hmmm....where else might this 12vdc have gone, where only 5vdc is meant to exist?...




    Damn superI/O chip..at it's core it's only a 3v3 device, but it ends up doing power management/supervision ('S' states etc), so they typically use some input for voltage sensing of various power rails on the mobo, and if one of these was tied to the same 5vdc that saw 12vdc, you'd wonder if that particular input is now dead =)

    If the 5vdc rail was shorted to ground, I'd suspect the ALC887 immediately ; if eliminating 5vdc supply to this chip resolves the short, but the 5vdc rail still doesn't come up, you're left with two obvious possibilities...

    1. whatever instantiates this 5vdc rail is busted...you'd have to buzz it out with a meter, but I'd be thinking here, just because this looks like a sub-supply block with p-chan mosfet on the left (max of 3vdc gate, so logic controlled), and linear regulator on the right ;




    2. whatever instantiates this 5vdc rail isn't receiving a signal from the superI/O to turn on, because the superI/O can no longer sense the voltage on that rail (see above), and doesn't send the 'turn on' signal as a result.

    If it's just the ALC887 pulling the 5vdc rail down and the USB 5vdc power comes back up, I'd just leave the pins I lifted to isolate the 5vdc rail to the chip unsoldered, and just use a USB sound device...why?..getting a new chip and doing the rework would be an order of 10 more expensive 8)

    If it's ever the superI/O, one is pretty much stuffed...even if you can get the IC itself, it has to be flashed with the correct firmware to suit the mobo model you have, and the OEMs usually buy/use OTP (one time programmed) ICs, and some have protection that turns them into black epoxy coated marshmallow if you try to dump/read the original firmware from them. Sometimes you can buy these chips on ebay and the like, but at the cost + time/effort spent doing the rework, it's not cost effective.

    If it's something other than what I mention here, then at least you've got some idea of what you're up against...

    End of the day though....new B85 mobo's are still out there for <$100, 2ndhand for around $80....and brand new LGA1150 mobos with more features, newer chipsets and a warranty are only around $120 ....and when I was doing this work for a job, it was costed at roughly $100-$150/hr for the pleasure (+ parts =). Don't get me wrong here ~ if one has the time, tools and the want to fix one of these mobos, it is possible in many cases ; one has to question whether it's worth it or not B)

    edit: * not to be confused with AmigaOS startup-sequence =)
    Last edited by wotnot; 11-07-22 at 02:08 PM.

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    Wow, that's some indepth stuff!
    I have only just seen your reply and it's after midnight so I won't go into a detailed response tonight but I will come back to this when time permits.
    Agreed, it's likely not worth repairing but I thought if I had just nuked a fusible resistor or two then it would be worth having a go.
    If it's any more than that then I'm just going to set it aside.
    It still boots up and runs as it should, except a block of USB ports on the rear (the ones where the offending 12 volts got in) are no longer working.
    I haven't checked whether that's because there's no 5 volts on them or if it's because there's 5 volts but the chips that detect something being plugged into them are nuked.
    I can upload a hi-res photo of the rear of the board.
    I found the P260 devices and all of them check out Ok.
    I didn't spot anything that looked like an SMD fuse near the relevant USB ports, but I may have to get the microscope out to look for them.

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    That'll be the fuse for the back block (with ethernet socket).....just above to the right of the 'L' in LAN, you see 2 diodes - check them as well.

    edit: actually, I think those are the TVS diodes for the ESD protection on the ethernet port
    Last edited by wotnot; 11-07-22 at 02:20 PM.

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