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Old 10-02-2019, 06:58 AM   #1
dicky96
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Default Asus P5S800-VM no POST

Hi guys, here is the latest motherboard I'm trying to fix just for the hell of it

ASUS P5S800-VM

It turns on/off with the ATX power switch, but it doesn't boot (POST 00 on my diagnostic card)

I tried another P4 CPU, no difference

I tried with or without DDR. No difference

I have Vcore 1.4V, Vram 2.6V, 3.3V to the BOIS, 5V, 12V etc. I'm using a known good PSU

I reflashed the BIOS - no difference

From looking around with my scope I think i have no system clock. I'm looking at the RTM363-655 which I am sure is the clock generator. The crystal doesn't seem to be oscillating (just some high frequency noise on one pin probably from the PSU). I have 3.3V supply to this chip. Nothing seems to be happening on the outputs.

I couldn't find the exact datasheet. I found this one https://datasheetspdf.com/pdf-file/7...K/RTM363-210/1 for the RTM363-210

Does anyone know if the -210 and -655 version of RTM63 have the same pin out & functions?

Are these clock generator ICs known to fail?

best regards
Richard
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File Type: jpg IMG_20191002_134620.jpg (931.6 KB, 22 views)
File Type: jpg IMG_20191002_134626.jpg (1,020.9 KB, 17 views)

Last edited by dicky96; 10-02-2019 at 07:00 AM..
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Old 10-02-2019, 08:51 AM   #2
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Default Re: Asus P5S800-VM no POST

Here`s some boardviews ,it might help with your repair.
Attached Files
File Type: zip P5S800-VM R1.01.zip (1.61 MB, 14 views)
File Type: zip P5S800-VM R1.03.zip (165.6 KB, 10 views)
File Type: zip P5S800-VM.zip (1.71 MB, 10 views)
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Old 10-02-2019, 09:45 AM   #3
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Default Re: Asus P5S800-VM no POST

Thank you. That's much appreciated
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Old 10-03-2019, 03:32 AM   #4
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Default Re: Asus P5S800-VM no POST

Sorry to be a pain, I have openboardview but when I open one of those files i just see a large square with a few capacitors (or similar) on it. How do I get the boardview with all the components?


I had a look at the RTM363-210 datasheet again - link in post #1

Assuming that is basically the same as the RTM363-533 then i have #PD is high 3.3V so according to the datasheet the IC should be active.

#RESET is low but the datasheet says that is an open collector output

According to the datasheet Block Diagram 4, there should be control inputs for #PCI STOP and #CPU STOP but those don't appear to relate to any pins on the IC.

On the Xtal I have 1V on one pin and 1.5V on the other pin but it isn't oscillating.
Clock Outputs such as pin 21 and 22 are low. So it isn't working.

Should I now order an Xtal and RTM363-533, or is there some other possibilities I am missing here?

Also i notice there are some control registers and a serial interface to program the RTM363 output frequencies. But how does that happen unless there is a clock to drive the CPU in the first place, unless it is supposed to oscillate at some frequency like 100MHz by default at power on?

Last edited by dicky96; 10-03-2019 at 03:35 AM..
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Old 10-03-2019, 03:52 PM   #5
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Default Re: Asus P5S800-VM no POST

BoardViewer will open .asc files .
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Old 10-04-2019, 02:58 AM   #6
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Default Re: Asus P5S800-VM no POST

OpenBoardView works fine with them too. You may need to flip the board pressing the space bar.
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Old 02-11-2020, 05:25 AM   #7
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Default Re: Asus P5S800-VM no POST

I've come back to look at this again. Thanks for the board view files - I got them to open in Boardviewer and they helped me.

As far as I can see there is no system clock, I've looked around the clock generator IC RTM363-655 using board view as a guide and it is enabled (reset is high) it has power on all the pins it should have according to board view, the crystal is oscillating, but nothing comes out of the IC. All the pins (other than xtal) are static either 0V or 3.3V

Unfortunately I don't have a datasheet for RTM363-655. I do have datasheet for RTM363-210 but it is clearly not the same chip, even some voltage supply pins do not match the board view. Also I can't find a supplier for this 655 chip on aliexpress. I found one on alibaba who would sell me five at a reasonable price but the postage is killing the deal, and this repair is only for educational purposes.

So I could possibly assume that chip is faulty. However I apparently have another problem. I've noticed that a power FET on the board gets quite hot and from board view it is clearly feeding VTT which I measured at approx 1.2V.

I measured the resistance from VTT to ground and it is 2.5 ohms ,with no CPU fitted.

I then got a couple of other LGA 775 boards I have lying around and on those ones VTT to ground is 125ohms on one and 187 ohms on the other with no CPU. Having said that, one of those has an intel P45 chipset (125ohm) and the other has intel G31 (187ohm). This motherboard is SIS and I don't have another one lying around to compare with.

As VTT also supplies the north bridge and south bridge I am wondering if the low resistance reading means that one of those is faulty? The south bridge is getting warm but not hot. Or is it as feature of the SIS661X/SIS964 chipset on this board?

I also can't understand how a faulty north bridge or south bridge would stop the system clock from running, and it seems odd to have two different faults.

Can anyone advise what resistance to ground I should see on VTT? Did I find a useful diagnosis test on boards what will power on but not boot (0000 on post card)?


best regards

Last edited by dicky96; 02-11-2020 at 05:33 AM..
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Old 02-11-2020, 07:47 PM   #8
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Default Re: Asus P5S800-VM no POST

You'd need a oscilloscope with a decently high bandwidth to be able to check those clock signals (they'll most likely be 24MHz, 33MHz, 48MHz, 66MHz, 200MHz and maybe others). Clock generator can fail as any component can fail, but not really the first thing I'd suspect.

Can you post a picture of the board identifying the VTT MOSFET you're talking about?

I'm not sure I have a board with those specific northbridge/southbridge but I'll try to take a look. I don't really like SiS chipsets…

Unfortunately there can be a lot of reasons for a board to power on but not to start POSTing. Usually in this case I check CPU VCore, CPU reset signal if I can find it easily, I flash the BIOS, and I try to feel around the board for anything getting abnormally hot. You already checked CPU VCore and tried to flash the BIOS though.
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Old 02-11-2020, 11:12 PM   #9
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Default Re: Asus P5S800-VM no POST

I also suggest taking off the stock Intel pushpin heatsink and testing with the heatsink just resting on the CPU. These stock Intel pushpin heatsinks are known to warp boards badly and can sometimes damage the BGA on the CPU socket. So once you take off the push pins, try booting the board. Also try applying some pressure on the CPU heatsink as you try to boot the system. Hopefully this might show if you have a bad socket BGA or not. It's rare, but it can happen.

As for the very hot MOSFET that regulates Vtt... if it's outputting the proper voltage (which 1.2V sounds like it is) and ripple is in spec (will need scope to measure that), then it's probably regulating properly and not getting overloaded.
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Old 02-12-2020, 09:52 AM   #10
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Default Re: Asus P5S800-VM no POST

Actually I was using my 20MHz LG scope but I will look again tomorrow with my 100MHz Tektronix scope.

Vcore and VRAM are OK. I will have a look to see where the CPU reset signal comes from. Probably the SIO?

I've added a pic as requested

I agree the hot MOSFET is likely outputting the correct voltage. a 2.7 ohm load would be about 500mA at 1.2V so it is not excessive - it just seems very low resistance compared to the LGA775 Intel chipsets I compared with

The FET doesn't get very hot but it's still 65C according to my temp probe and that is with the board doing nothing.

I'll try the suggestions with the heat sink
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Last edited by dicky96; 02-12-2020 at 09:57 AM..
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Old 02-12-2020, 01:49 PM   #11
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Default Re: Asus P5S800-VM no POST

Ok so I found a 661GX-M7 rev 1.1 board and the lowest resistance to ground I could find on northbridge/southbridge is 58 ohms. Board dead cold and no power at all since I didn't plug anything in, no CPU/RAM either.

I also found 2 649-M2 rev 1.0B boards and one has 35 ohms the other 30 ohms as the lowest resistance to ground around northbridge/southbridge.

CPU reset signal is typically issued by northbridge with Intel chipsets. I don't know SiS architecture well enough to tell if it's the same, I think there are some noticeable differences in the power/reset sequence.
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Old 02-13-2020, 06:26 AM   #12
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Default Re: Asus P5S800-VM no POST

Thanks momaka, piernov - you helped me some more

I tried running the board without the cpu heatsink. It isn't a problem as the CPU only goes up to 45C even after the board has been powered up for 20 mins

I tried pushing on the CPU when starting the board but it makes no difference

I noticed on my POST test card that the CLK LED lights extremely dimly

I don't have any clock, checked on my 100MHz scope. Using boardview to guide me I checked some clock signals. CPU Clock, SATA Clock, NBH Clock - all missing. The 14MHz crystal to the Clock Gen IC is running

I also checked some reset signals to the CPU (with CPU fitted)

CPURST# (LGA pin 532)
DBRESET# (pin 63)
HTRESET# (pin 154)

all low

I noticed that I can turn the ATX PSU on with the power on switch but can't turn it off again by holding the switch down.

Without the heat sink on the North bridge it gets quite warm 47C. The South bridge is cool.

And given the low 2.7R resistance to ground on VTT with no CPU fitted, the hot VTT MOSFET and what Piernov said about the CPU reset coming from the north bridge, and the fact you also found much higher resistance on VTT with similar SiS boards....

All this to me is pointing towards a faulty north bridge BGA. And maybe that is somehow also stopping the RTM363-655 clock gen from running (even though CLKRST# on pin 4 is high)? The lack of a datasheet for this IC is not helping me here :-(

I'm thinking if I remove the north bridge and the resistance on VTT goes up a lot this may also indicate a problem there

I can get a couple of new SiS 661FX BGA at reasonable price so I would like to try fix this to prove if I have managed to diagnose it correctly or not

Either that or replace the RTM363-655

Is my diagnosis well thought out?

cheers
rich

Last edited by dicky96; 02-13-2020 at 06:31 AM..
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