MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

Can't find how to edit my post, so I'll add this...

My question is, if it's not too much of a hassle, which pins do you check these voltages on U7 and U23, what should the voltages be and can it be diagnosed without a scope?

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

Before you suspect the board, I assume you have done the obvious checks, like:

Have you tried removing the BIOS battery, clearing the BIOS settings, and installing a fresh battery if the old one measures lower than 3v?

Have you checked that you do not have a PSU problem? A bad PSU or a PSU with no Power_good signal will cause this problem.

Is your case's RESET button stuck?



I'll have a look and let you know where I measured the voltages I checked. I suggest you go through the schematic though and check all the power rails because even if it is a power rail issue, you might have a problem with a different one than I did.

If you need a scope or not probably depends on what's wrong with it. That you can power the machine off suggests a different problem to mine already.


Also, did this just happen one day or were there other problems with it before this happened? Or anything else you can think of that might be related?

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

Thanks for the reply.

I removed the old and installed a new BIOS battery.

At first I was convinced it was the PS because I put in in another system and it did the same thing on the second and third boot attempt. So I ordered a new PS.

In the mean time I went back and tried that same PS on the second and a third system and it booted multiple times and ran for hours without any problems. The new PS arrived and obviously hasn't fixed the original issue with the MSI board.

The board was in use for at least three years as our HTPC. One evening last week it wouldn't post... blank screen.

Pushing the reset button seems to function because the fan speeds up for a few seconds.

Just checked... NB_VCC1P1 has 1.25v. VCC_DDR has 1.862v.

Could this mean something... the only voltage it's getting on USB is 5v's on pin one and two.

Thanks again.
Lee

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

The PSU not working immediately in another system is a bit odd.

It reminds me of a PSU I had which had a bad PWR_GOOD generation circuit, which worked on some motherboards but not others. On the boards it worked on, the floating input from the faulty PSU's PWR_GOOD line must have been going high by itself on those boards thus enabling them to work even when the PWR_GOOD signal was not present.

I would check out the PWR_GOOD voltage from both PSUs (Grey wire). It should be at least 3 volts, likely more. Try this with the PSU running without board attached and powered up manually (short PS_ON to GND with a piece of wire) and also try measuring it when connected to a working and the non-working board.

I wonder if there is some fault with the PSU or board which gives a marginal PWR_GOOD output from the PSU? A shorted input on that line on the motherboard could even damage the PSU.



Assuming the PSU is OK though, I would start with Page 27 of the schematic and the SB700 & RS780 POWER GOOD CIRCUIT, and the one next to it that gives the CPU VRM enable signal (VCORE_EN#)

See what the SYS_PWRGD and PWR_OK and VCORE_EN# lines are doing.

I have some explanation of how these circuits work in Post #9.

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

SYS_PWRGD is 2.6v (system on)

PWR_OK is 4.5v (system on)

VCORE_EN# is 2.94v (system off). As soon as the power switch on the motherboard is pressed, it drops to 7.7mv.

I did check the voltages on the original power supply before ordering the new one and the only one that was off was the -12v, which was -10.79v.. That was with the board powered on.

Since I've never taken any electronics courses, you'll have to bear with me. I kinda wing it as I go.

I was hoping to check the voltages on the IC's (listed on page 27) but the only one I could find was U109. Too bad there isn't a component locator page in the manual.

I ordered up a used similar M/board off Ebay so I could use this 9950 chip, so I'm prepared for the worse. I'd still like to fix this board if possible, as long as parts are available and not too expensive.

Thanks again for the help.

Lee

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

Sorry I can't be more specific right now as the board is in a PC and I can't easily look at it at the moment.

I know U1 for VDDA_25 was around the CPU socket somewhere, it's a very small SMD part.

The transistors for the power-good circuits are around the area with the front panel connectors and SIO chip, IIRC.

What may make it easier for you is knowing what you're looking for, note after each part number they have the package like "SOT23-8" or "SOIC8"

Look up the package type so you know what it looks like and you will find it easier to find the part on the board as you can look just at components with that package.

Sorry I can't be more help right now


Tell you what though.. your PWR_OK 4.5v and VCORE_EN# going to 7.7mV at power on looks OK, but SYS_PWRGD at 2.6v seems odd. I think it should be closer to 3.3v. I will compare with my own board later and see what voltage I have there.

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

Thanks for taking the time to help out. In the meantime, the other M/board arrived so the HTPC is now working again. But, if there's any chance of fixing the MSI, I'm willing to put in the effort.

Ok, the package type info helped and I found all of the chips on page 27.

I see, for example, SYS_PWRGD has a '15' after it. Or, PWR_OK has a '26,28.35' in front of it. What do these values mean?

On U16, I can see the v out voltages 1.2, 1.25 and 1.8 but on U1 is pin 5 actually .25 volt?

I do see where SYS_PWRGD should be 3.3v. Any idea what would cause it to be low?

Can I power up the board without the CPU installed to check voltages?

Thanks.
Lee

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

I think, those numbers are to show you which pages of the schematic those same named connections (or 'nets') connect to.

If you're getting only 0.25v on U16's Pin 5, that does not sound good. Are Pin 1,6,7 on U16 OK? If so, U16 is probably OK, maybe it could be something shorted on 1_2VREF line.

SYS_PWRGD being a bit low is a funny one. Check the VCC3_SB rail from the other side of R216 and make sure it's not low too.

If Q54 was shorted it could be pulling the SYS_PWRGD down toward the +1.8V_S0 rail, perhaps.
Same may happen if C607 or C694 were leaky, pulling it toward ground.

Or you may have some problem in the SB700 & RS780 POWER GOOD CIRCUIT (or a signal feeding it)
What voltages do you get on base of Q37, gate of Q38, and the cathode of D27, D17, and D18?


Yes you can power up the board without a CPU (won't damage it), BUT without the VID signals from the CPU to tell the VRM controller what voltage it wants, the CPU VRM will probably not run. This may or may not give false readings, I am not sure.

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

Oops... I made a mistake on U16 pin 5. It's correct at 1.2v. Sorry for wasting your time on that one. All voltages on U16 are proper. I did check the others and Q37 base is .662v, Q38 gate is 5.2v and the cathodes on D17 & D27 are 2.6v.

As to the SYS_PWRGD, R216 is 3.4v.

Pretty much everything on page 27 checks out ok voltage wise except U4 pin 8 which is 12.4v. The schematic shows 5v.

U1 - Pin 1 = 3.4v. Pin 4 = .8v. Pin 5 = 2.5v.
U4 - Pin 1&2 = 5v. Pin 7 = 5v. Pin 8 = 12.4v (???)
U16 - Pin 1 = 5v. Pin 5 = 1.2v. Pin 6 = 1.25v. Pin 7 = 1.8v.
U28 - Pin 5 = 1.2v.

All voltages were checked without the ram or cpu installed.

Not sure if this means anything but R51 was 12v on both sides and R23 was 5v on both sides.

Thanks again for you time on this. Hopefully it will be worth your effort (and mine)

Lee

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

U4 having 12.4v on Pin 8 is OK, because R51 is a pullup to the 12v rail since Q14 is used with the source connected to 5v.

With an N-Channel Enhancement-mode MOSFET (what Q14 is), the gate voltage has to be higher than the source voltage for it to turn on. There is a threshold voltage and the datasheet lists that for this APM3023NUC as 1v minimum, so the gate has to be at least 1v higher than the source before it will start to turn on.

If the gate only had a 5v drive, it would be the same voltage as the source, also at 5v, the difference would be 0v, and nothing would happen. With a 12v pullup, the gate is driven with 7v (with respect to the 5v source supply) which is plenty enough to turn it fully on.



The gate of Q38 measuring at 5.2v doesn't add up. If Q37 has 0.6v at the base that suggests it is turned on, and thus should be pulling Q38's gate signal low.

If Q38 has 5.2v on the gate it should be turned ON, and would be pulling SYS_PWRGD low, even lower than 2.6v (pretty much should be 0v if Q38 is fully on)

I might be missing something, but right now, something sounds odd.

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

One last shot before I give up and toss this thing...

Do you think one of those cheapee PCI diagnostic cards would give me any useful information?

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

You might want to check if Q38 or Q37 is not shorted or leaky, something maybe odd there.

A good PCI card might tell you something but I can't be sure, and those cheap ones probably wouldn't. In fact, I have seen cheap ones kill working motherboards...

[ChaosLegionnaire]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

if its a short, a pci post diagnostic card wont be of much help. thats because the short trips the power supply within 1 sec of power on. thats barely enuff time to initialise the chipset and bios bootblock. u wont get anything useful off the post card before the psu trips and shuts it down.

also, post cards require a certain amount of experience and "street smarts" to interpret what the error actually means and exactly what is causing it to happen/occur.

take for example, a board that constantly hangs at early post during the base memory test stage. this may seem to indicate bad ram. however, swapping the ram didnt fix the issue. so what now? are u going to declare it a bad ram slot or a bad chipset mch? turns out the board actually had bad caps around the dimm slot. recapping the dimm slot caps fixed the issue and the board now posts and works normally now. so thats an example of how "savvy" one needs to be to figure out wtf is causing the board to hang at that post checkpoint.

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

We're not talking about dead shorts across the main power rails, I am just theorising that there may be a shorted\leaky part in the circuitry that generates the system power-good signals.

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

For the hell of it, I picked up this cheap 4-digit diagnostic card: http://www.ebay.com/itm/380831560767

Plugged it into a PCI slot and powered up the PSU. There were four dashes on the readout.

The -12v, +12v, +5v, +3.3v and the 'CLK' LED's came on. Neither the 'IRDY' nor the 'FRAME' LED lit. When I pressed the reset button on the MB, it's corresponding LED lit on the card.

Pressing the 'power' button on the MB did nothing. I also tried another PCI slot and pushed the button on the card with no luck. I did not hook up the external wire that came with it as I wasn't sure where it went. No beeps from the speaker.

The CPU did get warm within a minute as before.

With that fine bit of information, you guys should be able to figure this thing out in no time;-)

I think it's time to give up on this before I spend more time and money on it.

Thanks for the help.

Lee

[arcman49]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

I just reread "Chas'" post and noticed him mentioning the possibility of a short shutting off the PSU.

If that were the case, wouldn't it show a low voltage on one of the pins?

[Agent24]

Re: MSI DKA790GX \ MS-7550 v1.21 - Do I have a short on NB_VCC1P1?

I just reread "Chas'" post and noticed him mentioning the possibility of a short shutting off the PSU.

If that were the case, wouldn't it show a low voltage on one of the pins?

If there were dead short across one of the main rails, the PSU would shut off, which in your case, it doesn't.