Testing VRMs

[andlcs]

Originally Posted by badman86

Hi!
It's an MSI K9A. I'd like to measure the resistance from the input and output of VRM to ground.

I think you can measure the input at the toroid coil near the ATX+12V conector.
And the output at the shielded inductor near the CPU socket. I'm not sure tho...

power off your computer first

[badman86]

Thank you for your answers, I think i got it now!

Now, what is not clear yet is this:

a. Measure resistance from power supply pin (+3.3, +5, +12, -5, -12) to input of VRM. Resistance should be zero (too low to measure).

Power supply pin means the ATX connector on the motherboard?

[coolday]

Admins and moderators please try to start a section on motherboard repairing. it would be very helpful for newbies like us. Maybe this would be the first website to do so.

[coolday]

so anyone can provide pictures .... it would help us a lot

[closetonanak]

First You Should Know Where Is Vrm Section On Board And How It Works, Which One Is Positive Line And Negative Line Then Mosfets Of P And N Lines, Check The Voltages, If No Voltages Means May B Mfets Are Open Thn Take Them Out From There Place And Check Also Check On Pads If Still There Shorting Means May Be Another Mosd\fet Has To Take Out From Board And Chk It Like That Go Ahead If U C That Capacitor Is Leak So Fst U Hav To Repalce, And In Ths Case Pwm Ic Of Both Line ( +, _ ) Have To Replace And Chk, Buck Control Ic( For Power Good Signal) , Io All Thing Should Be In Lone To Replace

[sam67]

Originally Posted by coolday

so anyone can provide pictures .... it would help us a lot

Would also like some pics if anyone has any or links ..This is really interesting and should be further explored ..

[alexanderjohn]

yes! especially laptop motherboards

[Pyr0Beast]

Laptop motherboards ... hmm.... if I have the time ...

//c. Measure voltage at the input to the VRM (+3.3, +5, +12, -5, -12). Anything less than the value obtained in step 6.b indicates a high resistance in the PC board traces to the VRM, and is probably not repairable, even with a schematic.//
Is repairable, by making a bridge with a wire. Usually from one capacitor to the next one. Never occurred to me unless it is a manufacturer design.

Basically, it is pretty simple. If mainboard does not POST, check voltages.
If PSU voltages are within range, check voltages on motherboard. Cpu has pretty much standard voltage. About 1.4V is fine for newer CPUs, 1.8V for athlons and P3's, 2V or greater for first celerons and P2 (Some early P3)
If there is no power (voltage) to the CPU, check if you placed capacitors the right way, observing the polarity. Same goes for other circuits, RAM, AGP etc. ..
If VRM IC is busted, it would be hard to find a new one. Sometimes (rarely) only one of the mosfets break, usually the upper one. When it does that, the lower one migh go 'poof' as well (protecting the CPU).
If one breaks, replace both. Check IC for damage, might be good to replace that one as well, sometimes it is the cause of this mess.

//VRMs (voltage regulator modules) are a specific class of MOSFETs; //
Far from being true. A VRM module consists of the controlling IC and MosFet's (if we focus only on semiconductors).
However, it is true that there are many type of FET's. Mosfet being the most common one. JFET is one of the other type.

[ivanlee05]

Source = Input
Drain = Ground
Gate = Output

[Pyr0Beast]

?
Source = Output
Drain = Input
Gate = Gate

Current flows from the Drain to the Source afaik. (When the gate is (ON))
Can also flow backwards once the gate is on, however, the body diode will conduct when the gate goes off, leading to rapid heating.

[ivanlee05]

Originally Posted by Pyr0Beast

?
Source = Output
Drain = Input
Gate = Gate

Current flows from the Drain to the Source afaik. (When the gate is (ON))
Can also flow backwards once the gate is on, however, the body diode will conduct when the gate goes off, leading to rapid heating.

thanks sir for the explanation sir... i have a motherboard with no power here, i already check every parts like processor, memory and psu, they are good.. i'm suspecting dead vrm...

[Pyr0Beast]

Check the output voltage then.
(With the component in place of course)

[ivanlee05]

I am having problem with my neighbor's PC... His motherboard (Biostar GF7050V-M7 SE) could not boot... It has a power but theres no display or even beeps... I already check all of the VRM mounted in the Motherboard, all does have output voltage that range more or less what the input voltage is...

My first tought was a Failed BIOS updates... But He doesn't even know how identify a computer parts... I still had to recover the BIOS thru AMIBOOT.rom BootBlock Recovery... No, the problem is not the BIOS...



Other components like ram, psu, hdd, odd and the processor has been verified in my PC except for the video card... He doesn't have any, He uses the Onboard Video...

Thanks for those who will reply...

[ivanlee05]

I think this is a chipset problem that I could not repair anymore... I went to test all the electrolytic capacitor, and all does have more or less same voltage as input voltage...

I have no time to check all the surfaced mount resistors, diode and capacitors... I'll just force him to buy another Motherboard...

His motherboard served him well for almost 3years...

R.I.P. Biostar "RedFox" GF7050V-M7 SE

Bought: 09/10/07
Died: 01/29/10

^_ _^

[Pyr0Beast]

Replace the board. Simplest and effective.

[Zero Hour]

I suggest sir as a last ditch effort before retiring it completely if it's not too late, baking the board. I have the problem as a common issue with geforce 6100/6150 chipsets. It's the closest I can get easily to a 'bga reflow' since I don't have a heat gun or a torch or patience. I know it's not a 'convenitonal' method, but for a board you've attempted everything else on and are set on pitching it, it can't hurt.

My method, strip the board of any heatsinks and battery and cmos chips if possible, remove all thermal paste and thermal pads, preheat oven to 384F, elevate the motherboard off of a pan (i ball up aluminum foil and leave a little pointy end for 4 mounting holes to support the board) and bake for 5 minutes. I don't suggest pushing past 6 minutes, it's never been beneficial in my cases and stinks like hell >.< But so long as you can vent the house afterward hey it's fun :P

Or if nothing else, I'd pay shipping on the board if you don't wanna go through the trouble of baking it

[cmosbios]

well you can use hot air rework station for reflow.

[cmosbios]

@Akor
you mean if I test the VRM, test each mosfet(VR) inputs and outputs? I am not sure if you are referring to VRM block (2 mosfets, ic driver, pwm) input and output circuit tests. what i know about the VRM is NOT the mosfet itself but the circuit. please enlighten me.

[styxbound]

Sorry, I'm old and senile. Can anyone simplify for me how to find the inputs and outputs on vrms? There seem to be any number of fets around the cpu on a lot of boards, and I can't figure out where the in and out is on the circuit.

[Pyr0Beast]

Ouput is past the choke on output capacitors
Input on input capacitors