Testing VRMs

[momaka]

Originally Posted by bianchi77

I removed CPU resistance to ground = 34.5 ohm...do you mean northbridge sharing the same Vcore ?

Yes. *Possibly*

Is this for the A7S333 motherboard from your other thread? Or a different motherboard?

Originally Posted by bianchi77

When I tried to measure one of the chip capacitor with my LCR45....I got value >20000uF

That's because of the low resistance of the circuit. It will make your meter think it is filling up a big cap.

That's why it's never reliable to measure anything in circuit, especially if you don't understand the circuit well.

Originally Posted by bianchi77

One test that I can do...is, unsolder all the capacitors about 16 small smd capacitors and test the resistance again....

Yes, do that.
This is by far the best way to isolate a short-circuit: keep removing parts until the short-circuit is gone. Unfortunately, this is also the hardest way. But the reward is the certainty you get.

[bianchi77]

Any thick traces going from CPU V_core to Northbridge? If yes, that means the NB is powered by CPU V_core as well, and that's what would be giving you the still-fairly-low resistance of 34.6 Ohms.

I didn't see any thick traces going from CPU V_core to NB....
I saw northbridge has its own FET....

so CPU is using V_core itself....

[budm]

One info that is still missing is the Output Voltage reading from the power supply. i myself never seen the Vcore pin of the CPU with that <10 Ohms.
Quote:
Originally Posted by bianchi77 View Post
When I tried to measure one of the chip capacitor with my LCR45....I got value >20000uF
That's because of the low resistance of the circuit. It will make your meter think it is filling up a big cap.

That's why it's never reliable to measure anything in circuit, especially if you don't understand the circuit well.

I was going to tell him to get something like 100uF cap in parallel with 100 Ohms resistor and have him test the capacitance to see what reading he will get to proof that reading cap in the circuit is never be reliable, but some people still insist on doing that.

[budm]

BTW, without the real diagram that matches the board you are working on then you really do not know what point 'A' is really connected to.

[bianchi77]

Originally Posted by budm

BTW, without the real diagram that matches the board you are working on then you really do not know what point 'A' is really connected to.

Where can we find Asus A7S333 motherboard schematic ? In there, we can see where are those capacitors located,
My thinking, they are V_core filter capacitor in each Vcc pins on CPU....

[bianchi77]

I got this one :
http://www.s-manuals.com/pdf/motherb...schematics.pdf

[momaka]

Originally Posted by budm

BTW, without the real diagram that matches the board you are working on then you really do not know what point 'A' is really connected to.

It's usually not that complicated for a *desktop* PC motherboard.

Typically the CPU VRM powers the... CPU (surprise, surprise ) and occasionally the NB on a few motherboards. The fact that bianchi77 got a low-resistance after removing the CPU strongly suggests that the NB is powered from the CPU VRM as well. I'm not saying that it can't be shorted ceramic capacitors, but it seems unlikely to me in this case, since the motherboard does turn ON for a few seconds. If any shorted ceramic capacitor was to take that much energy from the CPU VRM for a few seconds, it would have burned for sure.

Originally Posted by bianchi77

I didn't see any thick traces going from CPU V_core to NB....
I saw northbridge has its own FET....

Yes, the NB does have its own MOSFET, but your motherboard doesn't have a separate SB, so that means the NB and SB are both in one chip... hence you could have 2 or even 3 different voltage rails going to that chip.

On a typical motherboard with separate NB and SB, the SB usually has two rails: a stand-by rail (usually 3.3V or less) and an active-ON rail that comes ON only when the motherboard is turned ON (and it's usually 1.2-2.5V, depending on motherboard's vintage).