Replacing OST caps on Microstar KT6 Delta motherboard

[Agent24]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Might even temporarily just use the old broken choke, as i'm thinking that it should still work even if a section of the bottom of the ring is missing.

I wouldn't, in that application it could throw the circuit stability out of whack and cause more problems.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Yes, i definitley did short G to S
(at the time of typing that, i was thinking of the schematic, rather than the physical pins, and got mixed up - sorry)

Thanks

Will be on to this some time next week
Might even temporarily just use the old broken choke - as i'm thinking that it should still work, even if a section of the bottom of the ferrite? ring is missing.

[momaka]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Thankyou for that
i looked at that surface-mount inductor, and it looked quite different - so was skeptical.
But now i feel confident, and combined with your last statement, inspired, that is OK - unfortunately minimum order is 77

Lol, now that's a downer.
I'll look through my box-o-coils and PM you if I find any suitable ones. Maybe easier if I just mail you a few .

OK,
i shorted the gate to drain, and they all return a reading from 0.5v to 0.6v for "forward voltage drop of mosfet body diode" (voice in mosfet testing vid) - which is similar to the other two.

Sounds reasonable.
In these tests, I suggest shorting G to S, though, and not D. Shorting G to D may not always discharge the Gate properly.

Does that indicate any cause for concern?
i now notice that there is a consistant difference between both couples of mosfets of about 0.04v, and perhaps that is due to the gate still being soldered to the board?

Yes, quite possible.

i'm kinda getting the feeling that these mosfets are testing OK

Well, I do recall your motherboard was turning ON (or at least running the fans and the PSU, that is). If a MOSFET was shorted, that would usually shut down the PSU as well.
I have spare MOSFETs as well, though they are the smaller TO-252 ones and not TO-263.

[Agent24]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

It still doesn't hurt to have spare parts, especially if getting more quickly will be inconvenient or pricey. When I buy parts I typically buy more than I need.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

I guess it is better to grab a few spare MOSFETs than to ponder whether the ones on the board are currently shorted or not.

The mosfets are not shorted

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Thankyou for that
i looked at that surface-mount inductor, and it looked quite different - so was skeptical.
But now i feel confident, and combined with your last statement, inspired, that is OK - unfortunately minimum order is 77

Sorry for those mosfet readings - must have being getting a bit tired when doing that.
It wasn't ohms, and also wasn't the diode reverse bias - i was talking shit

OK,
i shorted the gate to drain, and they all return a reading from 0.5v to 0.6v for "forward voltage drop of mosfet body diode" (voice in mosfet testing vid) - which is similar to the other two.

What i initially should have said, was that after turning the mosfets on (+ve to G and -ve to S), and then returning the +ve lead to D, i got the following readings:
0.084v
and 0.039v
The other two (which, when the pins are soldered to the mobo, would have their drain connected to the input caps) give readings of around
0.509v
and 0.549v

Does that indicate any cause for concern?
i now notice that there is a consistant difference between both couples of mosfets of about 0.04v, and perhaps that is due to the gate still being soldered to the board?

i'm kinda getting the feeling that these mosfets are testing OK

[momaka]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Those readings were taken with the mosfet legs desoldered.
They both switch on and off OK using the multimeter diode test mode.
But when i tested the mosfet diode reverse bias, those were the reading that i got - so was suspicious, because one was double the value of the other one, and the other two mosfets (15N03L) in the VRM both give reading of around 0.5V

Did you discharge the Gate before testing the MOSFET diode? If shorting all pins with your finger doesn't do it, try shorting Gate to Source with a piece of metal, then try measuring again.

That was the right link.
It was provided it to show where i was looking for the choke and that there was none that had a similar inductance/current rating, and that perhaps i had interpreted the inductance wrongly.

Well, the schematic calls for 1.1 uH, so you did interpret it right.
Given that most inductors have 20% tolerance, going up a notch to 1.2 uH shouldn't be a problem at all.

Have provided a photo of the choke
It has 5 groups of 3 windings

The 3 wires are all grouped together to form one thicker wire. Looks like a 5 turn inductor.

i don't have any other broken motherboards with similar chokes
Just a gigabyte board with different sized chokes and windings (2nd thumbnail)

The chokes on the Gigabyte look like 5-turn as well. If the choke toroid is the same size (well, more or less), this may work, especially since that board looks like it's from the same era.
But IMO, that's more of a crapshoot than using the 1.2 uH coils. Plus, if you are buying the replacement MOSFETs, you might as well grab two of those coils (and I say two so that you can replace both on your motherboard, just in case something doesn't work out).

Thought that they would be reasonably common - but am now suspecting that they would have being custom made.

It may or may not be custom made. However, I often find that many motherboard manufacturers usually make one VRM design, and then they use it on almost every motherboard that they make in that given era. So it's very unlikely that the design on your specific motherboard model is unique. Most likely, other Microstar motherboards from that era use the same coils.

Short of using that surface-mount choke, would it be better just to put this board away until i find a broken motherboard with a matching choke?

It's unlikely that you will find a perfect-matching choke (actually, chokes may sometimes look a bit different, but they could still have the same inductance and current handling capability). The surface-mount choke should work... the motherboard just won't look quite "factory made". But there's beauty in that too .

[Agent24]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Was surprised that there was no chokes anywhere near the stated 1.1uH value - which usually means that i'm doing something wrong.
Thought that they would be reasonably common - but am now suspecting that they would have being custom made

Most likely, very often they are custom made.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Thanks

Those readings were taken with the mosfet legs desoldered.
They both switch on and off OK using the multimeter diode test mode.
But when i tested the mosfet diode reverse bias, those were the reading that i got - so was suspicious, because one was double the value of the other one, and the other two mosfets (15N03L) in the VRM both give reading of around 0.5V

That was the right link.
It was provided it to show where i was looking for the choke and that there was none that had a similar inductance/current rating, and that perhaps i had interpreted the inductance wrongly.
Shirt outa luck HERE also

Have provided a photo of the choke
It has 5 groups of 3 windings

i don't have any other broken motherboards with similar chokes
Just a gigabyte board with different sized chokes and windings (2nd thumbnail)

Was surprised that there was no chokes anywhere near the stated 1.1uH value - which usually means that i'm doing something wrong.
Thought that they would be reasonably common - but am now suspecting that they would have being custom made

Short of using that surface-mount choke, would it be better just to put this board away until i find a broken motherboard with a matching choke?

[momaka]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Can someone please help me find a replacement output choke and a couple of mosfets since i'm getting ready to make an order for parts

(reason for replacing these parts is that i broke the choke and the mosfets have diode readings of 0.034v and 0.072v, on diode test mode)

If those readings are taken with the MOSFETs on the board, you probably have something else affecting them as these don't look very meaningful. But anyways, I guess it is better to grab a few spare MOSFETs than to ponder whether the ones on the board are currently shorted or not.

i can't find any replacement choke
HERE, that is 1.1uF / 25A
(although there is lots of toroidal inductors that look like it)

The choke is labelled on the vrm schematic as 1.1uH / 25A

^Wrong link?
Whatever you linked to does not list 1.1 uH, 25A choke. In fact, the lowest I see for through-hole inductors is 5.6 uH, which is a bit too high. SMD does have 1.1uH, but the current is way too low. However, I did find an SMD inductor rated for 1.2 uH and 25.4A here:
http://nz.element14.com/bourns-jw-mi...-20/dp/1180547
That should work, though, you may have to solder leads to it (preferably very thick ones) to attach it to the board (actually, it would probably be easier to solder the thick leads to the board and then the inductor to them). Kind of ghetto, but should work.

Also, I suggest you count the number of turns of wire on the good inductor and post it here. Should help with finding a proper one easier... or worst case scenario, use that info to pull something from another dead motherboard. Whenever you see inductors on PC hardware that might say something like "aRb", that means a.b uH (where "a" and "b" are single-digit numbers). Similarly, "Rab" translates to 0.ab uH.

Or worst worst case scenario - run the board with just half the VRM phases - i.e. with the broken choke (and associated MOSFETs) removed from the board. But a high power CPU could overload the other working VRM phase... so yeah, best saved as a last resort.

Both mosfets have this printed on them:
D432AN
FDB
6670AL
i think that it is a fairchild mosfet

So, would it be OK to use THIS,
as, in the component description, there is a note saying that this is a replacement for it.

^Yup, that's a Fairchild MOSFET. Part number is FDB6670AL. The MOSFET you linked to is a bit under-specced on the DC continuous current, but it has a higher pulse current, and same Rds(on) resistance, so it should work just fine. However, do note that what you linked to is a TO-263-3 case and not simply a TO-263. That means, the middle pin on the MOSFET is not cut off, so you probably will have to cut it yourself. But that shouldn't be a problem.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Can someone please help me find a replacement output choke and a couple of mosfets since i'm getting ready to make an order for parts

(reason for replacing these parts is that i broke the choke and the mosfets have diode readings of 0.034v and 0.072v, on diode test mode)

i can't find any replacement choke
HERE, that is 1.1uF / 25A
(although there is lots of toroidal inductors that look like it)

The choke is labelled on the vrm schematic as 1.1uH / 25A

Both mosfets have this printed on them:
D432AN
FDB
6670AL
i think that it is a fairchild mosfet

So, would it be OK to use THIS,
as, in the component description, there is a note saying that this is a replacement for it.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

OK,
will be using panasonic FM range,
and will revive this thread when it happens

but after destroying the other board, the priority now is sorting out a couple of decent PSU's

[momaka]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Try the recap still.

Worst case scenario is that even if nothing works and you if don't get the board running no matter what, you can still pull your new caps out and put them on a new board or some other device that needs new caps.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

thanks for the reply,

OK,
The CPU is removed, and still hard short to ground (0 ohms)

[momaka]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

To keep things simple, let's do this one step at a time...

Have the bulged Ost caps around the CPU been replaced already? If not, replace them along with any other suspicious Ost and Teapo caps and then test the motherboard.

If it works, good. If not, then we go back to testing. I see the CPU VRM is spitting out about 1.6V, so it's probably working fine.

As far as the shorted ceramic cap you removed – don’t bother with a replacement if the traces on the board are burned. Just clean up the area as much as possible to remove any short circuit paths (carbon dust is a good conductor, so if the PCB is burned, scrape away as much as you can with a small screw driver or some other small sharp tool).

CPU VRM is a bit tricky to test, but nothing too complicated. Yes, it's normal to have half of the MOSFETs have their Sources connected to ground and their Drains connected to the inductors (i.e. CPU V_core rail). These are usually referred as the "lower" MOSFETs. The "upper" MOSFETs have their Sources connected to the inductors and their Drains connected to 12V.

With a CPU in the board, the lower MOSFETs will almost always appear as shorted, so this is normal. Without CPU, you should get a fairly high resistance (few hundred of Ohms or higher) or open circuit. However, some motherboards (very few actually) also have their Northbridge draw power from the CPU V_core rail, so even with the CPU removed, you may still get a low resistance reading. Looking at the pictures for this board, I don't see a separate buck circuit for the Northbridge, so it is possible this is one of those few boards that powers its Northbridge from the CPU V_core rail. Therefore, don't be alarmed if the "lower" MOSFETs appear shorted even with the CPU removed.

[Agent24]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Yeah I didn't think it was likely to, but it's one way to help narrow down where a short might be.

You will probably have to remove it instead then

This is assuming of course that your board uses the exact same schematic as that reference one you posted.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

no difference in readings

can't see how a choke such as this would introduce any noticeable DC resistance - pretty thick copper wire.

did post a good inline picture but it vanished, and there is only a 30kb one when the thumbnail is clicked on - which is way too small

will try again now

[Agent24]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Driver could be shorted, or something that rail is powering. What is the DC resistance of the choke itself? You may be able to find which side of the choke the short is on by measuring both sides of the choke with respect to ground, and the side with the lower reading will be the side with the short (since the other side will be measuring with the choke in series with the short)

Depends on the choke resistance and the sensitivity of your meter, though.

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Ok
Trying this:

There is a contact to ground ("0 ohm", in the picture below), that i'm thinking should not be present.

At the bottom of this post is a diagram of the current situation.
i have erased the parts of the circuit that are disconnected (at the fets) and where components have being removed (the 6v caps and the choke)

Can someone please confirm that the problem has to be a faulty HIP6602 driver earthing when it shouldn't,
(downloaded the diagram of the driver, and note a couple of spots where it could possibly short circuit)

or back beyond the driver, towards the PWM,

or a fault into the vcore terminal (on the right side of the diagram).

Should i remove the choke to prove which way the fault is?

(i broke the other choke, trying to wiggle it out - so, discovered that they are reasonably easy to break)


(had to post this jpg inline, because the gif that i attached got re-sized too small)

[socketa]

Re: Replacing OST caps on Microstar KT6 Delta motherboard

Test results:

Removed all 6.3v caps, and un-soldered the D&S legs of the 4 mosfets

Tested the (removed) 6.3 caps on ohms X 100 - needle rose, then slowly fell right back down - indicating charging up, then eventually no current flow.


Mosfets test, DMM readings (as in the quick & dirty mosfet testing vid)
All of them turn on and off:

These two mosfets switch current to chokes
Q28 = 0.044v
Q30 = 0.021v

These two mosfets (different types from the above) had source connected to ground
Q27 = 0.478
Q29 = 0.514

all mosfets have diode reading of about 0.5V

with the 12v input caps left in (not yet removed) , the meter shows a partial short across their rails,
so that seems to indicate a fault on the input side, because the mosfet drains are physically disconnected, and the only path for current reach earth, is via one of those caps.

am wondering if that other little 4.7uf ceramic is on it's way out (even though it looks OK), since it is reasonably close to the other one that got fried.