Broken Pentium 2 Motherboard

[momaka]

Re: Broken Pentium 2 Motherboard

Okay, transistor 6... the 1802... the only data sheet that I think matches what we have is 2SD1802. It's an NPN BJT, not a MOSFET. Regardless, it just doesn't make sense to me why there is 0V on both the tab (which is the Collector, or position A as we called it) and the leg (the Emitter, or position B). A larger transistor like this is always used for handling power to major components, and in such a configuration, it's leg wouldn't be connected to ground.

Let me think about this one for a little bit till tomorrow. Meanwhile, can you check if the tab is connected to any voltage rail on the PSU? To do this, disconnect and remove the power supply, and see if any of the pins in the ATX connector show zero Ohms resistance with the tab on the 1802 transistor. Also see if the middle pin of the fan hearder right next to the 1802 BJT shows low resistance to the tab.

At this point, I'm trying to determine what that transistor does. The way it has zero volts just doesn't seem right. You did measure the voltages on that transistor with the computer turned on, right?

[canadaboy25]

Re: Broken Pentium 2 Motherboard

Meanwhile, can you check if the tab is connected to any voltage rail on the PSU? To do this, disconnect and remove the power supply, and see if any of the pins in the ATX connector show zero Ohms resistance with the tab on the 1802 transistor. Also see if the middle pin of the fan hearder right next to the 1802 BJT shows low resistance to the tab.

The tab on the transistor is not connected to any rail on the psu.

There is no continuity between the middle pin on the fan header and the tab of the transistor.

You did measure the voltages on that transistor with the computer turned on, right?

Yes, all of the voltage measurements were done with the computer on.

[momaka]

Re: Broken Pentium 2 Motherboard

The tab on the transistor is not connected to any rail on the psu.

There is no continuity between the middle pin on the fan header and the tab of the transistor.

Looks like another brain fart on my part.
What I should have had you check is if the tab is connected to one of the two outer pins on the fan connector. From the pictures, it looks like it should be as I see a trace running there.

If it is, then I need not worry about it more. But if it isn't, then I really don't know what other purpose it is serving... and being this big, it's definitely so that it can handle a fair ammount of power - in which case, 0V on both Collector and Emitter probably isn't normal.

But anyways.

I did some resistance measurements on an old Pentium II 440BX motherboard of mine (Zida BXi98-ATX) just to get more familiar with how the power runs. The layout is different, but the design is pretty much the same. Looks like transistor 5 is for the North Bridge. But I can't confirm if that voltage is right for it or not (the one on pin 3) as I would need a working Pentium II board for that.

I know I have a Pentium II CPU somewhere deep in my closet along with another Pentium II motherboard. I'll try to pull it out either this or next weekend to do some voltage measurements on it so I can confirm the above with transistor 5.

Other than that, I don't know what else it could be. Last week, I was working on a Bestec'ed motherboard (the Bestec PSU went over-voltage on 5VSB and killed a few components) and wasn't able to revive it, despite finding the components that went bad and fixing the problem. So it could be that the over-voltage from the PSU did take your motherboard, too.

[canadaboy25]

Re: Broken Pentium 2 Motherboard

Looks like another brain fart on my part.

No problem, I have had my fair share of those too.

What I should have had you check is if the tab is connected to one of the two outer pins on the fan connector. From the pictures, it looks like it should be as I see a trace running there.

Yes the pin on the fan connector closest to the transistor has continuity to the tab.

So it could be that the over-voltage from the PSU did take your motherboard, too.

Well I hope that is not the case.

[canadaboy25]

Re: Broken Pentium 2 Motherboard

So I assume that it is no good and I should scrap it or should I hold on to it then?

[ratdude747]

Re: Broken Pentium 2 Motherboard

So I assume that it is no good and I should scrap it or should I hold on to it then?

Scrap it.

[goontron]

Re: Broken Pentium 2 Motherboard

its scrap mate!

[momaka]

Re: Broken Pentium 2 Motherboard

Yes the pin on the fan connector closest to the transistor has continuity to the tab.

That transistor controls the AUX fan voltage then. Very rare for an old motherboard to have that. But not non-existant as this proves.
Therefore, that means there is nothing wrong with voltages on that transistor (unfortunately).

So I assume that it is no good and I should scrap it or should I hold on to it then?

Well, I hate giving up, but at the same time I don't want to waste your time by making you try more things that likely won't solve the problem.

So at this point, the motherboard might as well be considered dead. Should you want to further troubleshoot it, it would be purely experimental (call it a "rainy-day", "nothing better to do" project, if you like). The only things really left to try are: get a POST card and check if the motherboard is spitting out any codes, re-flash the BIOS externally (either buy a new BIOS chip or reprogram the old one with a home-brew serial/parallel port programmer), or change the Southbridge and Northbridge from another Pentium II motherboard. But let me point out - note of these are even close to being trivial (the last one requires at least some experience with BGA work - which requires a lot more specialized equipment and expertiese). And there really is no guarantee the problem will be solved by these either.

If you do decide to scrap the motherboard, at least pull your new caps from it. I usually save the whole board as sometimes I get other dead boards that need parts (MOSFETs, transistors, ceramic caps, etc.). I fixed 3 monitors alone by salvaging ceramic caps from dead PC hardware.

[canadaboy25]

Re: Broken Pentium 2 Motherboard

Well I quite enjoy testing my skills on random things and also hate giving up.

I just spent the week getting my 1996 asi 9000 pentium-s running with windows 95 lol so I really dont have anything better to do.

If you have anymore things to try please post them.

[momaka]

Re: Broken Pentium 2 Motherboard

If you have anymore things to try please post them.

Start with the post card, I suppose. See if it spits out any codes (I doubt it will, though).

Next step after that is the BIOS programmer and re-flashing the BIOS. But the BIOS programmer is a project in itself.

I think you will have the best luck with finding a non-working donor motherboard that has the same Southbridge chipset and then transferring that onto your board (although let me note that if you do find a donor board with such a chipset, chances are it will work or will be easier to fix than what you have). But, this requires quite a bit to do. You will need at the very least:
hot air station
electric stove
type-k temeprature probe
flux
solder balls
reball jig and stencils (unless you want to do the reball manually ball by ball - good luck and kudos if you do!)
sharp tweezers (or seweing needle and a very steady hand)
LOTS of patience

So as you can see, it takes quite a bit to do that last item.

I just spent the week getting my 1996 asi 9000 pentium-s running with windows 95 lol so I really dont have anything better to do.

Sounds similar to my weekend .
I swapped some dead BGA RAM chips on a 7600 GT video card several weeks ago, and yesterday I was toying with the RAM voltages a little bit. Finally got the video card to work! Still artifacts a little in Windows (and a lot in BIOS)... but it works well enough as a test card.
^ and all of that work was needed because some crappy caps failed in the RAM VRM circuit and killed the original RAM chips.