Sapphire 3870PCIe no video signal

[alindumitru46]

A new video card problems: Sapphire 3870PCIe.
The fan spins, the pcb heats gently in the GPU zone, but the board did not give video signal.
As I learned something now, I went to check MOSFET empirically, as @momaka advised me here: https://www.badcaps.net/forum/showpo...41&postcount=7
Surprise 1: exactly the same type MOSFET: AO D452 and AO D472 ; a good thing because I still have 4 or 5 pieces.
Surprise 2: exactly the first MOSFET is in short circuit: AO D452.
Specifically all AO D452 mosfets give continuity to the measurement between Source and Drain pins.
The problem was solved by replacing the previous one mosfet D452, the first.
What to do now, to do the same?

LE:
Figure 2 - Correction position of mosfet.
I measured an identical videocard but AGP slot: the same MOSFET with the difference that the group of eight chokes are grouped opposite!
AO D452 mosfets show continuity on AGP card, although the board works perfectly !!!

[momaka]

Re: Sapphire 3870PCIe no video signal

...
Specifically all AO D452 mosfets give continuity to the measurement between Source and Drain pins.
...
I measured an identical videocard but AGP slot: the same MOSFET with the difference that the group of eight chokes are grouped opposite!
AO D452 mosfets show continuity on AGP card, although the board works perfectly !!!

And now you are about to learn something new again .

Like I mentioned in the other thread (but probably in too complicated language), when you measure resistance or continuity across MOSFETs, some of them will be in parallel with GPU. Therefore, when you measure them, you are also measuring the resistance of GPU. And GPU resistance is always low - typically 1-10 Ohms. So your multimeter will beep and show a short-circuit for the MOSFETs in parallel, even when there is none.

On your board, it looks like the AO D472 MOSFETs are across the GPU (i.e. between GPU V_core and ground), whereas MOSFETs AO D452 are between 12V rail and GPU (i.e. 12V rail and GPU V_core).
Are you sure that the AO D452 show continuity between Source and Drain, and not the AO D472 MOSFETs?

The problem was solved by replacing the previous one mosfet D452, the first.
What to do now, to do the same?

No.
First verify the problem. Check resistance between ground and 12V rail pins at the 6-pin PCI-E power connector. Also see if that "R10" round inductor has continuity across its leads.

The fan spins, the pcb heats gently in the GPU zone, but the board did not give video signal.

And here is a sign that this is likely NOT a MOSFET problem. A shorted MOSFET will usually make the power supply trip and turn OFF, like it did on that Leadtek 7300 GT after you had the burned coil replaced, but had not yet replaced the bad MOSFET.

I will let you confirm the above questions I have before we move on, though.

In general, when the video card turns ON but there is nothing on the screen, this is usually a BGA issue - in other words, the solder under the GPU chip or the RAM has gone bad. The fix is to reflow the solder on the GPU.

By the way, I have two Radeon HD 3870 video cards as well, but their MOSFETs are in a different package from yours, so they may not be connected the same way as yours. But if you need working voltages for the GPU and RAM, I will be able to provide those as my cards are both working.

[alindumitru46]

Re: Sapphire 3870PCIe no video signal

There is no rush toward the end of the week, when I get to deal with the issue.
Now is Christmas, it's normal to have other concerns.
Happy holidays to all and especially to my mentor's, @momaka.

[momaka]

Re: Sapphire 3870PCIe no video signal

Happy holidays to you and your family, friends, and relatives as well!

[SuperDuty]

Re: Sapphire 3870PCIe no video signal

Mmh, everytime there was a shorted MOSFET on on of my video cards, the computer wouldn't boot at all. Just a half second spin of the fans. It actually froze the power supply every single time.

[alindumitru46]

Re: Sapphire 3870PCIe no video signal

I have to make a statement because I was not able to properly define*the situation right here:"The problem was solved by replacing the previous one mosfet D452, the first".
I have not made any intervention on board Sapphire, I referred only to the first mosfet replaced on Leadtek board . I have tried to anticipate the solution: replacement of the first mosfet, as in that case.
Sorry I was not clear enough.

[alindumitru46]

Re: Sapphire 3870PCIe no video signal

You guessed right @momaka, d472 is showing continuity!
My mistake, although I edited the post to the correct position MOSFET, I forgot to correct the text.
Also, R10 round inductor has continuity. Cannot measure the resistance between ground and 12V rail pins at the 6-pin PCI-E power connector because the values is in continuous change. I think capacitor is charged and I fail to empty them...