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-   -   Strange resistance readings on Radeon RX6800 VRM (https://www.badcaps.net/forum/showthread.php?t=98915)

dicky96 09-11-2021 04:28 AM

Strange resistance readings on Radeon RX6800 VRM
 
Hi guys
I can't quite get my head around the resistance readings on this GPU

On phase of Vcore VRM reads higher resistance than the others

This 3 mins video shows exactly what I mean
https://www.youtube.com/watch?v=NZR4otU77VE

I can't think any way this could happen unelss one phase is not directly connected to the others, but is conetced via a low value resistance

Can someone unconfuse me please

Cheers

momaka 09-12-2021 10:07 AM

Re: Strange resistance readings on Radeon RX6800 VRM
 
TBH, I'm not too familiar with these newer gen GPUs, so perhaps it is possible that this one phase is not connected to the others. Have you traced the MOSFETs for that phase to see if they are exactly connected like the MOSFETs on the other phases (and to the same controller)? I think that might be the first clue in telling if that's part of the GPU VRM or some secondary GPU VRM. I do agree that it's strange that only this one coil reads higher resistance, though. I'd expect to see at least another phase in parallel with it, if it was to be some secondary GPU VRM rail. On the other hand, if all of these phases were to be connected together, then the only way that higher resistance would be there is if the copper trace from the coil to the GPU chip has burned and gone high resistance... which I think would be highly unlikely.

dicky96 09-12-2021 10:51 AM

Re: Strange resistance readings on Radeon RX6800 VRM
 
Hi Momaka
Thanks. It shouldn't be too difficult to see if this phase goes to the same controller as the other phases, for example if there is an 8 phase VRM controller

But I couldn't find a datasheet for the coil driver IC either so that does not exactly help :(

Because this one phase still has a very low resistance to ground it also seems odd to me that it would be a singleton as you mentioned.

I could just try powering the card up and measure the voltage on this and other inductors on the VRM - but I really would like to understand what I am looking at first.

dicky96 09-17-2021 04:51 AM

Re: Strange resistance readings on Radeon RX6800 VRM
 
OK I pretty much got to the bottom of this - at least I have a plausible theory for the odd resistance readings on the VRM

The fault on this card appears to be elsewhere

It's easier to watch than for me to explain in detail what I worked out

https://www.youtube.com/watch?v=ooefNfC8Lxg

Rich


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