Strange resistance readings on Radeon RX6800 VRM

**dicky96** · 09-11-2021, 04:28 AM

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

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

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

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

09-11-2021, 04:28 AM	#1
dicky96 Sun Seeker Join Date: Mar 2017 City & State: Sunny Gran Canaria My Country: Spain Line Voltage: 220V 50Hz I'm a: Professional Tech Posts: 1,399	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 __________________ Follow me on YouTube ------------------ Learn Electronics Repair https://www.youtube.com/channel/UCFX...R8UZ2vg/videos

09-12-2021, 10:07 AM	#2
momaka master hoarder Join Date: May 2008 City & State: VA (NoVA) My Country: U.S.A. Line Voltage: 120 VAC, 60 Hz I'm a: Hobbyist Tech Posts: 11,149	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.

09-12-2021, 10:51 AM	#3
dicky96 Sun Seeker Join Date: Mar 2017 City & State: Sunny Gran Canaria My Country: Spain Line Voltage: 220V 50Hz I'm a: Professional Tech Posts: 1,399	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. Last edited by dicky96; 09-12-2021 at 10:55 AM..

09-17-2021, 04:51 AM	#4
dicky96 Sun Seeker Join Date: Mar 2017 City & State: Sunny Gran Canaria My Country: Spain Line Voltage: 220V 50Hz I'm a: Professional Tech Posts: 1,399	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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