MP 1,1-5,1 Odd power issues on cMP 5.1 (flashed)

[terminatermule]

Hi all,

Hoping for a bit of help with my cMP 4,1/5,1.

This machine has been running fine for years, in its current config. Suddenly it started shutting down when loading up the GPU heavily. After much investigation, I have found that my AUX power connector A is the cause of the issues. Afterburner shows that it triggers a shutdown at ~130 watts (total draw including slot power) whereas if I now use AUX B I can crank the poor card for all its worth to 150 watts total combined draw without any issues at all. And I do mean ANY, that is to say, this may have been a long-festering issue as I have always struggled to OC this card at all, and now on the alternative AUX power plug, it runs great at speeds it could never have dreamt of before..!!

So the question is, what can cause this?

The power supply itself can supply the current and then some, I even removed a CPU (and swapped them for one another) in testing and that liberated quite a bit of power (that I shouldn't be struggling with regardless) and of course, it made no difference, but at that stage, I hadn't cottoned on to what was happening.

I suspect that the current sense circuit for AUX A is wonky, has anyone experienced this before on the 4.1/5.1 cMP? Does anyone have any info or experience diagnosing and repairing this circuit? I am immediately suspect of the 5L0 current sense resistor used on these two circuits but would like to hear input from anyone that has experience with this before I get the scope out and start probing this thing....

Cheers Glenn

 

[krakman]

There could be a whole myriad of possibilities, if it is one of the traces in the motherboard then you will have to replace the motherboard.

Another alternative is to get a EVGA power link adaptor which takes power from both AUX A+B and feeds it to the GPU - there is a whole topic about this on the forum: https://forums.macrumors.com/threads/evga-powerlink-pin-connectors.2197274/

or use dual mini 6 pin to 8pin cable.

The simple solution is if AUX B works then just use AUX B, no????

 

[MIKX]

Information about the EVGa PowerLink

EVGA PowerLink with AMD GPUs

Just got the EVGA PowerLink working with my Sapphire Dual-X 7970 3gb. IMPORTANT : The power input sockets on the PowerLink are 2 X 8 Pin female and cannot be changed. The EVGA PowerLink ensures that exact power demanded by a GPU is supplied. That said, one must still supply enough power to...

forums.macrumors.com

You should use a good light and a large magnifying glass to ( shut down first ) de-dust around the Mini 6 pin sockets with a soft non-conductive brush to ensure that you have no knocked anything off the backplane PCB near the sockets . .more common than you think.

 

[terminatermule]

Hi guys,
Thanks for the prompt replies.

Thanks for the advice, I should have mentioned that I have already ordered an EVGA Powerlink, but I see this as an aid or bandaid, not a fix. Same with the cable, neither is a fix to the original issue.

I will inspect the traces again with the machine outside on the bench tomorrow, however, nothing has moved/changed in the machine to be able to cause physical damage to the PCB. I will clean the entire machine out and inspect, and report back if I can see anything at all. Dust seems minimal, however, I do have some fur from my Husky floating around, but nothing alarming and mainly only a few strands on the fans.

Given that both AUX connectors are supplied by the same 12 rails that feed everything else I can safely rule out the power supply I would suggest. I think I may crack the probes out of retirement and measure the output of all of the pins at the mini 6 pin header on AUX A under load to ensure that there isn't a bad trace/supply issue, if the voltage holds steady then I know it is a sensing issue, not a supply issue, which is where my gut tells me this problem lies.

Cheers Glenn

 

[MIKX]

More than a few people have 'scraped off' tiny motherboard components in the act of removing GPU cables from the Mini 6 pin sockets.

 

[terminatermule]

Hello everyone,

I purchased HW Monitor today, I'm sure I've already purchased that in the past but oh well.

Anyway, with its excellent monitoring of the Mac' sensor data, I was able to confirm that there isn't excessive current draw on any port, and the same draw between the two AUX ports when swapped back and forth. Thorough cleaning and blow out didn't get me anywhere either. I measured the two current sense resistors in circuit and they measured the same, but I went the extra step of bypassing the AUX A resistor and checking all the readings to ensure that they decreased the measured draw, which it did. However even with the A port only showing a draw of ~10 watts and the PCIe slot itself showing about 46 (real wattage from the AUX A port was actually about 75 watts) the machine would still trip into safety shutdown if pushed to a combined draw of about 140 watts or just under, which equals about 110 on the AUX port.

I should note that I can only get that much draw in Windows as the GPU stress tests available on Mac OS can't push the card hard enough to trigger a shutdown, neither do/can DX11 games seemingly, Vulkan and DX12 though can do it easily.

Starting to look like I may not be able to get a permanent repair of the existing circuit, which, is quite disappointing.

I should also mention that I carefully inspected the circuits for matching sets of components and can confirm that nothing is missing/knocked off.

Cheers Glenn