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  #1  
Old 12-04-2019, 10:03 PM
Yumi Yumi is offline
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Default RM850x GPU compatibility? + other questions

I've been running a Cooler Master RealPower 550W power supply in my PC, which I just replaced with a new Corsair RM850x, but am having a few issues.

When I first plugged everything in, the system would power on but not boot. My GPU (Sapphire RX570) needs an 8-pin connector but has an optional 6-pin, both of which I had connected. I tried disconnecting the 6-pin and the system booted fine.

The next day, the system wouldn't boot. If I disconnect the 8-pin on the GPU, the system does boot, but there's no video output (obviously). Plug the 8-pin back in, and the same thing occurs - power but no boot.
I've tried unplugging harddrives, as well as plugging the 8-pin PCIe port (PSU side) into a different modular slot, but it doesn't make a difference - if the GPU is powered, it won't boot. I've even tried using a 4-pin molex in the PSU, with a 2x molex to 8-pin PCIe adapter (which I had to use on the old PSU because it didn't have 8-pin PCIe power), and it didn't work.

To see whether the GPU has had something odd happen, I plugged the old PSU back in, and the system boots with the GPU powered on.
So, it seems that the RM850x doesn't like the GPU for some reason. I've never really heard of a PSU<->GPU incompatibility (other than not enough power or connectors), but does anyone know what could be the issue?

---

Also I noticed that when the system powers on/off, there's a click noise from the PSU. It's just a once-off and doesn't seem to be indicative of an issue, but I've never seen a PSU which does this, so am wondering: is this normal?

Another thing, when I tried turning off the PC on the first day, after powering down, it immediately turned itself on again. Repeated the process and confirmed that it isn't just a once-off thing. As such, I had to turn it off at the switch to prevent the PC auto powering itself on. Would the PSU be tripping on something, causing this behaviour?


Thanks for reading!
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  #2  
Old 12-05-2019, 12:53 AM
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When you swapped out the PSU, did you also swap out all of the cables?
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  #3  
Old 12-05-2019, 02:26 AM
Yumi Yumi is offline
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Quote:
Originally Posted by DevBiker View Post
When you swapped out the PSU, did you also swap out all of the cables?
Yes I did.
If it helps, the old PSU isn't modular, so there's no cable mixing of any sorts either. I haven't tried running the 2 PSUs at the same time (but may give it a go), so the tests above were all one PSU or the other.

Thanks for the response.
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Old 12-05-2019, 02:52 AM
jonnyguru jonnyguru is offline
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The click noise is normal. It's a relay in the PSU.

Is the 8-pin you're plugging into the graphics card the 6+2-pin? You're not accidently using the CPU power 4+4 power instead, are you?
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  #5  
Old 12-05-2019, 03:55 AM
Yumi Yumi is offline
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Quote:
Originally Posted by jonnyguru View Post
The click noise is normal. It's a relay in the PSU.
Ah, thanks for the info!

Quote:
Originally Posted by jonnyguru View Post
Is the 8-pin you're plugging into the graphics card the 6+2-pin? You're not accidently using the CPU power 4+4 power instead, are you?
Yes, it's the 6+2 pin, with PCI-E labelled on the side.
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  #6  
Old 12-05-2019, 07:27 AM
Yumi Yumi is offline
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Quote:
Originally Posted by Yumi View Post
Yes, it's the 6+2 pin, with PCI-E labelled on the side.
By the way, that's on the GPU side. I presume the 8-pin plugs on the PSU side are the same for both CPU and PCIe.

Did some more experimentation, with interesting results.

Plugged in a Radeon HD 6450 GPU, which doesn't require external power, in alongside with my main GPU. Interestingly, the system fails to boot even if the 8-pin is unplugged in the main GPU. If I pull the main GPU out, and run only the secondary GPU by itself, the system boots fine.
So it seems that the PSU doesn't like something when the main GPU is fully powered, or it's alongside a second GPU.
I did also try the GPUs in different slots, but that didn't make a difference.

I decided to try the 2x 4-pin molex -> 8-pin PCIe idea a bit more. After removing the secondary GPU, plugging this adapter into the 8-pin GPU in again, the system failed to boot, as expected. So I decided to plug the optional 6-pin in, and disconnect one of the molex plugs (so essentially, there's 1 out of 2 molex plugs connected to the adapter, which is plugged into the GPU's 8-pin socket).
This didn't work, so I tried switching the molex plugs around, and... it booted. I'm not sure how the adapter works when there's only one molex plugged in, so perhaps it acts like the 8-pin isn't plugged in?
I tried plugging the second molex plug into a different molex socket (on the same cable though), and it seems to have booted, surprisingly.

I have no clue what the issue is, but it seems, so far, that randomly re-plugging in the molex adapter into different sockets (all on the same cable, by the way) can make things work. Dunno if this thing persists though - will have to see next time I try booting the computer.

Once the system boots, it seems to work fine though.
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  #7  
Old 12-09-2019, 08:16 AM
Yumi Yumi is offline
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After trying it a few times, it's definitely not reliable.

I'm not sure if this was always the case, but I've noticed that it seems to be more likely to succeed after a quick power cycle. That is, it won't boot when you first power on the machine, but if you quickly flick the PSU switch off/on (so that it actually powers down), and then press the PC's power button right away, it generally boots up successfully.

My motherboard has a debug LED display which makes this easier to observe. In the won't-boot state, the display gets stuck on "00" (not documented in motherboard manual, but "01" is "power on / reset type detection" so presumably it's the state before that) since powering on. If it does boot, the first number I see on the display is "06" ("microcode loading") and it changes rapidly.

I still get the issue where the PC doesn't shutdown properly (PSU immediately turns the PC back on after shutdown).

Last edited by Yumi; 12-09-2019 at 08:19 AM.
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