CGMINER - SCRYPT - Why does adding a third GPU kill my hashrate

[minerbird]

Hi all,

I am new to this game (but luving it already), and have made some good progress in putting a rig together, however I have a problem that I just cant seem to solve ... any suggestions would be most welcome.

I have a single motherboard MSI-B85M-G43 with 2GB RAM and Pentium G3220 CPU. The MB has 2 PCIe x 16 and 2 PCIe x 1 slots and I am running windows 7 64 bit and the latest AMD drivers dowbnloaded from the net. I use CGminer 3.7.2

My intention was to populate the MB with 4 Saphire R9 280x TOXIC GPUS.

The first GPU gave a steady 750 Kh/s
Adding a second GPU gave an additional 750 Kh/s for a total of 1500 Kh/s
Then came my problem, Adding a third GPU causes the rates on all three cards to drop to approx 580 Kh/s !!!!

I have tried everything I can think of including:
- Powered and un-powered risers
- Swapping PCI-e slots
- re-tuning CGminer settings
- Changing power supply (I have corsair 1050 Watt)

my miner settings are:

Intensity 13
Engine clock 1100
Memory 1500
Thread concurrency 8128
Worksize 256

No matter what I try I get the same results - the rig seems not to want to go above 580 per card with three in together ! 
Has anyone else had or heard of this sort of issue and find a reason for it ?






 

[foodies123]

tl;dr

either:
driver is outdated/badly installed
psu problems not enough watts.

[G K G]

I think its PSU issue. check the power usage at the wall first with 2 cards and then try again with 3 cards. maybe u'll need to undervolt it

[neflihim]

Could be PSU.  I hear those Toxics can be power hungry.  I have 2 MSI 7970 Twin Frozr and a MSI 280X GAMING on a 1050AX Corsair that is no problem though.  What is the brand of PSU?  Rating? 

Also, try undervolting a bit and then dropping the core clock down maybe to 1050 each.  I run all mine at 1040, anything over that I get less hashrate.

[minerbird]

Could be PSU.  I hear those Toxics can be power hungry.  I have 2 MSI 7970 Twin Frozr and a MSI 280X GAMING on a 1050AX Corsair that is no problem though.  What is the brand of PSU?  Rating? 

Also, try undervolting a bit and then dropping the core clock down maybe to 1050 each.  I run all mine at 1040, anything over that I get less hashrate.

Thanks for the feedback, I think you guys may be right about PSU

Im using a corsair 1050W job, however I've read that the toxic can pull 330 each, when you add the MB, cpu etc thats probably too much for the PSU. Ill try something bigger.

Unfortunately it seems that the toxic cant be undervolted (if anyone know different then pls let me know). Guess I choose the wrong card 

Talking of PSUs - ive seen loads of Blade server 12 2000W units going for next to nothing on EBAY, anyone used these as a power source ?

 

[Wipeout2097]

Not saying it can't happen, but I've never seen a PSU slowing down components. Shutdown, instability, lockups, blue screens, yes...

Unless the voltage regulators and control logic inside the Sapphires throttle the devices when they see that the PSU is providing way below 12V. What do the "Sensors" tab of Gpu-z show? Can you post screenshots while the 3 cards are mining?

try to undervolt by bios modding with vbe7: http://www.techpowerup.com/forums/threads/vbe7-vbios-editor-for-radeon-hd-7000-series-cards.189089/

[bouncemeister]

Did you find a solution to this problem?

I was thinking of buying this motherboard, but i want to be able to add four 280X cards to is (using powered risers).