Problem solved by buying a new mobo. It seems it was a simple incompatibility.
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Specs:
GPU: Radeon RX 470 PowerColor Red Devil, 4 GB, 8 pin power connector, brand new video card
CPU: Phenom II X4 955 (3200 MHz)
Motherboard: Gigabyte GA-M55S-S3 rev. 1.0, PCI-E 1.1 (GPU-Z says so), nVIDIA nForce 550, latest BIOS version F13B
RAM: 4.5 GB DDR2-533: DIMM1: Kingmax KLBD48F-A8KI4 - 1 GB DDR2-533, DIMM2: Kingston 99U5316-028.A00LF - 1 GB DDR2-800, DIMM3: Patriot PSD22G8002 - 2 GB DDR2-800, DIMM4: SK Hynix HYMP564U64CP8-Y5 - 512 MB DDR2-667
PSU: Chieftec "A135 APS-550S" 550 W, does have two 6+2 PCI-E power connectors
Problem:
The system starts properly with my old video card - Radeon HD 6850 (Sapphire, 1 GB, PCI-E 2.0, 6 pin power connector).
With the new RX 470 my PC won't start - it keeps rebooting every 2 seconds right after being powered on.
At lest I hear the motherboard's sound similar to rebooting (but no error beeps). No video signal (HDMI port). Can't enter the BIOS setup. All the fans and HDD are spinning normally. Fans on RX 470 are slightly increasing their speed for a split second right after reboot sound, but keeps silent. Southbridge become slightly warm as usual.
What I did and tried:
1. Disconnected HDD. I don't have any other devices connected aside of the Ethernet PCI card
2. Leaving only one RAM stick, tried everyone of them. Also tried leaving only two sticks
3. Plugging back Radeon HD 6850 and resetting BIOS to default
4. Clearing CMOS with the jumper
5. Connecting the different 6+2 PCI-E power connector
6. The motherboard does have a weird PCI-E molex connector on it, I tried connecting it to PSU. Motherboard manual just recommends to use it if the video card needs it. I didn't hear about any such video card ever.
7. Starting the system without video card but with RAM gives beeps
8. Starting the system without RAM but with video card gives beeps (I think they are just long beeps in both cases, well they are seems ok, because I tried this out with my old video card, same beeps)
I know that this motherboard is old, but PCI-E 1.1 and 3.0 should be compatible? Everyone on Google searches says it's ok.
I doubt it's the PSU problem, well at least it's working ok, cool and quiet even when I overclocking my HD 6850 to 956 MHZ and running Furmark for quite some time.
The recommended PSU for my HD 6850 is 500 W and for my new RX 470 is 450 W. Their TDPs are almost identical: 127 W for HD 6850 and 120 W for RX 470.
Sorry, but I don't have an option to test another PSU or motherboard at the moment.
----------------------
Specs:
GPU: Radeon RX 470 PowerColor Red Devil, 4 GB, 8 pin power connector, brand new video card
CPU: Phenom II X4 955 (3200 MHz)
Motherboard: Gigabyte GA-M55S-S3 rev. 1.0, PCI-E 1.1 (GPU-Z says so), nVIDIA nForce 550, latest BIOS version F13B
RAM: 4.5 GB DDR2-533: DIMM1: Kingmax KLBD48F-A8KI4 - 1 GB DDR2-533, DIMM2: Kingston 99U5316-028.A00LF - 1 GB DDR2-800, DIMM3: Patriot PSD22G8002 - 2 GB DDR2-800, DIMM4: SK Hynix HYMP564U64CP8-Y5 - 512 MB DDR2-667
PSU: Chieftec "A135 APS-550S" 550 W, does have two 6+2 PCI-E power connectors
Problem:
The system starts properly with my old video card - Radeon HD 6850 (Sapphire, 1 GB, PCI-E 2.0, 6 pin power connector).
With the new RX 470 my PC won't start - it keeps rebooting every 2 seconds right after being powered on.
At lest I hear the motherboard's sound similar to rebooting (but no error beeps). No video signal (HDMI port). Can't enter the BIOS setup. All the fans and HDD are spinning normally. Fans on RX 470 are slightly increasing their speed for a split second right after reboot sound, but keeps silent. Southbridge become slightly warm as usual.
What I did and tried:
1. Disconnected HDD. I don't have any other devices connected aside of the Ethernet PCI card
2. Leaving only one RAM stick, tried everyone of them. Also tried leaving only two sticks
3. Plugging back Radeon HD 6850 and resetting BIOS to default
4. Clearing CMOS with the jumper
5. Connecting the different 6+2 PCI-E power connector
6. The motherboard does have a weird PCI-E molex connector on it, I tried connecting it to PSU. Motherboard manual just recommends to use it if the video card needs it. I didn't hear about any such video card ever.
7. Starting the system without video card but with RAM gives beeps
8. Starting the system without RAM but with video card gives beeps (I think they are just long beeps in both cases, well they are seems ok, because I tried this out with my old video card, same beeps)
I know that this motherboard is old, but PCI-E 1.1 and 3.0 should be compatible? Everyone on Google searches says it's ok.
I doubt it's the PSU problem, well at least it's working ok, cool and quiet even when I overclocking my HD 6850 to 956 MHZ and running Furmark for quite some time.
The recommended PSU for my HD 6850 is 500 W and for my new RX 470 is 450 W. Their TDPs are almost identical: 127 W for HD 6850 and 120 W for RX 470.
Sorry, but I don't have an option to test another PSU or motherboard at the moment.
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