New system failed bench test, one beep/no video. trouble diagnosing.

I am assembling my components of a self-built computer, doing a bench test (mobo on cardboard box), getting one beep and no video output. Tried multiple cables (dvi/hdmi), nothing. Positive the monitor works, and tried restarting that too.

Everything seems to be working like normal otherwise. Fans spin, get the "all clear" quick single beep.

Tried with no RAM, get three long repeating beeps. Tried resetting CMOS batteries, nothing. Tried different ram sticks in different slots, no difference. Tried the other PCI-e slot for the video card (there's no onboard graphics), nothing.

Running out of ideas.  

Specs as follows:

Processor: AMD Phenom II X4 965 Black Edition Deneb 3.4GHz 4 x 512KB L2 Cache 6MB L3 Cache Socket AM3 125W
RAM: G.SKILL Ripjaws Series 4GB 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Cas Latency: 9 Voltage: 1.5V
Motherboard: ASRock M3A770DE AM3 AMD 770 ATX AMD Motherboard
PSU: CORSAIR Enthusiast Series TX750 V2 750W ATX12V v2.31/ EPS12V v2.92 80 PLUS BRONZE PSU
GPU: XFX HD-687A-ZNFC Radeon HD 6870 1GB 256-bit DDR5 PCI Express 2.1 x16 HDCP