I’ve had a similar behavior after building the 1.5 kit: after powering up the OSSC the green LED and the LCD backlight lit up, but then the backlight got dim and nothing else happened. No text on the LCD, no HDMI output – nothing. However the FPGA seemed to work, since i was able to flash the latest firmware via USB blaster after powering the OSSC up for the first time. But that didn’t change anything.
The culprit was a solder bridge on two pins of the TVP7002 (pin 79 & 80 if i remember correctly). Hadn’t seen it under the microscope, but measured it after painstakingly testing all adjacent pins on all ICs for continuity. Maybe in your case the problem is kind of the the same?