All ground plates are soldered for sure.
Does the test pattern rely on the TVP (where is that actually generated – i think it might be the ITE chip but I’m not sure!)?
My guess is that the TVP needs to work in order to get the Red ‘Sync’ led to go out (as it is then sync’d and locked to 59.59Hz on some input).
Because both the test pattern AND the sync’d input from scart are not outputting anything, I’m guessing it’s something downstream of the inputs.