As mentioned, resoldering the inter-board connectors is a good thing to try first.
Additionally, if the boardset coins up and can be played, but no video.... Sounds like it is a problem within the analog section. Depending on board revisions (and they are different ones). Usual suspects are the TL081 (8 pin), or the MC1495L, or the AM6012PC (DAC312) chips for either the X or Y analog outputs. When Analog is working, you should see up to +/- 2V fluctuating on the X and Y test lugs. If it measures around 0v on either, then it is dead and the spot killer on the monitor will be triggered. If dead, you can look at the OpAmp or D/A ICs for that axis (chips mentioned above).
One check would also to see if the AM6012PC is getting digital inputs and subsequent analog output (I believe the analog comes out on pin 15). If you are not getting any digital inputs to one or both of the D/As, then the problem is more likely back in the vector generation area which is harder to diagnose.
Since, you can coin up and hear a game, this tends to mean the program runs and thus your program and vector RAMS/ROMs are functioning properly. A completely dead board would need a different approach to debug the problem - starting with the 6502A processor and whether it is being continually reset because program code is not clearing the Watchdog...... which could be bad data/address buffers, bad ROMs/RAMs, bad sockets, etc. etc. etc.