Andrew,
have you already looked at the sync pulses that come from the sync separator PCB? When you connect your oscilloscope to the line sync output while running a video signal from one of the club CDs, you should find very stable sync pulses of 400Hz (2,5 msec). First see that the contrast pot is in its optimal position, i.e. 1,4 volt peak tot peak video going to the LED-driver board. Then adjust the 1k trimming pot to the centre of stable sync pulses. This should not be critical.
Then connect it to the frame pulse output and you will see slower pulses, 12,5 per second. If this is Ok the PCB is working fine.
Then make the connection from the line pulses to the clamp pulse input of the LED-driver. After having done this you might connect the select clamp input of that board to +12. I have a small switch connected to be able to select both active clamp or DC-restoring. Simply switching from ground to +12.
DC-restoring works too when no sync pulses are present, also when the contrast setting is not Ok, with unknown video signals. But with standard and clean CD-signals the active clamping is working better.
The line pulses are also needed for the synchronisation circuit of the disc motor. This is the reason for you to look into this now.