After using my logic probe running over the video section of the PCB I found what I though could be an issue. The MB14241 custom IC had data being fed into it and the clocking/enable signals all seemed to be pulsing away, but all of the Data Output pins were not showing as having any signals on them.
Reading online about the Custom IC I found mention of it having tri-state outputs which could have been the reason my IC didn't show any activity. The IC in question is a video shifter which was used to assist with drawing of sprites in early Taito and Midway games.
Part of the schematic diagram shown below which includes the Custom IC which i suspected was faulty, The scan of the original schematic is difficult to read, the IC pin numbers are very blurry which makes working out issues more time consuming.
Whilst probing around i put my logic probe on pin24 which is the supply voltage pin, the pin snapped off from the IC! Clearly it was weak and corroded, I filed the IC down to expose the contact so I could solder my own 'IC leg' back on. I was hoping this would have been the issue, no supply voltage to the IC would match up with all the output pins being 'dead'.
Well this didn't fix the problem but the image of the display slightly changed. A couple of the output pins were now stuck high or low but most were still showing as being 'dead'.
My next option was to swap the IC with a known worker to see if this was the issue, luckily the custom was already socketed so I just needed to find a replacement and plug it in. Cue me asking a collector friend for a loan if he had one, luckily a Zenitone Invader Revenge PCB was found that had the same IC needed.
Once popped into my PCB the game fired and and played perfectly! I now need to procure a replacement IC and return the loaned IC.
Another 35 year old game lives on to be played!