I have a ninja spirit on my collection since more than 10 years. I play it regularly but the other day it once powered up the foreground layer had some vertical black bars crossing the screen for the complete height on the right side.
After a while also the left part started to blick glitching ( cannot be seen on the screenshot below unfortunately)
The board is an irem M72 and the background and foreground layers are handled on the bottom board. The circuit for both playfields is completely the same.
Schematics are available for Rtype which is the same hardware minus the C board.
I checked immediately the circuit which draws the foreground and decided to check a couple of TTL
With some luck I immediately found one 74ls283@10A (IC 78) which was glitching badly the left part of the screen when testing with the probe on pin 10.
74LS86@7B ( IC50)was also glitching the screen when testing with the probe on pin 6
The HP logic comparator gave me bad outputs on some pins of these TTLs.
After desoldering them I tested on my TTL tester and they were declared good!
I decided to solder back new ones to be on the safe side and to my surprise the game was fixed.
Turns out that the TTL tester didn’t catch the subtle faults of these ICs ( not Fujitsu parts!)