The game was running fine for the first few days but by Sunday the game wouldn't coin up. Having had problems with the credit PCB in the past, I replaced it with a spare that resurrected the game for the remainder of the show.
On the bench the credit PCB coin counter output was working but the credit output was not. Using a scope confirmed that transistor Q4 (BC548) appeared to be not working. On the schematics Q4 was listed as BC171 and having neither in stock I found a BC237 was an adequate substitute that fixed the credit output.
First power after winter break found an all red rolling picture and HV leakage that subsided as the game warmed up. Swapping in the spare RGB drive daughter PCB confirmed that the red fault was caused by the RGB drive PCB. The rolling picture was corrected with a small tweak to vertical hold, however previously this game had tended to loose vertical lock over time, an issue that I already knew wasn't related to any of the KT-3 daughter PCBs :|
Since the RGB PCB was relatively simple the plan was to fault find it on the bench using a multimeter. I compared all the in-circuit measurements for the transistors, diodes and resistors across all the three colour drive circuits. For the transistors I used the back-to-back diode check. Transistor TS242 (BF422) registered a diode voltage of 1.38V across B-C that was not present on the other two. The schematic confirmed the anomalous reading was in the red drive. Removing TS242 & TS244 (BF422) to check out of circuit gave Hfe readings that seemed OK. Testing a few reclaimed BF422 didn't give consistent results so I elected to order new ones to get a matched pair of known good.
With TS242 & TS244 replaced the in-circuit B-C diode voltage was gone and the PCB was working properly in the cabinet.
I realized that my set of spare KT-3 daughter PCBs was missing input PCBs. There were two versions of the input PCB, one for inverted video and one for normal video. Though I did have one spare of each in my parts box they were both labeled faulty. In cabinet (normal in Scramble and inverting in Quasar) they both gave a blank white screen. The input PCBs are simple with a single TDA2560 IC used for chroma/luma. I'd seen the bright white screen before when this IC fails.
On the bench all the resistors and transistors were consistent according to the multimeter. After a cap replacement and new TDA2560 ICs (socketed with some pin-bending to adapt for the Z-line package) the normal video PCB was working properly but the inverting PCB still gave a bright white screen. Suspecting one of the new TDA2560 may be bad I swapped them between the two and the bright white screen persisted on the inverting PCB ruling out a fault with the IC. This put the fault in the passives and I realized that the one thing I hadn't checked on the bench was the brightness pot. Sure enough the inverting PCB 2.2K Ohm brightness pot measured ~9K across the track. I was only able to locally source 2K Ohm replacements in the right form factor but it was close enough to fix the PCB.