The power supply in a black JP DDR cabinet is very finicky if you go to replace it. In the above 1st Mix manual, on page 58 you can see an exploded view of the PSU box. Basically it contains a fuse, some AC distribution and four individual PSUs powering various pieces of the cabinet. In the original configuration, all four PSUs are isolated from earth ground and have their own DC grounds. Three of them supply 12V to various parts of the cabinet and one supplies 5V to the JAMMA edge connector.
As specified in the manual, the top PSU boards are a 5V 30W DC power supply and a 12V 50W DC power supply. The bottom PSU boards are twin 12V 150W power supplies. In order for various game boards to function correctly (especially Bemani PC mixes), each PSU needs
to be electrically isolated from the others and earth ground. Do not
connect the earth ground (FG) of any of the PSUs to the earth ground cables inside the PSU box. This means you can't cut corners and replace multiple PSUs with one beefier one, as it would tie the grounds together for those circuits. Also, make sure that they are isolated from each other and the metal PSU box by using mounting tape or electrical tape, as the metal casing is connected to earth ground. When swapping out the individual PSU boards, you can test to make sure you haven't inadvertently connected one of the PSUs to earth ground by testing for continuity between the FG terminal and the metal side of the PSU box itself. Make sure the FG terminals on each PSU has no continuity with another PSU or the PSU box, and make sure the GND terminals on each PSU have no continuity with other PSUs.
If you neglect to ensure that each PSU is isolated from earth ground as well as has its own DC ground not connected to any other PSU, you will notice strange behavior with X kits (Bemani PC). Your monitor won't sync properly, the stage lights and start button lights won't go out all the way when flashing off, and there will be an extremely loud buzz on the speakers. Even if you leave the FG terminals empty, but let the PSUs rest directly on the metal PSU box itself, you will see occasional issues when lights turn off. Essentially the X kit expects each PSU to be floating relative to the others, and any connection to earth ground on any of them creates a loop in the cabinet that throws off the P3IO inside the X kit.