The black one sure looks like a PSU too.  Check the voltages on that one as well.  Do both machines have 2 x PSU?

There looks to be some funky wiring with the tape around the silver one too.  I'd be careful around those and make sure the repairs are OK.

The 5v should be checked at the PSU and at the corresponding wire at the JAMMA edge connector as well.  WIth machine off - make sure your earth's are clean and not rusted.  I think you can use a DMM on ohm and check the earth points to the earth on your power plug (unplugged).  In the back of my mind I have a figure of less than 2ohm is what it should be - but I really am running out of RAM on that so I'd check with someone who's got more experience in that area than I.