Hello - I might be able to help, I do have a working Domesday System and VP415, repaired last month after the PSU exploded last year! I also have a "spare" working VP415 which I cobbled together last year from a broken VP415 and a fully working VP410. The VP410 is identical to the VP415 except that it doesn't have the SCSI/Genlock "Domesday Sandwich" at the bottom. I've also got a working VP380 but I'm not sure how similar the insides are to the VP4xx players.
The VP415 user's manual and service manual are online somewhere or other, I can get them to you from my local copies if you can't find them.
Since I repaired the VP415 at the beginning of July, I've been doing as much work as I can with the Domesday System before it conks out again, in time for the 25th anniversary in November.