reg/rec getting really hot.

hi all,
i'm in the final stages of getting my xj600 bobber style bike on the road, i've got the bike running after a complete nut and bolt rebuild.
but i noticed ,after only a few minuets running ,the regulator rectifier is getting really hot , so hot i'm unable to touch it, this is an old reg rec off of my other bike ,that charges a 17 volts, so i changed it on my other bike.
i've got a new one on the way, but could this be the cause of it getting hot.
the alternator puts out about 50volts on all 3 phases at around 3000 revs.
i dont want to damage the new one when it comes,
is there anything else i can do to test the circuits ,
im pretty good at sorting electrical issues, as it was my job.
but everything seems to test ok, no shorts to earth, and no bad connections ,as far as i can tell
anyone any thoughts?

 

up-date, so , tested all ignition / charging circuits on bike and everything checked out ok, new reg/rec arrived, fitted on bike ,and all good. at 3000 revs voltage reads 14.6.
reg / rec still gets warm, but not unable to touch it like it was,