Sequential circuits samplers

>>By the way, I know the transformer and everything before it is good
> because if I disconnect the transformer from the main board and turn
> the machine on, then it does not blow the fuse.<<

then it might be the bridge rectifier(s) between the output of the
transformer & the regulators you just changed. it could still be the
remaining regulator, but as you say this is now unlikely- the -12V
rail it supplies is quite a low draw. also look at the smoothing
capacitors & any tantalum bead capacitors around the power supply.
can you be sure that the main boards themselves aren't the problem?
can you disconnect them from the power supply? I will get the lid off
my 2k2 this weekend & have a look to remind myself.

d.