What I did was replace the silicon rectifier diodes
with Schottky
diodes, which have a much lower voltage drop. Problem solved. Assuming
your existing transformer can actually produce enough current from the
secondary, that might be enough for you too.
It would be worth finding out just where the voltage is dropping (between
the lightly-loaded and fully-loaded configurations). If the RMS AC voltage
from the transformer secondary is going down
significantly, then I would
suggest that the transformer is being overloaded,and I
wouldmn't try just
fiddling with the rectifiers or smoothing capacitors. Nor would I try an
external boost transoformer in this case. You don;t want to burn out the
computer's mains transformer.
On the other hand, if the secondary voltage is still OK, then I would try
using schottky rectifiers and/or increasing the smoothing capacitance
(parallel up some more large electrolytics).
-tony