G4 power supply circuit

A replacement unit is VERY expensive considering the value of the computer. Can anyone help.
soli wrote on :

G4's (desktop) have had a problem for some time ie, the AC Bel power supply! I have searched the net for two days but have not seen anything pointing to the problem nor the solution (overheat and shutdown) . A replacement unit is VERY expensive considering the value of the computer. Can anyone help.

Thanks in advance