It sounds like you are getting hosed by your mechanic. The diodes (6 of them) does the full wave rectification. There will be small ripples which won't hurt anything. The voltage regulator regulates charging voltage and does not remove ripples. Put a DVM on the lowest AC scale and measure charging voltage. If you see a few milli volts AC you do not have any problems. If you see 1 volt AC or higher , one or more of the 6 diodes is bad. Adding a capacitor will not help. The bad diode (s) need to be replaced.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked