Re: Is this dead or can be repaired ?
There is a huge market in diodes depending on who you get them from. The places that actually sell at wholesale, usually will only sell to a business. Sometimes you can find a way to buy from them.
Usually they will sell a "diode frame", the mounted complete set of diodes, and not single diodes. Single diodes often must be press fit into place, and if something damaged one (as opposed to it failing from a flaw) there is reason to replace the entire frame.
So you can try to find one diode from an electronics house, or you can try to find a complete diode frame set from a wholesaler. Either way will take some time and sleuthing unless a local alternator shop can hook you up. If you can find a tv or electronics repair shop, they may be able to locate the one diode for you as well.
If the diodes have any numbering or marking on them, that can be looked up to find out what they are.