Hello. I have a 1965 C337 and I know exactly where to look for the diodes. Open the avionics panel (the large panel outside-top of the front engine cowling) look outboard along the inside left sheet metal side of the fuselage, you see some large shunts about 1 1/2" wide and about 6" long, they have a wavy appearance. Just forward of this are two diodes screwed to the side of one of the shunts. To test these diodes your mechanic will have to cut the wire on the top of the diode and check it with an multi-meter WITH A DIODE SCALE. The meter should read flow through the diode with meter leads connected one way and NO FLOW with the leads connected the opposite way. The diodes are hard to remove but doable! Remove the two 6-32 screws to get at the nut on the bottom of the diodes and if you're smart your have your A&P mechanic re-locate these diodes for better access. The diode PART NUMBERS: original-6F20-D, superseded by 20H3P, superseded by (1N3881 this one is more common in non-aviation appliances and can purchased at
www.mouser.com for about $6 each.)
I hope this helps, I've attached a charging system diagram the diodes are ITEM 10. This diagram is for the 38 AMP alternators the diodes are the same for the 30 AMP alternators!
regards, BILLS