Diode boards generally fail by having one or more diodes ‘open’ internally, “open’ means they do not exist electrically. Rarely do they short-circuit. There are small diodes on the diode board with two types of functions. Early boards, such as on the /5, did not have as many diodes as later boards, the later boards had a stator windings center-tap, which used the extra diodes to improve output wattage. Failure of any of the small diodes will cause strange output problems. Usually, a diode board failure is at the solder joint of one or more of the six large power diodes; or, that diode opens. It is quite rare to see shorted large diodes on the board. If a large diode solder joint becomes lousy or opens or the diode itself opens, the alternator will still produce current, but it will be quite considerably lower in total wattage output. The typical result of one large diode opening (or, solder joint failure) is that the battery will show near proper voltage during riding, but when the headlight is turned-on, the voltage will sag rather considerably.
Diode board testing and information about repairing the faulty Wehrle-manufactured ones (I’ve seen a few Bosch boards with the same problems with non-bent-over large diode leads and soldering failures) is covered in depth in the June 1999 issue of AIRMAIL, an article by the late great BMW Guru Oak Okleshen. Airhead owners may want to read the article. The Oak-recommended & difficult to do board modification could be done, with considerable effort, for reasons of solder joint failures, more likely if you have a R100 engine, which develops more heat (worse if an RS or RT). I’ve had good luck with a much simpler method, which is the removing of the ‘paint’ with gel type paint remover (often it has no effect though and abrasion is needed). I always enlarge the solder pad area first, whether the gel remover works or not, by careful scraping with a sharp Xacto thin blade hobbyist knife (don’t remove copper material), & then re-soldering using a high temperature solder (50-50 plumber’s solder and rosin flux) and a extra hot soldering iron. No re-painting or re-coating is needed, but certainly could be done. My repair method is vastly easier to do because drilling the PC board & adding wires & soldering per Oak’s method is quite difficult & best done by total disassembly, which Oak did not get into and is a huge PIA! Yes, it can be done with long tweezers or forceps, but that is quite tricky. I cannot recommend Oak’s method, and I won’t even do it myself. I’ve also seen several Bosch branded boards with this problem, perhaps Wehrle made them? There have been some other Bosch and Wehrle labeled products that made me think either could have manufactured them.