Drying out some expensive electronics

November 17, 2007

The Busch & Müller Dymotec S12 is almost waterproof.

I didn’t take the pictures that I should have. Oh well.

I bought a B & M S12 to run about 5 watts of LED lights (very bright, for the power, and with multiple lights and lenses I can get a nice color and pattern). With the steel brush roller, it’s a great generator, quiet, and low drag, especially for the power it produces. I had hoped that it would be completely bulletproof, but Thursday night I rode it home in a heavy rain and discovered that it was not. It got drenched, and because of how it is mounted (nearly horizontal axis) I think it was more vulnerable to water getting in through its connectors. B&M confirm that this is true; roller up is best.

Just behind the electrical connectors are some interesting-looking electronics that serve to limit output at high speeds or when a bulb is out (it is designed to detect this). Getting it wet caused it to malfunction in a very peculiar way; at low speeds, the lights would produce low-speed brightness; at medium speeds, still low-speed brightness, and at a medium high speed (14 mph) the lights would drop out completely. Not fun, for cycling in the dark. I tried to blow and dab it dry and shield it with some duct tape, and the lights-out speed increased to 19mph, but it never came up to full brightness.

I tore down the whole system, front to back, and verified that the only wet was in the generator; I could swing it back and forth in my arm and water would come out. There’s not much help on the web for this sort of thing, and the generator is expensive ($285), but I decided that if I didn’t get the water out (and soon) things would not get better. B&M say, don’t do this, instead put it on a heater for three or four days. So, I screwed up my nerve, and carefully backed out 5 little phillips-head screws from the back side of the generator. This loosened the cover up near the spring-tension adjuster, but the rest of the cover was still held tight by waterproofing. I pried it loose, and opened up about a 1/4-inch gap, with enough wires and stuff in it to suggest that I should stop there. So, I blew out the water, and left it sitting (open, so the air could get in) in front of a fan next to our furnace.

Last night, I reassembled it, carefully greasing the screw threads with beeswax and lanolin, and with a thin bead of silicone sealant around the gap. B&M say, avoid sealants with acetic acid (like silicone) because it could be bad for the electronics. Except it turns out I used a “Silicone II” compound, which outgasses methanol and ammonia.

It went together fine, and then I smeared the excess back into and around the joint. Then, I gooped up the connector covers with a lanolin film and put them in place, and then squirted silicone seal into the assembly, and let it set overnight. This morning, I opened up two connectors, put it all together, and it worked wonderfully. I made a shield out of duct tape to keep the generator somewhat drier in the future.

I plan to move it to a vertical-axis mounting, with the connectors pointing down, and I expect that this will work even better. B&M say, yes, vertical axis roller-up is best. Current sticking point is getting a screw-in stud for the fitting in my fork — local bike shop has nothing on hand, will probably figure out what to order tomorrow.

Despite my mistakes, I succeeded in not damaging my generator, at least not yet.

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