Saturday, July 30, 2011

Yet Another Circuit Repair

So last night I lightly sanded the inner perimeter of the Rockler bearing with 150 grit sand paper, to help give the dome drive wheel some traction. Of course, I covered the race with masking tape to protect it from the dust.

After cleaning up the dust, I removed the masking tape, and the surface looked sufficiently scuffed.

I reinstalled the bearing on the droid and tried it out. It was working great, no wheel slippage, everything was fine... until I heard a loud pop! and smelled the wonderful odor of some burnt electronic component. I immediately powered off, too disgusted to diagnose what blew up this time.

Today I pulled the electronics board out, and discovered the problem. Neither Mike nor I realized this past Tuesday that one of the two decoupling capacitors (the 0.33µF) was a tantalum capacitor, meaning it has polarity, unlike the 0.10µf ceramic capacitor, that has no polarity. Of course, odds being 50/50, the capacitor was installed backwards and survived a few days until it could take no more. Thus, the source of the bang and smell.

Fortunately, I purchased two of these guys, so I pulled the spare out of the package. Yup, one lead is longer than the other, and there is a "+" on the markings.

I actually managed this minor surgery on my own. I replaced the capacitor, using the proper polarity this time.

I hooked everything back up in the droid, and the circuit ran flawlessly and the dome wheel never slipped for forty-five minutes, until I got tired of hearing him and turned it off. So until the next time something blows up, I hope to leave the electronics alone for a while.

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