Thursday, December 14, 2006

Weird troubleshooting...

I don't often write about work, partly because it's boring to relate stories about repairing computers, and partly because management may not look kindly on detailed stories about aircraft maintenance. It's actually a good job and I'd like to keep it until retirement if possible. It feeds us.

But sometimes there's a truly weird event to relate, and today must have been my turn. I've been working on a 'dog' unit – one that has serious problems that are difficult to eliminate. This particular unit has a -12 volt monitor circuit that failed repeatedly. Oh, the voltage was within spec, but it had a lot of noise on it that caused the failures. I traced the problem from the monitor circuit back to the power supply. Everything on the power supply board tested OK, leading me back to the input transformer. It looked OK, but on another unit I'd encountered a similar problem due to a cracked transformer core. I decided to 'augment' the core with another piece of magnetic material. I looked around the bench. The most suitable thing was a refrigerator magnet of my hero Pepe Le Pew. So I stuck Pepe on the outside of the transformer and ran the test again.

It worked!

The extra magnet damped down the noise, allowing the unit to pass the acceptance test. Now, before you think that I sent the unit out to the field that way, please think again. It's staying in the shop until I get a replacement core assembly, though I'll admit the idea of using Pepe Le Pew refrigerator magnets to fix a computer has a certain perverse attraction. The FAA, our management, and our own QA department would stomp me with hobnailed boots. Sometimes you just have to do something a little odd in order to troubleshoot down to the component.

Bicycles can be that way too, though fortunately they're not as complicated and don't give weird failures very often. This week, my commuter bike began making a thunking noise in sync with the crankset. It could be: the bottom bracket, a pedal, a cleat, a loose crankarm, the saddle, or even the handlebars. Mysterious clicks, creaks, and thunks can be traced to any of them. What makes this one more annoying is that it comes and goes without any apparent reason. I checked the chain, chainrings, and cog without finding anything. This weekend I'll overhaul the bottom bracket. It's overdue anyway.

I had a similar problem a few years ago. There was a clicking noise coming from the bottom bracket, or so it seemed, and it only happened at night. How would darkness have any effect? It was maddening. I checked the usual suspects as listed above, but found nothing. Then the weather got really cold and I had to ride home with a sweater and anorak. That afternoon, I heard the clicking again. I stopped to look for the cause, but again, found nothing. The click occurred right at the same cadence as my pedaling. It just HAD to be the bottom bracket or something attached to it. But why would it click only when it was cold?

That's when I noticed the metal zipper pull on the anorak. It swung in and out right along with my cadence, and each time it tapped the zipper, it clicked! I felt like an idiot. But at least I was an idiot with a well-maintained bottom bracket!


Blogger Fritz said...

Mystery bicycles noises ... know the feeling. Ever use a stethoscope or rubber hose to diagnose a bike noise problem?

I had a similar "click click click" in time with the cadence noise. Turned out to be the plastic end of a shoelace hitting the crank arm with each stroke. I've also learned the hard way to ignore mystery noises at my own peril.

11:22 AM  
Blogger Paul Tay said...

DAMN the mystery noises. Santa simply rides the bike, til it don't ride no more.

12:17 AM  

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