Wednesday, March 10, 2010


PM was astonished that I still had a bottle of Cramolin in my toolbox. It was a popular cleaner for electronics once upon a time, and as I found with my commuter bike, it prevented corrosion and intermittent contacts between the bike computer and its mount.

Cramolin was replaced by DeoxIT, a similar appearing liquid that has a different odor and slightly higher viscosity. Where Cramolin smelled slightly acidic, DeoxIT is far less pungent. Also, DeoxIT's higher viscosity makes it a little more difficult to remove after cleaning an assembly.

And therein lies a story.

When I hired into the navigation electronics shop at this very large airline, most of the fleet was composed of Boeing 727s and McDonnell Douglas DC-10s. The controls for those aircraft were primarily electro-mechanical systems. When you look in the cockpit and see all those dials, switches, and indicators - those were a big part of my job. Since much of my previous work involved small mechanical devices, working with gears, levers, and springs was second nature. That's how I ended up on the pitch control panel for the DC-10.

Pitch is the up or down motion of an aircraft relative to a horizontal line. The pilot would set his cruising altitude on the pitch panel by turning a knob connected to a gear train that moved both a mechanical counter and a mechanical encoder assembly. These days, encoders are optical devices, very small and very reliable. Your car probably has an optical encoder on the frequency dial, particularly if it's speed sensitive. Turn it faster, and it makes bigger changes.

Those old mechanical encoders weren't nearly as reliable. They consisted of two spools with a pattern of make-and-break contact areas. Tiny cat-whisker switches rode on the spools. As the spools turned, the cat whiskers touched on gold plated conductive areas or insulators. the pattern determined the particular data being sent to the flight computer. Naturally, this was a redundant system as there were two identical spools sending the same data. If they differed, the computer flagged the panel as faulty.

Corrosion wasn't a problem due to the gold-plated contacts. But dust intrusion and the eventual wear on the cat whiskers, combined with the loss of tension over the years, made these panels a bit tricky. A tiny amount of dust on the spools would build up under the cat whiskers. Under high magnification, it looked almost like a miniature snowdrift blown up against the wire. But it was just enough to lift the switch out of contact with the spool. Once one side went intermittent, the computer threw up its hands in disgust.

The usual procedure was to disassemble the panel, remove the altitude encoder assembly, and then disassemble it. Care had to be taken to mark it in order to make re-alignment easier. The spools were cleaned with Cramolin (See? I got around to Cramolin again) to float any dust up out of the spool's surface. The excess was wiped away and then the spools were cleaned again with denatured alcohol. I really wanted grain alcohol for a cleaner as it leaves no residue behind and it makes a dandy technician lubricant when mixed with some fruit juice. That's an excellent way to get fired around here, incidentally.

After cleaning, the whole shebang went back together again, sometimes aligned properly on the first try. When I was learning to do this, the initial unit took about 2 hours to disassemble and clean, and then it took the rest of the week to get it back together and working properly again. There were lots of interesting ways to do it wrong. I found most of them.

This morning I wanted to see how DeoxIT compared to the older Cramolin. I had a circuit board with push button switches. These have only one moving part besides the push button cap. There's a small coil spring formed in a circle around a center post with a groove to retain it. When the button is pushed, the spring is forced out of the groove and it makes contact between the center post and a circuit trace. When the force on the push button goes away, the spring goes back into the groove. It's very simple.

Since this control is in the cockpit, it sometimes gets liquids spilled onto it. There a gasket between the push button cap and the switch itself to prevent liquids from entering the unit, much like a computer keyboard. Still, they can get very dirty and this one was no exception. I cleaned it up and decided to try the DeoxIT on the switches.

Using a cotton swab, I daubed a bit onto each spring. After a few minutes, I washed it off with alcohol, or rather, I tried to wash it off. The stuff is tenacious! It took three washings to get the board clean and I still had to wipe some DeoxIT away with a small Kimwipe.

I'm thinking this stuff would be better for connections exposed to weather, like my bike computer, or the electrical connectors for trailer hitch lamps. But in future, I will use it very, very sparingly on these control panels.

One last thing - because I went looking for information on Cramolin yesterday, I discovered that our shop doesn't have a material safety data sheet for the stuff. Without it, I don't think we can legally have that chemical in the shop, and since we are regularly inspected by the FAA and our internal audit people, I'll probably have to dispose of my bench stock.

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Blogger zek said...

Lots of interesting stuff. Trying to correlate findings. More on my cleaner page about contact cleaners, and differences between Cramolin and Caig.

1:10 PM  

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