Old Plotters Never Die

Posted: April 1, 2012

They just need their track ball fixed.

By: John Beatty

The 13-year-old chartplotter on our 13-year-old Krogen 42 stopped cooperating with me a while ago. The cursor, controlled by a built-in track ball, would not move. It doesn’t take much to figure out that when a device with only one mechanical part quits, the moving part is the likely culprit.

I mentioned this to my wife, Linda, telling her the plotter was so old that it was not worth having it repaired. One of her concerns was what to put in its place on the pilothouse console, and the other concern was how much it would cost. We have a laptop that displays our chart software, and it is our primary reference for position. The old plotter was a backup and is used to show the big picture.

I was afraid that if I took the back off the unit, springs and gears would fly out in all directions, and reassembly would be impossible. But, if I did dig into it and could not fix it ... oh well, no loss. I could just push the whole mess into the garbage can. 

I removed all six screws, and nothing flew out when I opened it, so I continued. Inside, there were several circuit boards, including a smaller board where the track ball should have been. I took three more screws out of that piece, and I found the ball. There were two small rubber-coated rolling pins that rode on the ball — one pin for the X axis and the other for the Y axis.

Cleaning the Mouse

Remember the days before the optical computer mouse? Periodically, you had to disassemble the mouse and clean out the cat hair and dust inside. My situation wasn’t exactly like that, but close. Here is how I explained it to my better half: “You know how a tire can develop a flat spot on the bottom if the car sits idle for several months? When you drive the car, the tire returns to normal. When our old plotter sat unused, the two rubber-coated rolling pins that ride on the ball developed flat spots that could only be fixed by rotating the rubber.” 

“That is just like rotating the tires,” she said. Smart girl.

As far as I could see, the rolling pins were each held by a springy wire at one end and a “sensor wheel” on the other. With my tiny screwdriver, I moved the spring wire off the end of the rolling pin and slipped the rubber sleeve off the pin. Then I swapped the rubber sleeve end-for-end and slid it back on the pin, giving the ball a new surface area with which to spin the pin. I did the same for the other rolling pin.

Then I laid the pin-holding board back down where it had been attached. The pins contacted the ball just like before and were ready to get back to work. The cursor should go where I wanted it to.

As luck would have it, the cursor did go where it should when I spun the ball. With the track ball working again, the plotter functioned normally. No need to chuck it and buy another plotter and a bunch of new charts just to use as a backup to our primary display. I was glad my wife understood the processes, because when we really need a new plotter, she will know why. And I got a few points for fixing something.

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