Tuesday, December 04, 2007

Repairing the Meade LX200 RA Drive

Finally, we've gotten some good, dark steady skies. Just in time for the 10" LX200 Classic to go belly-up. Well, maybe not belly-up, but it wasn't happy. At start up, when the RA drive is turning to align the worm gear, I was getting an awful "clackety-clackety" sound. Then once the drive setteled into RA tracking, it would skip or hiccup every 7 to 9 seconds.

A couple of months ago I had disassembled the entire RA assembly and re-lubricated it. I had been getting a high-speed vibration while tracking that I was attributing to "old grease." (Now, I'm not so sure that was the problem, but it's nice to have the worm and RA gear re-lubed anyway.) When I heard the noise, my first thought was that I had somehow made a mistake when I reassembled everything. Didn't seem likely though, as the scope had been running just fine. Then it occurred to me--this latest problem only showed up after the weather had gotten cold. Just the same, I removed and reinstalled the RA motor/gearbox assembly. No change. But blowing the hair dryer on it for a few minutes would cure it! Auugh.

So it's off to the internet and the Meade Advanced Products User Groups (MAPUG) archives, where I find a description of exactly what I was experiencing. Hooray! The fix? Adjusting two potentiometers on the motor control board. "No problem!" I thought. Then I read where the job requires a dual-channel oscilloscope. Drats.

But I kept reading. A little further down the page I found a link to Bruce Johnston's Astronomy Pages, and his instructions on repairing the LX200 drives using just a multimeter. Hey, I have one of those!

A short trip to Radio Shack for a few bucks of supplies, and I was off. I was getting pretty good at removing the motor assembly by now, and soon I had it sitting on the kitchen table. I followed Bruce's instructions and connected two batteries and the multimeter, and sure enough, the numbers were out of whack. Adjusting the two pots was simple, and after disconnecting all the wires, I reassembled everything back in the base of the scope. Guess what?

It works! Just as soon as I finish this, I'm off to the observatory to attempt making this year's first Mars image. Wish me luck.

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