LX200 major dec fix!

Jun 6, 2003

Enough is enough…

I’ve resorted to drastic measures with my 10” LX200.

The beginning of the saga took place last year when I stepped away from the computer to find my LX200 strangling itself with its cords. The fuse on the motherboard that is susposed to blow during such happenings, didn’t. The result was a fried motherboard and a call to Meade for the $200.00 replacement kit, which included all electronic components and drives (not a bad deal, incidently).

BTW, replacing the 2-amp fuse on the motherboard with a 1-amp fuse is NOT a bad idea.

Installation of new motherboard was simple. It was the drives, especially the declination drive, that posed a problem. For those faint of heart, unwilling to dig into the guts of their fancy SCT, you should know that it takes a lot of experimentation trying to get the drives working again in such a way that the motors are not stressed. It requires precise positioning of the worm drive against the gear, then appropriate lubrication.

To make a long story short, I’ve had problems with the drive ever since.  Sometimes the two alien bolts that hold on the drive works themselves loose, a sign of two much play between the gear and drive. Sometimes the drive binds up because there is not enough play.

Most recently, the drive has not responded to guiding corrections sent from my autoguider, first with the Meade 208xt and then the SBIG ST-7e.  Handpad movement in the slowest speed works intermittently. Not good.

Therefore, I began a complete shakedown of my LX200, a “supercharge” of sorts. Not only have I taken all drives and gears apart for a degrease and re-lube, I’ve decided to do a major fix of the declination system, as seen here in the MAPUG archives, which includes a complete bearing replacement.

Like I said, NOT for the faint of heart. But an expensive telescope that doesn’t allow me to take pictures isn’t of much use to me. Afterall, if I want a scope solely for visual use, I’d just borrow one of the club’s dobs.

The bearing replacement is the tricky part. It involves complete removal of the forkarms from both the base and the OTA. That includes removal of dec clutch assembly, whose rear plate is held on with Loctite. Once I crack the two bolts holding on the plate, the arms and OTA can be pulled apart.

This reveals the posts on the OTA, each covered with a thrust bearing, a couple of thrust washers, and a rather silly nylon “bearing,” shown in picture.  This allows the OTA to spin in the hole in the fork arms. This nylon bearing is the source of much friction, which is not good when you need your dec drive to work as stress-free as possible. When replaced by needle roller bearings, a near frictionless movement in declination is possible.

But replacing the nylons bearings requires re-boring the holes in the fork arms to accommodate the larger outside diameter of the needle bearings.  This type of precision will require more tools and expertese than I have available at the house so I’ve taken the fork arms and bearings to a local machinist. Hopefully in a week, and a hundred bucks later, I’ll have the fork arms back ready for reassembly, which will include regreasing and installation of the declination drive.

One thing about the declination drive. LX200 owners are probably aware of the amount of backlash and hesitation in the dec drive. This occurs because of the amount of play, or movement, between the worm and the gear. If the worm is too loose, the gear has to take up more slack before the drive can engage the gear. If the worm is too tight, the gear can either cause tremendous stress on the whole system or even bind completely.  So any excessive movement in the system adds to the backlash, which incidently can confuse a CCD guider completely.

While watching the drive work prior to disassembly, I noticed that there is a small rubber “O”-ring between the dec gears and the worm. When I engaged the drive with the keypad, this joint would compress, adding movement to the system that should NOT be there. Taking a hint from the MAPUG archives, I decided to pack epoxy resin around the “O”-ring.  The resin should harden enough to make the joist immovable.

As a result, when I get the fork arms back and I am able to reassemble the scope, the OTA should move in the fork arms like a free swinging pendulum (once the Dec clutch is disengaged) and the regreased Dec drive should have very little excess movement.

Oh, I know what you are thinking. Don’t worry! Re-aligning the forks and positioning the OTA isn’t that difficult…I’ve had tons of practice! Incidently, poor GOTO performance is often symptomatic of poor alignments of either the fork arms and/or the OTA, that is, as long as your scope is level and properly aligned in the first place.

Anyway, I’ll post the results of my MAJOR scope fix right here in a couple of weeks!