FIXING ETX-90 ENCODER OUT OF POSITION
Last updated: 27 December 2012
Sent: Tuesday, December 25, 2012 09:54:18 From: W3DX@aol.com (W3DX@aol.com) I'm reporting the resolution of a problem with an ETX90PE, in case you want to use it on your website. I purchased an ETX90PE fork so that I would have spare parts for my trusty scope, since Meade no longer provides parts for that model. The fork was represented to me (very fairly) as being non-working with some sort of RA motor fault. The symptoms were as follows: 1) During auto align, after leveling, the scope would start rotating in RA and would not stop. A "reset" on the AutoStar did not resolve the problem. 2) Even with the non-computerized ETX90EC handbox (the one with the four lights), the RA always moved at high speed, even when set to the slower speeds, while the DEC appropriately changed speeds. Further, pressing the RA control resulted in random movements right and left. When I opened the instrument, there were no loose wires apparent inside the mount. I found the ground wire to the battery box had been broken (easy to fix). And I noticed a curious thing: the beam from the optical detector device did not seem to pass through the encoder. The encoder appeared either too small or out of position to break the beam of the optical detector. This can be seen in the first two photos below.
To fix the problem, I considered a few options: 1) Find a bigger encoder (not likely) 2) Make a bigger encoder disk out of paper, and paste it over the encoder (challenging) 3) Adjust the motor assembly closer to the optical detector (the motor assembly is fixed), or 4) Adjust the PC board with the optical detector closer to the encoder and motor assembly (doable). I decided on option 4 as the most feasible. I reasoned that I could remove the mounting screw nearest to the optical encoder, and put a shim in place to do the job, but I wasn't sure if the PC board had enough flexure. If that failed, I thought I might remove two of the three mounting screws, and rotate the PC board into position. I began by removing the screw closest to the optical encoder. Voila! Once the screw came out, the PC board, having been under tension, immediately moved into place without the need for further shimming. The third photo shows the resolution of the problem.
The screw can be seen as being removed (center of photo, to the left of the encoder), and the encoder and optical detector are now in position. If you look closely, you can see that the PC Board hole is not quite concentric over the plastic socket in the base of the telescope. The board shifted (slightly) down and to the right. I tested the non-computerized handbox, and it worked fine. Changing RA speeds resulted in appropriate change of speed in the RA motion of the telescope. Then I tested the AutoStar 497. Still not working perfectly. A change in speed on the controller did not result in a change of speed of the telescope. I reasoned that this must be an AutoStar problem, not a hardware problem, because the non-computerized handbox was working. If it was hardware, both handboxes should present the same problem. So I did a simple AutoStar "Reset" in the menu system, and the problem was completely resolved. While the mount was open, I resoldered the black ground wire to the battery box. From there, the ETX90PE properly ran through its paces and was able to autoalign and perform all other functions properly. Clear skies, Rob Capon, Stillhouse Mountain Observatory
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