repaired Nicole's CGEM mount

Fixed Nicole's CGEM. I'm convinced now that the issue was the motor assembly was not in the correct position. Or had moved since assembly. It's working fine now after my reassembly and check of the transfer gear meshing.

I began the inspection and disassembly last night. The first thing I noticed, while setting up, was that the tripod azimuth peg was loose. It must be tightened. Easy (low priority) fix.

I noted that the body and casing of the CGEM is such that you can NOT remove things and have a look-see. I wanted to visually inspect the worms and ring gears. There's no way to quickly do that. And I wondered where the motors were. On the Super Polaris and Great Polaris and the AVX, they are clearly outriggers near the worm gears. The CGEM does not have bumps there. Where are the motors? Joined the CGEM Yahoo!Group to get some ideas. Partly out of curiousity, I removed the control-port panel. There they are!

RA motor is on the left; DEC on the right. Wasn't sure what the red-sheathed harness was for. With a flashlight I could just make out brass (transfer) gears on the far side. Pleased to see metal brass gears on the motor assembly and beyond. In fact they reminded me of the brass gears on the SPC8. Now I understood the mechanics. Noticed the slotted holes in the mounting plate for adjusting the assembly's position...

The panel is secured with 4 Philips screws. All are quite short. The one at the bottom left was slightly longer.

While removing the panel, the first thing I spotted actually was the coin battery. A lithium CR 2025. That must preserve data... somewhere. Huh. Why the mount? Surely there's a battery in the hand controller, where the brains are. No matter. Just a data point in the tear-down. And a future heads-up, I guess, that this battery might need to be replaced.

I also noticed the white stuff! Ironically, Bill and I had been talking about that the night before. He had spotted this white residue inside his Celestron mount. I suspected it was residue from water, as in dew. Damn dew everywhere! Didn't think it would cause a problem. But I'll ask the electrical experts...

[ed. Update. Peter H thinks is it residue from solder flux. I didn't notice it until he mentioned it that the goo is near the through-the-hole soldering points and not around the surface mount parts...]

OK. Now I wanted to pull the motor out. I could inspect the gear carefully. First I marked the position of the bolts on the motor assembly plate. Then I popped open the car tool box! 5/32" hex on a long extension with the socket wrench driver. The flat washers were deformed slightly...

The brass gear is big. Larger than the ones on the SPC8. It looks scored and scratched on the face but that was just the grease. I immediately looked for broken or missing teeth. All there. I looked for flattened or rounded off or damaged teeth. They all looked perfectly fine to me. Hold the phone! I spotted a metal shard. About ¼" long! White metal... Aluminium? What the hell is that doing there? Where did that come from? From another gear?! From the housing? Sheesh. That's not good.

Looked at the motor system from the other direction.

Hey. What's that?! I spotted little pieces of copper filaments! A dozen or so. Like from a stranded copper wire. Perhaps that had been shredded or cut. Where'd that come from? Had a wire been too close to a gear and been cut into? Very strange.

Discovered what the red harness was for: the encoder sensor.

So, I was happy with the condition of the big brass gear. But I realised there was more to the motor assembly. I could see the output shaft into the gear was off-centre. The bit between the mounting plate and the motor (in the foreground of the above photo), the less shiny round cylinder with the seam (to the right of my thumb) must be the gear box, the transmission. I could see that when the plate was removed, I'd be able to access some small screws. I wondered if the plate and transmission were glued together.

Two grub screws with small hex heads hold the brass gear on the keyed output shaft. The encoder wheel, black plastic, rides on the gear, with slots for the grub screws.

Two small Philips screws (like the plate ones) hold the sensor to the plate. Three flat-head Philips screws hold the plate to the transmission. Two long small Philips hold the tranny to the motor. Gently opened the gear box.

All good! No broken teeth. No rounded teeth. No debris! Lightly oiled. Bearings looked OK. So, more good news. I was not happy to see plastic here but it looked to be in working order.

I checked the web for a replacement, if necessary. Didn't specifically find a motor assembly for the CGEM but the Celestron "motor assembly - RA - Advanced GT CG-5 series" with the product number "C-CG5-F00-2" looked to be an exact match. USD $132. I buttoned up the transmission. Now I was interested in powering the motor and taking it for a spin! It was at this point I noticed something else funny.

It hadn't fully registered as I was setting up the mount. I had looked at the metal screw-on connector attached to the mount panel when I unpacked the heavy mount and set it up. But it just didn't click at the time. That it shouldn't be there.

It finally dawned on my when I grabbed the DC car CLA adapter power cord and tried to plug it in. Oops. It looked like the plastic housing had been ripped off, pulled violently off, the metal connector. Instead of unscrewed. Oh oh. Where's my soldering iron.

That was last night, into the wee hours, and I was getting very tired.

§

Coffee brewing, I began phase two. First task: power the mount.

Retrieved a suitable female plug from one of my electronic parts bins. Grabbed the 3 amp CLA adapter from the TV in the basement (that's another story for a different blog).

Verified it is centre-pin positive. Triple confirmed. Checked with the digital multimeter.

Connected everything. Powered up. Yeh! Red LED. Hand controller showed "Verifying packages" and then went to a ready state. Huh? No message about missing the RA motor. Whatever. Powered off. Connected the motor and rested it inside the casing. Powered up. Tapped the Up and Down buttons and the DEC motor spun up. Played with the hand controller slew rate (which I noticed they've reworded as MOTOR SPEED). Set to a low rate and tapped the Left and Right buttons. The RA gear turned. Good. All working as it should. Huh? No message about the missing encoder. Whatever.

Rebuilt the motor assembly and remounted it in the casing. Removed the straight flat-head screw opposite the motor so to inspect the transfer gear mesh. Looked good. Tightened the motor assembly bolts. Now look at that...

The motor assembly was sitting lower now, according to my Sharpie pen marks, than when I received it! Maybe by 1 or 2 mm on the right side. Very telling. This makes sense. If the motor assembly was too high, the transfer gears might have hopped.

Was ready for a full test. Powered up. Slewed in RA at higher and higher speeds, working good, and then the other w—

It stopped! Gar. What happened? Oh. No power. OK. My adapter was not quite fitting correctly in the mount socket. Ha. Michael Swanson had warned of this on his NexStar Resource Site power page:

[The] plug for the scope end has an inside diameter of 2.1mm... Note that the more common connector is 2.5mm inside diameter. This will not provide a positive connection and leads to many problems with operation of the scope as momentary power interruptions cause misbehavior of the electronics of the scope.

Power it up again. Continued slewing in the same direction for a ways. Yes. Then started slewing in the other dir—

It stopped. What?! Power was OK. I backed up and tried again. Stopped. About 30° from horizontal. Oh no. A flat spot in the gears? Binding? Stiction? None of that made sense. What's this mean?! And then it hit me. Michael's note might say...

This will not provide a positive connection and leads to many problems with operation of the scope as momentary power interruptions cause misbehavior of the electronics AND MECHANICS of the scope.

I had reached a programmed stop point in the mount. The mount was trying to protect itself. To prevent physical collisions. The CGEM mount won't swing past a certain point to help reduce the chance of the OTA colliding with the tripod. When I had powered it up after the interruption, it messed up the home position.

Grabbed the "networking" tool box from the garage. Found an old phone extension cord. Plugged it into the AUX port. Wire-tied the power plug to the phone cord. OK. Now it was secure and I could try again without interruptions. Homed the mount. Powered up. Slewed fully to the east. The mount stopped on its own just past horizontal. I was expecting this now. The slew limit reached. Slewed fully to the west, 180° (or more) of movement. No hopping. No weird noises. Smooth movement. No binding. Stopped at the west limit.

I concluded the RA motor assembly was too high for Nicole. It had either been installed that way at the factory and she had just been lucky in terms of weight and balance and other conditions. Or the right hand bolt of the RA motor assembly was not tight enough and the motor had been gradually shifting upwards. Between shocks and vibration in assembly, disassembly, and transport, operating loads, maybe balance issues, it had worked it's way higher and higher until the transfer gears stopped meshing well. When she noticed the "jumpy" behaviour during her last observing session, the gears were starting to miss each other. When she tested it, as recorded in her video, without counterweights, in a very unbalanced state, the gears couldn't hold the weight.

And suddenly I realised where the aluminium ¼" shard came from! It's like a murder mystery. The motor assembly was rising too high in the casing. This made the motor transfer gear started hitting the inside of the casing! Must be very shallow clearances. It shaved off a piece!

Still no idea where the copper filaments are from...

Grabbed the camera tripod. And shot a couple of movies for Nicole. The main one showing successful slewing from home to east, east to west, and back to home. Both in AVI.

• repaired CGEM - part 1 - ready to test (83 MB)
• repaired CGEM - part 2 - the sequel! (131 MB)

Nicole is gonna be very happy, I thought.