Celestron 127SLT mount slew issue

[jeffwjz]

I have a Celestron 127SLT with its computerized Alt-Az mount, I think this is the same mount for a series of small Celestron scopes like 102SLT, 114LCM etc. I used the mount for another scope of mine, but this scope is a bit heavier and may have overloaded the mount.

Recently when I was trying the mount out I noticed that the slew is acting weird. When I select the slew rate between 6 - 10 the Alt and Az axis can move with no problem. However when I select any slew rate between 1 - 5, both axis are unable to move. I took the mount apart and examined its components, both motors are working normally - the motor speed changes when I adjust slew rate between 1 - 9. I found that it is the gear box between dual axis motors and Alt/Az rotating gears that is faulty. At slew rate 6 - 9, the out shaft of gearbox rotates with motor rotation, but at rate 1 - 5 the gearbox shaft stops rotating even if the motors are still running.

I suppose this suggests something wrong in the gearboxes. How can I fix it? Any ideas?

Thank you for reading.

[jeffwjz]

No replies?

[Cosmic Geoff]

I don't know the answer, but it does not sound like a motor/gearbox problem. There is no mechanical reason why the gearboxes should run at some (software-controlled) speeds and not others.

At motor rate 1 to 5 the movement will be too slow to see unless you are looking in the eyepiece. I have never, ever, selected rates 1 and 2.  There may be backlash in the gearboxes which means that if you change direction nothing will move at the output till the cogs all catch up with the motor. The slower the rate, the more pronounced the effect of backlash will be.

The motor/gearbox assemblies for Alt and Az are identical so you could swap them over if you think it is going to tell you anything. (it probably won't).

If the above notes don't help, I suggest you check that you have an adequate power supply and that the power plug is making proper contact.   Then check the motor control firmware. Celestron offer a program that allows you to flash the handset and motor control firmwares, and there is another one that is just for the motor control and allows you to verify the motor control firmware chips and also flash them with new firmware.

The exact procedures are too involved for me to detail here.

Sorry I don't recall the exact names of these things (I can look on another PC if nobody replies), but the 'Nextarsite' should point you in the right direction.

Edited June 13, 2019 by Cosmic Geoff

[jeffwjz]

On 13/06/2019 at 10:58, Cosmic Geoff said:

I don't know the answer, but it does not sound like a motor/gearbox problem. There is no mechanical reason why the gearboxes should run at some (software-controlled) speeds and not others.

At motor rate 1 to 5 the movement will be too slow to see unless you are looking in the eyepiece. I have never, ever, selected rates 1 and 2.  There may be backlash in the gearboxes which means that if you change direction nothing will move at the output till the cogs all catch up with the motor. The slower the rate, the more pronounced the effect of backlash will be.

The motor/gearbox assemblies for Alt and Az are identical so you could swap them over if you think it is going to tell you anything. (it probably won't).

If the above notes don't help, I suggest you check that you have an adequate power supply and that the power plug is making proper contact.   Then check the motor control firmware. Celestron offer a program that allows you to flash the handset and motor control firmwares, and there is another one that is just for the motor control and allows you to verify the motor control firmware chips and also flash them with new firmware.

The exact procedures are too involved for me to detail here.

Sorry I don't recall the exact names of these things (I can look on another PC if nobody replies), but the 'Nextarsite' should point you in the right direction.

Hi Geoff,

Thank you for your reply. I understand all your points and I have actually tested against the possibilities you mentioned.

I understand that speed 1 - 5 can be very slow, however what I did is to disassembled the motor and gearbox and check their rotation directly. What I noticed was that at speed 6, I can see the gearbox output shaft is indeed rotating, although slowly. But when I change to speed 5 or lower the shaft stopped completely while the motor's encoder is still running, suggesting the motor shaft is still rotating. So I guess maybe there is some coupling issue in the gearbox.

Also it's very strange that the same condition happened to both RA and DEC motors. I don't think this is firmware related as I can see the motors are running as per demand. I'm really confused.

I have emailed Celestron and I'll see if they can pinpoint any causes.

[michael8554]

12 hours ago, jeffwjz said:

the shaft stopped completely while the motor's encoder is still running,

I would guess that the encoders are geared up to run faster, so you may still not be able to detect movement of the shaft.

Make a mark on the shaft with a Sharpie pen and you might perceive movement after a while. 

Is Rate 6 too fast for fine adjustment? 

Michael