GOTO AZ motor slipping?

[Kon]

The current diagnostics are that removing the cable from the board to the stepper the motor runs fast; with the cable back on it's running at low speed as expected. I suppose it rules out a faulty stepper. Unless it's not giving enough torque, partly faulty?

I tightened the central bolt to the point that it was very firm base but no tracking.

What I can't get my head around is, why the wormwheel and brass turn but not the large metal disc. Where else should I be looking? I took the whole clutch system off to check but nothing. 

Any other suggestions?

[michael8554]

Have you tried with a better power supply yet ?

But Geoff mentions that this is a stepper motor.

Skywatcher describe it as a "DC Servo motor".

As I understand it, a DC Servo motor only has a single pulse drive and feedback loop from the encoder, unlike a stepper motor, which has two phased inputs.

Could be wrong.

Although a pulsed signal, you might compare the voltages with a cheap Digital Voltmeter at slew and tracking rates.

Can you swap with the Alt motor ?

Michael

[Geoff Lister]

26 minutes ago, michael8554 said:

But Geoff mentions that this is a stepper motor.

I had assumed a stepper motor, but it may well be a straight DC motor.

Geoff

[Kon]

40 minutes ago, michael8554 said:

Have you tried with a better power supply yet ?

But Geoff mentions that this is a stepper motor.

Skywatcher describe it as a "DC Servo motor".

As I understand it, a DC Servo motor only has a single pulse drive and feedback loop from the encoder, unlike a stepper motor, which has two phased inputs.

Could be wrong.

Although a pulsed signal, you might compare the voltages with a cheap Digital Voltmeter at slew and tracking rates.

Can you swap with the Alt motor ?

Michael

I am not an electronics expert, so looking online it was described as stepper but I could well be wrong.

I thought of swapping the motors around but as my Alt is perfect, do I mess with it? But it will be a good diagnostic.

Yes I changed the power supply, not as fancy as the one you suggested but I will invest on very good one.

A voltmeter would be good. I assume I should have similar power?

[Peter Drew]

Referring back to the original photo there is probably another clutch between the motor and the worm shaft, this is to protect the motor should something restrict the rotation of the wormwheel.  It is the bushing next to the motor output.  There are two rows of grub screws forming the connection, one set possibly fixed to the motor shaft and the other connecting the worm shaft with some facility for slippage in the event of a problem.  If this is the case it is another area for unwanted drive variation.      🙂 

[Kon]

13 hours ago, michael8554 said:

Can you swap with the Alt motor ?

So I swapped the encoder and the problem persists; if it is the correct part (the green chip in this photo). I did not want to mess more with the Alt as it is tracking very well. It seems to be a faulty motor. I will look into sourcing a new one (I have not found anything yet). I might start a new post for a motor 🥲

Edited April 20 by Kon

[michael8554]

"A voltmeter would be good. I assume I should have similar power?"

The voltmeter on DC Volts range may show the average voltage of the pulses when tracking.

I'd guess the motor is powered by the red and black wires.

The board may have the encoder and feedback voltages.

Compare with the Dec motor when tracking.

Michael

[Geoff Lister]

I measured the current requirements for some of my mounts. This was with a DVM, and would be an average value.

I tend to use a 12V, 1.5A or 2A plug-top mains supply, particularly with the Skyliner 250.

Geoff

[Kon]

I looked at both axis motors and it seems the az motor is faulty; no electrical but the output shaft has a lot of play compared to the alt. It seems I need to source a new one.

[Cosmic Geoff]

While I have not examined one of these mounts, I am surprised that the azimuth motor should have developed significant bearing wear. My experience of small motors is that this doesn't happen.  If it has, one might expect that under load it would fail to engage with the gearbox at all.   If it IS a motor fault, getting another might prove troublesome and if you can't definitely identify it as a generic motor you can buy on Ebay or from China, your only other recourse is to buy another subassembly or scrap mount.  Or find a craftsman prepared to fit another bearing bush to your motor.

A useful means of diagnostic where the altitude and azimuth drives (etc) have similar parts is to swap bits over and see if the fault follows the part.   The altitude  and azimuth motor/gearboxes in the Celestron SLT mount are the same (same part number) and I swapped them over while chasing a fault. (It wasn't in the motors).

If you attach tell-tales (bits of paper or magic marker marks) to slow moving parts, you can see if anything is moving (slowly) or not.

[Kon]

12 hours ago, Cosmic Geoff said:

While I have not examined one of these mounts, I am surprised that the azimuth motor should have developed significant bearing wear. My experience of small motors is that this doesn't happen.  If it has, one might expect that under load it would fail to engage with the gearbox at all.   If it IS a motor fault, getting another might prove troublesome and if you can't definitely identify it as a generic motor you can buy on Ebay or from China, your only other recourse is to buy another subassembly or scrap mount.  Or find a craftsman prepared to fit another bearing bush to your motor.

A useful means of diagnostic where the altitude and azimuth drives (etc) have similar parts is to swap bits over and see if the fault follows the part.   The altitude  and azimuth motor/gearboxes in the Celestron SLT mount are the same (same part number) and I swapped them over while chasing a fault. (It wasn't in the motors).

If you attach tell-tales (bits of paper or magic marker marks) to slow moving parts, you can see if anything is moving (slowly) or not.

Thanks for the suggestions Geoff. I swapped the encoder and gears and the problem persisted. I didn't swap the motor but I noticed that the shaft of the azimuth motor has a lot of play, going back forwards when I press it. The alt one doesn't seem to have it. I also noticed the az makes a squeaky noise (even without gears) whereas the alt is quite, thus why I thought it is the motor. If the PCB has a fault I would expect the motor control to be dead? The wormwheel and brass turn extremely slowly but no transfer of movement to the large disc so the motor is engaging with the gears ok. I checked and tightened for slippage. I am running out of ideas what to check next.

[bosun21]

On 22/04/2024 at 22:02, Kon said:

I looked at both axis motors and it seems the az motor is faulty; no electrical but the output shaft has a lot of play compared to the alt. It seems I need to source a new one.

Kostas have you checked all the relevant grub screws and Allen screws which position and hold the various plates of the clutch assembly? The video posted earlier in the thread shows these various screws and where they are located. I ask this due to your description that the brass worm gear is turning and it is tensioned against the teeth on the drive plate yet it's not moving the axis. To me it appears that the clutch is slipping and this could be due to the screws mentioned. Tightening the main tensioning nut on top of the assembly will have no effect due to this. I would watch the video and check the screws on yours at each disassembly stage. Good luck 🤞.

[Kon]

6 hours ago, bosun21 said:

Kostas have you checked all the relevant grub screws and Allen screws which position and hold the various plates of the clutch assembly? The video posted earlier in the thread shows these various screws and where they are located. I ask this due to your description that the brass worm gear is turning and it is tensioned against the teeth on the drive plate yet it's not moving the axis. To me it appears that the clutch is slipping and this could be due to the screws mentioned. Tightening the main tensioning nut on top of the assembly will have no effect due to this. I would watch the video and check the screws on yours at each disassembly stage. Good luck 🤞.

Thanks Ian. I will try again tonight with the clutches. I found a replacement motor with encoder etc, new, but I don't want to fork £100 if I am missing something with the clutch etc.

[bosun21]

36 minutes ago, Kon said:

Thanks Ian. I will try again tonight with the clutches. I found a replacement motor with encoder etc, new, but I don't want to fork £100 if I am missing something with the clutch etc.

Watch the video and disassemble it in stages like he did and check the various sets of securing screws. Good luck.

[Kon]

20 minutes ago, bosun21 said:

Watch the video and disassemble it in stages like he did and check the various sets of securing screws. Good luck.

Done it a couple of times already but I will try again.