Looking for a Video on How To Adjust Dec Belt Tension on an iOptron CEM70G - Forums

Topic starter

28 days ago

Checking Youtube, I was surprised when I saw how few videos there are on maintenance of the CEM70-series mounts.

PHD2 Guiding Assistant is suddenly reporting significant backlash in DEC (and I'm seeing it in my guiding as well). There is no mechanical backlash that I can feel. Nice and snug, no back and forth movement that I can hear, see, or feel.

The next step I am told is to check the DEC belt for tension. Anyone know of a good video they can share? If a picture is worth a thousand words, a good video is worth 100,000 words. :)

Thanks,

Tommy

Tom van Peer

28 days ago

You can also check the videos for the CEM40 or CEM26. Principle is the same, maybe a screw more or less to get the housing off, but you will get the idea. Like this, rather lengthy, video for the CEM40: https://youtu.be/fbLgkYgvH-w?si=8f8REE7xYSaJWB5d

I had the same symptom with my CEM26. I felt no backlash, inn the end it turned out the worm was sitting a bit too tight, so it was more like stiction than backlash.

Scott Badger

27 days ago

There’s this one, https://www.youtube.com/shorts/YjgDn9ZMsJE, but it doesn’t really show how it’s done. Do you have this document?
CEM70_Belt_Tension_ADJ.pdf

It doesn’t show how you actually increase the belt tension after loosening the three screws, probably because it’s pretty low tech….you just use a flat head screw driver to pry the motor away from the gear. I found, and I think it’s common, that you need to push it as far as it’ll go.

Also, one of the screws holding the cover on is pretty buried and a real PITA to re-install…..

Adjusting the gear meshing might help too. Those instructions are in the manual.

Cheers,
Scott

Tommy Mastro

Topic starter

27 days ago

Tom van Peer · Apr 19, 2026, 03:00 PM
You can also check the videos for the CEM40 or CEM26. Principle is the same, maybe a screw more or less to get the housing off, but you will get the idea. Like this, rather lengthy, video for the CEM40: https://youtu.be/fbLgkYgvH-w?si=8f8REE7xYSaJWB5d
I had the same symptom with my CEM26. I felt no backlash, inn the end it turned out the worm was sitting a bit too tight, so it was more like stiction than backlash.

Maybe I’ll try that first. The process only requires adjusting the two hex screws accessed from the outside (no covers to remove). I’ll back it off a smidge and see if that helps.

Tommy Mastro

Topic starter

27 days ago

Scott Badger · Apr 19, 2026, 10:01 PM
There’s this one, https://www.youtube.com/shorts/YjgDn9ZMsJE, but it doesn’t really show how it’s done. Do you have this document?
CEM70_Belt_Tension_ADJ.pdf
It doesn’t show how you actually increase the belt tension after loosening the three screws, probably because it’s pretty low tech….you just use a flat head screw driver to pry the motor away from the gear. I found, and I think it’s common, that you need to push it as far as it’ll go.
Also, one of the screws holding the cover on is pretty buried and a real PITA to re-install…..
Adjusting the gear meshing might help too. Those instructions are in the manual.
Cheers,
Scott

Thanks Scott. I do have that document. The issue is that the images are so close up, I’m not really sure what the arrows are pointing to. And to your point, the instruction says one screw is concealed but doesn’t explain how to access it (or maybe it does and I’m just not getting it).

I’m surprised with the popularity of this mount there aren’t more how-to videos. I have a dead hand controller and there is no troubleshooting videos for it. I’m just going to ship it to iOptron and let them figure it out. Hopefully they won’t rake me over the coals.

Tommy Mastro

Topic starter

27 days ago

Tommy Mastro · Apr 19, 2026, 10:11 PM
Tom van Peer · Apr 19, 2026, 03:00 PM
You can also check the videos for the CEM40 or CEM26. Principle is the same, maybe a screw more or less to get the housing off, but you will get the idea. Like this, rather lengthy, video for the CEM40: https://youtu.be/fbLgkYgvH-w?si=8f8REE7xYSaJWB5d

This was a cool video. This guy is brave. And he does it all bare-footed in a cramped little shop.

Scott Badger

27 days ago

Here are pics of the screw positions (please excuse the pollen and duct tape!…..). Note that the ‘hidden screw’ pic is with the mount slewed over to the east side, and taking the picture from the west side, so you’re looking at the bottom of the scope.

For better or for worse, I’ve had a fair amount of experience with their support, and though quality control is an issue, I’ve always found the technicians to be very helpful. I even spent a full day, 6:30 to 4:30, working remotely from their conference room (I live 2 ½ hours away) while their chief engineer spent his whole day chasing down a couple issues.

Cheers,
Scott

📷 screw 1.png📷 hidden screw.png📷 screws 3 and 4.png

Scott Badger

27 days ago

BTW, if you adjust the gear meshing, like the manual says, if you over-tighten the adjustment screw, you can easily damage the plunger, which in turn can create a stiction issue — part of why I sat in their conference room all day…. Anyhow, I wouldn’t go beyond barely snug.

Cheers,
Scott

Tommy Mastro

Topic starter

27 days ago

Wow, you’re the best Scott! I see it now. It all makes sense now!

I have it apart and the belt feels pretty good. I’m snugging it up ever so slightly.

Now more than ever I think the gear mesh might be too tight (stiction). I meant to try that first but I got excited and fast forwarded. :/

Tommy Mastro

Topic starter

27 days ago

Well that was relatively easy. I tightened the belt a smidge and backed off the gear mesh a smidge (like 1/10th to 1/8th of a turn).

And of course clouds rolled in so probably no testing tonight.

Mission accomplished. Thank you Scott and Tom!

Scott Badger

27 days ago

Post the results!

Hoping I built up some karma to appease my own sky gods tonight!…Ha!

Cheers,
Scott

Related discussions

(Worm Gear) Mount Advice

Recently had to do some maintenance to my EQ6-R Pro Mount due to guiding issues and thought I'd share for anyone who may be struggling with similar issues. ———— I’m using an EQ6-R Pro with an EdgeHD 8, OAG, and ASI2600MM. Mount is about 2 years old a...

Mount maintenance and guiding issues similar to your CEM70-series backlash problem.

Dec 16, 2025

Astrophotography on a Budget: My €341.12 ($399) DIY Harmonic Mount Review & PHD2 Test

V3 Mini Performance: My Results After Two Months of Testing (Solar & Deep-Sky) Hello everyone, Lately, my inbox has been flooded with questions about the performance of my V3 Mini mount. After more than two months of rigorous testing, I wanted to...

PHD2 guiding performance testing and mount-related astrophotography issues discussed.

Sep 11, 2025