Just thought that I would contribute my experiences with a similar problem with my iOptron CEM40. Initially I noticed a periodic error in guiding with a deterioration every 750 secs. I suspected the belts so tightened up the tension but the problem persisted. Next the calibration started to fail with similar issues to those described here. I could still get away with guiding but later the calibration failure was what forced me to take the belts off and closely inspect them. They looked perfect from outside but on inspection some of the teeth were flattened and the fibres of the reinforcing were starting to fray. The mount is about 18 month old with medium use and no pier strikes etc.
I sourced a replacement belt locally (not an ioptron one but these belts are standard ones) and everything now works perfectly.
I also did firmware updates when the problems started but the only thing that made a difference was replacing the belt. So I think that Jasem's comment about considering hardware/mechanical causes is appropriate.
I was suprised that the belts would need replacing after such a short time but maybe they are a weak point in these mounts and will need replacing regularly. The replacement process is pretty simple and are illustrated in pdfs available on request from ioptron and I think you can find them though some of the US and UK astronomy forums.

Cheers

Mike

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