I have an AZ-EQ5 that I've used maybe a dozen times, always in EQ mode. While its tracking is an improvement over my previous ALT-AZ mount I haven't been able to get exposures longer than about 90 seconds without oval stars. My scope has a 1250mm focal length and I know that exacerbates the problem. I recently started using PHD guiding and the tracking graph looked pretty awful and I could feel a bit of backlash in the RA axis, so I thought I'd go through the backlash adjustments / gear cleaning procedures that are shown in some posts and on youtube.
One thing I found is that with the motor removed so that I can spin the worm gear easily by hand there is a very noticeable 'rumble' as the counterweight shaft moves toward pointing straight down, from either side of the pier. Moving it up is smooth, but moving down is not. Pushing down on the shaft as it moves doesn't stop the rumble. It does it both before and after cleaning the gears and adjusting the backlash, and whether I spin the worm gear fast or slow. After reassembling the mount I can't tell if it's doing it while tracking since the movement is so slow and increments so small, but I suspect that it is.
Has anyone seen this before, or have any advice on how to cure it?