[JA]

Hi Michael,

As far as diagnosing the root cause of the intermittent star trailing problem you are "LUCKY" in a way that the problem relates to an apparent intermittent, but recurring, lack of tracking. Why do I say "lucky"? Because you can dispense with the time consuming testing required with mount setup, aligning, guiding, imaging etc.... All you need to do is focus on the mount itself for the moment. Later when the cause is narrowed somewhat, and some changes potentially made, retest it for tracking using imaging.

We only need the mount, its power supply and means of starting its tracking. Perhaps a hand controller would be easier than the ASI Air which might inject more variables. Later we could retry to initiate tracking with the ASI Air to confirm.

Call this Home diagnostics 101 for your issue:
1. Remove the HEQ5 Pro side panel to expose the RA and DEC drive motors and gears.
2. Inspect the gear drive for anything untoward, excess wear, tooth problems, debris in the drive train,etc...
3. Check that the RA pinion gear and RA driven gear are locked tight on their drive shafts.
4. Check that the DEC pinion gear and DEC driven gear are locked tight on their drive shafts.
5. Check any backlash present.
6. Power the mount ON and go through the hand controller start up procedure and set TRACKING ON.
7. FOCUS your attention very closely on the RA pinion gear. Based on Skywatcher's specified 705:1 gear ratio I would expect the RA motor pinion to make 705 revolutions per 24h in which time the RA axis will revolve once. If we factor that down further the RA pinion gear will move ~29 revs per hour or ~0.5 revs per minute or ~3Degrees every second. And that's the kicker. We know from previous tests that the mount looses tracking for about a second every few minutes. I think that , although difficult, and perhaps with a magnifier or video you should be able to see the RA motor pinion STOP for about a second every 2-3 minutes as per your previous findings. Bear in mind the RA motor pinion is, I'm guessing, something like 10mm in diameter then if it stops for ~ 1 second a point on its circumference will not have moved the requisite ~0.3mm. That's the size of the change you should focus on, hence possibly use a magnifier.
8. Observe whether the issue also exists with the DEC pinion as per step 7.
9. If you have been able to see the fault as described (the intermittent stopping of the tracking), then you are lucky because you now have a test bed on which you can make changes to help you search for the root cause. Try different power supplies, reseating any motor connectors You need to remove another cover for that). If you are electrically inclined or have a friend who is you could look at the motor drive voltage/current and see what happens to it during a stoppage of drive. Also check the power supply voltage as a function of time. A storage oscilloscope function would be great for those tests. I could imagine that the mount crashing in to the tripod may have overloaded the drive electronics and potentially damaged the output drive, fried an output transistor(?) depending on the circuit protection.(???)
10. Retest above with ASI Air control of tracking ON to the mount incase somehow there is an intermittency to the cable????

The fact that this loss of tracking is presumably occurring in RA and DEC suggests a common (probably electronic) cause in the drive circuitry or problems with the internal mount power or power supply.

Best
JA