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Vixen GPDX rebuild...


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Having spent a bit of time recently building an imaging rig around a Samyang 135mm lens, I tried to test it in the field last night - and fell at the first hurdle! My (formerly) trusty GPDX kept stalling in RA and simply would not slew. About 2 years ago I had fitted a Skywatcher EQ5 upgrade kit to it and since then it had been running and guiding pretty well under EQMOD.

Quite why it decided to play up now was a mystery. It had last been used a month or so ago to handle a C9.25 in cold and damp conditions with no issues, so I didn’t think that a colder temperature had caused the problem.

The RA axis seemed really stiff, even with the clutch off. So this morning I took the motor and worm gear off, unscrewed the polar finder and tried to loosen the RA bearing retaining ring. This is locked in place with 3 x 2mm hex plugs accessible through the hole where the RA setting circle ring lock goes. Disconcertingly, these were quite loose already, yet I still couldn’t shift the retaining ring. Eventually I managed to get it off using a pair of circlip pliers, though I did make a little bit of a mess of it as the retaining ring appears to be made of aluminium.

After a bit of persuasion with a rubber mallet, I managed to get the RA bearing out. All of the bronze machined surfaces looked fine, thankfully, so I took the opportunity to clean everything up and regreased with some decent CNC lithium grease.

I think what had happened was that the loose locking plugs on the bearing retaining ring had allowed it to tighten itself up to the point where it pulled the bearing up and seized it. Once back together, the whole thing moved a lot more freely. I retightened the bearing retaining ring (which only seemed to need to be finger-tight to keep the bearing solidly in place whilst retaining free movement) and made sure the locking plugs were firmly tightened down this time.

I refitted the RA worm gear and spent quite a while adjusting it to minimise backlash whilst allowing the drive wheel itself to be rotated by finger pressure. I then refitted the drive motor and bingo, it seems to slew smoothly - and indeed, it seems a bit quieter than it was before. There’s a bit of backlash but it doesn’t seem too bad.

The proof will be how the autoguiding behaves, but at a 135mm focal length it doesn’t need to be brilliant, and I think I’d rather have ropey guiding than a stalling RA motor!

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