LewisM
03-09-2013, 06:08 PM
Since I have owned a Vixen GPD2 pretty much from new (original owner used it a handful of times), the RA axis has always been stiff, and the Dec head showed only MINOR sideways movement (with a scope on) and a little gloopy. The RA issue was becoming worse, as I could no longer get an accurate balance - it was that stiff. I HAD to do something, so today, I disassembled the head.
The Dec was very easy to fix - I disassembled the motor (I have a Synta GOTO kit on it), and then took off the Dec worm (4 allen bolts). It was festooned in exceedingly sticky, THICK grease - worse than the stuff Synta usually uses. It REALLY showed a lot of resistance with this stuff on it, and around the shaft. Degreased it, regreased with premium lithium-PTFE grease.. Kept going, and took the saddle off, and degreased the dec gear carefully, and regreased that too. Reassembled itall, and the improvement was SIGNIFICANT.
Now to the RA axis, and this involved unscrewing the polar scope, then removing the setting circle scale. I removed the setting circle locking screw, to access the axis nut's 3 locking grub screws - I didn't have to loosen them - they were LOSE! So, it seemed that with no locking, the shaft nut was tightening itself with the rotation of the RA axis. I modified a set of circlip pliers, and backed it off a minuscule amount (less than 1'32nd of a turn off). The RA axis almost breathed a sigh of relief - it now is nice and free - not loose like the Synta knock off clones, but SO much better! So, time to reassemble (I didn't feel like the ardour of taking the RA motor assembly off with it's very delicate circuitry just to degrease and regrease the worm and gear there). Locktited the grub screws AND the nut into the position it is currently in :)
Reinstalled the RA setting circle, and rechecked - stiffened up again. Hmmm. Degreased the inner ring of the circle, and regreased - not much difference. So, it seems the RA setting circle scale is binding somewhat with the axis nut and possibly bottoming out abutting the RA tailstock. So, I have left it all off (never used the circles or the polar scope anyway). Will have a mate turn up an aluminium cover for the back end (I know it will be cheaper than the legit GP cover :) )
Plugged it in, and tested it (after getting a nice mesh with the dec motor), and a MAJOR MAJOR difference. Put back the counterweights (to pre-marked positions) and the scope how it was "balanced" last time out. Not even close to balanced :( So, last time out was so grossly unbalanced - no wonder PHD threw hissy fits all night at me and failed guiding after 2 minutes.
So, fingers crossed, back to 20 minute exposure round stars again, with PHD making MINUTE corrections, just like it used to be. May even try my luck unguided - it's done it before without much hassle.
The Dec was very easy to fix - I disassembled the motor (I have a Synta GOTO kit on it), and then took off the Dec worm (4 allen bolts). It was festooned in exceedingly sticky, THICK grease - worse than the stuff Synta usually uses. It REALLY showed a lot of resistance with this stuff on it, and around the shaft. Degreased it, regreased with premium lithium-PTFE grease.. Kept going, and took the saddle off, and degreased the dec gear carefully, and regreased that too. Reassembled itall, and the improvement was SIGNIFICANT.
Now to the RA axis, and this involved unscrewing the polar scope, then removing the setting circle scale. I removed the setting circle locking screw, to access the axis nut's 3 locking grub screws - I didn't have to loosen them - they were LOSE! So, it seemed that with no locking, the shaft nut was tightening itself with the rotation of the RA axis. I modified a set of circlip pliers, and backed it off a minuscule amount (less than 1'32nd of a turn off). The RA axis almost breathed a sigh of relief - it now is nice and free - not loose like the Synta knock off clones, but SO much better! So, time to reassemble (I didn't feel like the ardour of taking the RA motor assembly off with it's very delicate circuitry just to degrease and regrease the worm and gear there). Locktited the grub screws AND the nut into the position it is currently in :)
Reinstalled the RA setting circle, and rechecked - stiffened up again. Hmmm. Degreased the inner ring of the circle, and regreased - not much difference. So, it seems the RA setting circle scale is binding somewhat with the axis nut and possibly bottoming out abutting the RA tailstock. So, I have left it all off (never used the circles or the polar scope anyway). Will have a mate turn up an aluminium cover for the back end (I know it will be cheaper than the legit GP cover :) )
Plugged it in, and tested it (after getting a nice mesh with the dec motor), and a MAJOR MAJOR difference. Put back the counterweights (to pre-marked positions) and the scope how it was "balanced" last time out. Not even close to balanced :( So, last time out was so grossly unbalanced - no wonder PHD threw hissy fits all night at me and failed guiding after 2 minutes.
So, fingers crossed, back to 20 minute exposure round stars again, with PHD making MINUTE corrections, just like it used to be. May even try my luck unguided - it's done it before without much hassle.