Mount issue or power issue

LewisM · #1 08-07-2015, 08:04 AM

I set up last night for imaging, and when I had the heaters and mount plugged in to the 12 V DC battery, the battery pack started chirping, indicating it was near drained. No problem - I thought - and plugged it into the charger. It stopped chirping, and everything seemed to be running just fine.

Alignment went well, but then when slewed to target, I noticed RA was still drifting after GOTO (and GOTO had beeped, indicating completion). Thinks I, not backlash, as I had recently readjusted, probably a little too tight in RA if anything. Anyway, it settled soon enough, and I got 90 minutes on target with good tracking.

I then slew to M8/M20, which was almost at zenith at the time. The RA issue returned, this time coupled with the hand controller intermittently flashing like a loose power cable. I tightened all the power connections, just in case a slew had loosened one, and readjusted counterweight bias slightly to load the gears, but no good. PHD couldn't calibrate, and the star drift started increasing in speed.

I tried recentreing target. RA started, then the hand control started dimming again and RA ground to a sudden halt and wouldn't go. I unlocked clutches and put it back in park, and tried RA and Dec movement with the hand controller - Dec seemed normal, but RA started normally again, then stopped. Tried again, and it didn't even try - the hand controller almost went off. Turned it all off and went toprocess what I had gotten, and figure out the issue in the morning.

Now, as I mentioned, I HAD readjusted RA and Dec recently. I had eliminated all backlash in both axes, but the RA seemed to every so slightly bind in a couple spots. When I had adjusted it, I ran the motors full circle (loaded), and no NOTICEABLE binding or even change in motor note.

So, I am hoping to eliminate mechanical, and hope it was merely electrical. Would it be reasonable to suggest that RA required MORE grunt seeing it not only has to rotate on it's axis, but has ALL the weight to push, whereas Dec only has to push the telescope (and NOT the mass and inertia of the counterweights) and SHOULD be less loading on the motor. Therefore, I assume the RA motor is going to draw more from the circuit, and if that circuit is compromised by a low power supply, will show as a problem? (slewing in Dec during the error assessment was NORMAl, perhaps slower than usual, but normal)

I really feel that even though perhaps RA is a TOUCH tight (and I don't really want to fiddle with it again seeing PA is SPOT ON), that the fault lies with the power. I will try a spare battery supply today before tonight, but will also try my regulated AC-DC converter.

Appreciate any thoughts

John0z (John) · #2 08-07-2015, 08:19 AM

If the RA axis is tight, presumably the RA motor would have to work harder in order to move it. Generally though, this should not make a lot of difference if the RA axis is reasonably balanced. When pointing to zenith, perhaps balance might be the problem, hence the RA has to work even harder?

Second point - if the battery is low, and you are essentially running off the battery charger - could the charger be unable to supply the current needed to drive the entire setup? I would connect a multimeter in series to measure the current that is drawn when everything is running. If you are drawing 2 amps and the battery charger is charging at 1 amp, then 1 amp must be coming from the battery which if low, means that the voltage will also be down.

LewisM · #3 08-07-2015, 08:23 AM

That is my assessment too John.

Perhaps I should slacken the RA mesh a touch, but like I mentioned, there is NIL backlash, so not sure I want to!!!

Anyway, off to Jaycar later to sort the issue. Thank goodness my wife presented me a $500 Jaycar giftcard for Christmas last year!

John0z (John) · #4 08-07-2015, 08:29 AM

Can you check the balance of the system when pointing at zenith? I would also put a meter in series to measure current anyway, so you can see how much it is taking when pointing somewhere, then move to zenith and see if the current drawn increases. I haven't use dew heaters, but every heater does take power, and maybe that was causing some of the problem for the battery pack.

I have used the cheap 7AH batteries with my Losmandy GM8 and they are ok for a few hours - I haven't tried running it until it goes flat though. But I recently changed batteries in one of my UPS's and ended up with two 17AH batteries that were still good which I now use.

Shiraz (Ray) · #5 08-07-2015, 09:19 AM

my guess is that the RA is too tight and is binding. You can run it at high speed and it will work even if slightly over-tight, but at low speed the added load of stiction starting to appear will stop it. You always need at least some backlash.

the power supply would have been labouring as the motors drew extra current to try to overcome the extra load. If you put on a super heavy duty power supply, you may risk damaging the motors or drive circuits.

LewisM · #6 08-07-2015, 09:20 AM

Just confirmed diagnosis.

Plugged the spare battery into the mount. Loaded just like last night. NO problem anywhere.

Went inside and unplugged the recalcitrant one from the charger where it had been since 11pm. Didn't recharge. So, that pack is dead. Might keep it just as a shop light - that at least seems to work. Don't have a multimeter to check what it is outputting (will buy one today), but I doubt it is mount-sufficient, especially with those big resistors we call dew heaters!

I have slackened RA a touch too.