Last updated: 8 March 2004
Date: 2/29/04, 19:37 From: R&JS (email@example.com) For the past several months, I have been extremely frustrated by an erratic problem with my ETX-90EC and its 497 controller. The problem was random slewing -- the scope would be tracking an object when suddenly the altitude motors would switch to high speed and the scope would slew up or down, usually down, by 10 to 20 degrees. If the scope were tracking an object near the horizon, the tube would slew down to where the tube was pointing downward where it was stopped by the altitude stops. Occasionally, the random slewing would be up -- the scope would slew up from the object being tracked until it hit the stops. I could move the scope back onto the object being tracked by simply holding the UP or DOWN arrow keys to slew the scope back into position. When the scope slewed like this, the Autostar was not aware that the scope had moved. For example, if the scope behaved normally and I told the Autostar to PARK SCOPE, the scope returned to the home position -- pointed north, tube level. For example, if the scope slewed down by 15 degrees, and I recentered the object and did a SYNC, then when I parked the scope, it pointed north, but the tube was parked pointing down by 15 degrees -- the scope did not know it had slewed down by some amount -- the Autostar thought the tube was horizontal when it was actually off by the amount of the random slewing. The problem was random -- it happened or did not happen regardless of temperature or battery voltage; it happened with the scope running off external batteries or the AC adaptor. However, the problem occurred every time I used the scope -- sometimes after 10 minutes of operation, sometimes after an hour or so. I issued several pleas for help and received a lot of suggestions including replacing the Autostar; ensuring that the cables were all plugged in tightly; RESET, CALIBRATE, TRAIN. Nothing worked. I replaced the Autostar, jiggled and wiggled cables, replaced cables, all to no avail. Finally, one suggestion solved the problem -- the problem is in the external power connector on the telescope. One individual replied to me that he had seen this behavior in other scopes and it was due to an instantaneous interruption in power through the external power connector. His explanation was that as the scope tracks, the relationship between the external power cable and the external power connector changes and at some point there will be a momentary interruption in power -- not enough to cause the scope to power down and back up, but enough to cause a memory loss and random slewing. To test his theory, I ran my scope for two weeks off the internal battery pack -- power from the internal battery pack does not go through the power connecter, instead, the internal battery pack is wired directly to the scope's electronics. During the two week period, I experienced not a single instance of random slewing -- the scope tracked perfectly and behaved itself in every respect. I then modified the scope as follows: -- I purchased an automobile cigarette lighter extension cord -- the AC adapter and my battery pack have an automotive cigarette lighter jack. -- I opened the bottom of the scope and drilled a small hole in the base plate beside the battery compartment. -- Then, I ran the lead from the lighter extension cord through the hole and soldered the respective power cord leads to the positive and negative terminals of the battery compartment. -- I installed a strain relief clamp on the bottom of the battery case and tied down the power cord so it would not pull loose. --This now gives me a power cord that feeds external power directly to the scope electronics, bypassing the external power connector. I plugged this cord into my external battery pack and tested the scope -- it tracked normally for 18 hours without interruption. I have been using the external power cord for one week and the scope has performed flawlessly -- previously, I could not go through a 30-minute observing session without the scope going haywire. It appears, then, that there may be a problem with contact between the plug on the end of the external power cord and the external power jack on the scope -- at least, it seems that this was the case on my ETX-90. I did notice one other problem. After installing the direct external power cord, I ran the scope for a couple of days using my external battery pack. Then, I switched to the AC adapter running through the direct power cord -- and after about 30 minutes of tracking, the scope slewed downward. I went back to the external battery pack and the scope performed normally for six hours. Back to the AC adapter, after 20 minutes, it slewed down for no reason. Back to the battery pack, no problem for four hours. This led me to suspect that there was a momentary interruption of power from my AC adapter. I had a friend put the AC power adapter output on a logging meter that logs the voltage output from a device over a period of hours. Sure enough, over a period of four hours, there were two momentary interruptions of voltage from the AC adapter -- the interruptions were approx one one-thousandth of a second each -- as though someone had flipped a power switch off and back on in the space of .001 second -- but this seemed to be enough to interrupt the memory of the Autostar while not causing it to turn off and back on. I intend to replace my AC power adapter with a real 12 volt power supply, not another "wall wart" AC adapter. If there is enough interest in this modification, I will open my scope and take some pictures to post for all to see. Hope this is helpful. ---- Joe Schlatter Bristol, TN
Subject: random slews Date: 3/5/04, 13:32 From: Flundberg77@wmconnect.com I am responding to comments on random slewing sent to you by Joe Schlatter shown in your telescope tech tips. I also experienced numerous cases of random slewing with my ETX 125. Unlike his main problem, I did not experience a problem with the power connection. I traced my problem to the 110v power supply because I never experienced any random slews using an external 12v battery hooked to the power connection. I tried a second Mead 110v power supply and a Radio Shack regulated 110v to 12v power supply and still experienced random slews with both. I have concluded that my problem was attributable to pertibations in the 110v power provided by my electric power provider. I have relied entirely on my 12v external recharcable power pack for over a year now without a single problem. I hope this helps. Forrest LundbergMike here: Yes, indeed, local power source fluctuations can cause this.
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