Last updated: 8 March 2004

ETX-90 random slewing problem traced to external power plug

Date: 2/29/04, 19:37
From: R&JS (
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

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

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
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 Lundberg
Mike here: Yes, indeed, local power source fluctuations can cause this.

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