Last updated: 31 July 2007
Subject:	ETX 125 PE:  Be careful with AC adapter
Sent:	Tuesday, July 22, 2008 18:00:07
From:	Laurence Hecht (hecht3@verizon.net)
I bought an ETX 125 two months ago, and had a wonderful first 2 weeks
with it. I was using it on my deck so much, that I decided to get the
Meade 541 AC adapter to save batteries. After using the adapter for
about -hour, the motor drive stopped working in the up- and
right-direction. It never recovered. I decided to test the adapter
output, and found 16.64 volts (rated at 12 volts). Curious, I tested
five other wall adapters I had around the house for portable drills, an
ozone machine, and sundry other things. All of them were producing from
20 to 40 percent over the rated voltage. (An electrical engineer told me
you had to measure with a load attached, so I tried a 1K and 100-ohm
resistor on the Meade adapter, and found no change.)

I would recommend no one use the AC adapter, as the circuit board or
some other component seems to be sensitive to voltage input. Use a
portable battery instead. By nature, they are incapable of producing
significant excess voltage.



After sending you the message above, I just read the July 11 Feedback
message from Achilleas in Greece (manahil@otenet.gr), who experienced
the same problem as I did with excess voltage from his Meade AC adapter.

Given the apparently widespread nature of the problem, I suggest you
post a PROMINENT WARNING  somewhere on the ETX site, so that more users
do not have to experience the disappointment of frying  the circuit
board (or whatever it is) on such a nice telescope.

I wonder if you could also prevail on Meade to look into the problem. I
had no success in advising various people in the Consumer Service Dept.,
and seemed to just antagonize one of them by raising it. I had to send
my scope back to them, and have been six weeks without it. They covered
shipping as it was less than 30 days old, but I am getting the runaround
about when it comes back. I thought I did them a favor by reporting my
voltage test to them, and offered to send back the adapter, but they
weren't interested. I am beginning to suspect there is something
seriously wrong in the internal operations of this company.


Subject:	re: ETX 125 PE:  Be careful with AC adapter
Sent:	Sunday, July 27, 2008 23:26:46
From:	richard seymour (rseymour@wolfenet.com)
Welcome to the wonderful world of unregulated power supplies.

The 16.8 volts is actually -normal- for these things
(i wrote a note quite similar to yours to Mike back in 1999)

Your electronics friend is correct: it SHOULD drop down to near
its rated voltage under a load... however your 100 ohm resistor
should have been enough to show a drop.  The scope is more like
a 30 to 50 ohm load, dropping to 12 ohm when slewing.
The electronics of the Autostar and scope -should- be able to
handle in excess of 18 volts for short periods.
(the lowest-rated components i've found in there are rated
for 25v, so the 18 is a safe margin.)

Why 16.8v?  Because DC voltage is achieved by rectifying and
filtering the sinusoidal AC waveform... and the -average-
voltage (if you chop off the peaks of the sine wave and
use that energy to fill in the troughs) is 12v (120v at
your wall plug -really- gets up to 168v at the peaks)).
(168 is 120 times the square root of two, as 16.8 is to 12)

It's possible your Meade supply has a faulty filter capacitor,
so that it's not storing the peaks to fill in the troughs.

The Meade supplies (as are most cheap "wall warts") are unregulated
supplies... they're far cheaper than regulated, and -usually- it works.
I've been using a (RadioShack) unregulated supply on my ETX90 for 9 years,
It's over 16v unloaded, but drops near 12v when slewing.

If you want to -avoid- the problem, use a "JumpStart" type battery.
Mike covers quite a lot of them here:

good luck with the repaired scope
From:	Laurence Hecht (hecht3@verizon.net)
Thanks, Dick,  for the explanation. I should have known it from all the
book on electricity I have read, but it shows the difference between
book knowledge and practical. I hadn't realized that I had verified the
root mean square equation by my measurements of five or six wall warts I
had around the house. I've already concluded that a battery is the way
to go. I've never seen a 12-volt battery put out more than about 13.5 v,
and that only for the short time it is at peak charge.

I'm curious to see what explanation Meade repair gives for the
damage--if the scope ever comes back. (Their Customer Relations leave
something to be desired.) I hope they are smart enough to take these
wall warts off the market, or put some kind of extra filtering on the
circuitry of the scope itself, as they are costing themselves a lot in
repairs, and frustrating a lot of customers--if that is indeed the cause
of the problem that I and others have had. ETX with GoTo  is a very nice
scope, when it works.


Subject:	541 A/C Adapter Problems
Sent:	Monday, July 28, 2008 08:08:24
From:	Steve Hollar (phoneman@hollar.com)
I've been reading some of the problems a few people have reported about
the a/c adapter. I just bought the Meade 541 with the 607 cable. I have
not used it yet as I intend to use it on my new 125PE when it arrives.

This makes me a bit nervous. What is your opinion about these adapters?
Looking over your web site, it appears only a very few have experienced
problems. This leads me to believe it was either a fluke, or, perhaps
they did something wrong. I guess I'm just looking for some reassurance.

I might add, on my ETX 70AT and my DS130 I use those adjustable voltage
and polarity adapters and have had no problems at all.

Steve Hollar
Lake Elsinore, CA 
Mike here: From the few reports I've received over the last few months there does seem to be a small number of bad units out there.


That's not good. Would just testing the voltage on the adapter confirm
whether it is good or bad?
Mike here: If you put it under a load as described above by Dick Seymour.

Go back to the Warnings Page.

Go back to my ETX Home Page.

Copyright ©2008 Michael L. Weasner / etx@me.com
Submittals Copyright © 2008 by the Submitter
URL = http://www.weasner.com/etx/warnings/ac_adapters.html