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 (firstname.lastname@example.org) 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. Larry
After sending you the message above, I just read the July 11 Feedback message from Achilleas in Greece (email@example.com), 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. Larry
Subject: re: ETX 125 PE: Be careful with AC adapter Sent: Sunday, July 27, 2008 23:26:46 From: richard seymour (firstname.lastname@example.org) 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: http://www.weasner.com/etx/buyer-newuser-tips/powersupplytable.html good luck with the repaired scope --dick
From: Laurence Hecht (email@example.com) 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. Larry
Subject: 541 A/C Adapter Problems Sent: Monday, July 28, 2008 08:08:24 From: Steve Hollar (firstname.lastname@example.org) 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, CAMike 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.
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