Weasner's Meade ETX TechTips

ETX TECHNICAL TIPS
Last updated: 6 September 2000

Power Source Modification

From: Ron Mariotti (rmariotti@excite.com)

Just to let you know what I found... to recap... I had a big problem with the Autostar unit suddenly going off in to space while trying to hone in on an object. I could even "see" what was happening using the Star Navigator II program. It seemed to loose track of where it was, I could see the Telescope Field moving suddenly somewhere else on the map. Been working with Meade for the past 2-3 weeks. I have new drive motors. Two new Autostar Controllers (they thought there might be a hardware incompatability with the "new" drive motors and the "old" (original) Autostar that I purchased). One day I decided to use the battery pack (what a revelation!). Up until this point I was using the AC Adapter option. It behaved quite differently; it was actually working! Well, having isolated that the AC Adapter was the source of my problem, I called Meade to inform them of this discovery. Unfortunately, they couldn't offer any solutions to my problem. They, basically, told me to use the battery pack (whatever). So, I took it upon myself to come up with a solution.

I figured noise or interference was my problem to I purchased an RF filter (Radio Shack sells them) and clipped it on the end of the cable where it plugs into the mount. Didn't really have much of an affect. So I experimented...

I rigged up an adjustable regulated DC power supply and discovered that if I lowered the input voltage to the scope, it behaved. I proceeded to build a power supply regulator circuit that just happens to fit quite nicely into the AC Adapter unit. I purchased a prefab circuit board, cut it to shape, mounted the components, (made sure that the IC I used had a heatsink and was mounted in a place where it would not contact any other components) and mounted it in the AC Adapter with a couple of dabs of hot glue. I adjusted the output of the circuit to around 10 VDC. What a difference! I believe that the Autostar does not like any power fluctuation or surges, probably due to the drive motors.

I have attached a schematic of the circuit I used. (Disclaimer: you can try this at your own risk. Don't blame me if you release any smoke from the chips!). It has been working well for me. The LM317 IC also has a current limiting circuit of 1.5amps and thermal protection. Might protect those drive motors some day. I forgot to mention that I also broke a drive motor. It seems that the bearing in the mount siezed up enough to break the gears inside the motor. Meade recommends "Sewing Machine Oil" as a lubricant. No WD-40 or Light Household Oil (3 in 1).

I think I've gone on enough.

Ron Mariotti Gainesville, VA

[9/6/00]

From:	watt@okisemi.com

I did some research and found an example schematic on Motorola's site
and have made modifications to Ron's circuit.  The resulting circuit as
shown works great for me.  Using the unmodified Meade power adapter, I
was able to get no more than 10-15 minutes without an errant Dec slew.
Last night I ran this for 3 hours and it worked great.
 
I am an electrical engineer, but do not take responsibility for this
circuit.  Anyone wanting to try this should be very careful to assure
it's proper operation before connecting to their scope.  Ron mentioned
in an email to me a couple of very good ideas that I want to share:
Take a 12v light bulb and use this as a trial load.  Also make sure that
the diodes and fuse inside the power adapter have no chance to come
close to this circuit.
 
Here is the resulting schematic.

Best Regards, -Phil Watt

And:

From:	rmariotti@excite.com (Carrie & Ron Mariotti)

When you get a chance, could you please update the power supply schematic
with the attached picture on this page? It won't work the way it was
drawn.

Thanks Watt!

and thanks Mike.

Cheers!
"Beauty is in the eye of the beerholder"
- Anonymous Refrigerator Magnet

Mike here: Image updated.

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