ETX TECHNICAL TIPS Last updated: 26 February 2000

DS Scopes Drive Modification

I have been fooling with a DS scope for a few weeks now (actually a Short Tube 80 on a DS60 mount. Problem I was having is that when the scope was slewing to an object, as it approached the object and it appeared in the field the motors would then begin their slowdown for final positioning, suddenly one or both motors would suddenly run full speed bringing the scope 5 to 10 Deg off and it is there it would finally come to a halt and beep. This was happening very often and once it happened the only thing to do was to start over and re-align. I called Meade and they are sending new motors. I believe I have heard others saying they are experiencing a similar problem or other unexplainable problems. In the mean time I put the Autostar in the status mode to display the ALT-AZ coordinates in degrees and used the arrow keys to slew back and forth and up and down while watching the numbers increase or decrease. What I found is that when running at the fastest speed everything was fine. At the slower speeds the display would become erratic and suddenly jump from say 5 Deg down to 2 or 0 but the amount and direction it jumped was just random and unpredictable. Obviously this could cause some strange things to happen when slewing, tracking or aligning. The problem is that to obtain the slower speeds, the motors are being pulsed on and off rapidly. When power is applied and removed to any DC motor there is a resulting startup or inrush current at power on and a resulting surge or spike when power is removed from the collapse of the magnetic field in the motor. It seems these surges were interfering with the electronics in the motor boxes upsetting the encoder counts. To correct the problem I added a 3 ohm resistor in series with the motor to reduce the inrush current and a 0.01 uF capacitor across the motor to "snub" the surge. I do not have schematics of these motors and do not know if this is the best fix for the problem but mine is working flawlessly now. The dates on my motors are 9930 (30th week of 99). Whether or not newer motors suffer from this problem I do not know but I will see when a new one arrives. For anyone who is electronically inclined and has had unpredictable things happen this may correct the problem or may be a way of salvaging older motors as spares as I don't think that Meade asks for the old ones back when they send out new ones. If anyone has had this problem with their motors feel free to contact me.

Tony George

http://www.egroups.com/files/meade-ds/Fixes+for+the+Meade-DS/Motor+modifications/Motor-Modification.html

Thanks
Tony George

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