Last updated: 12 March 2003
ETX, modification of electronic circuit
The ETX RA speed is not very accurate. The speed is not adjustable and due to the simple mechanical construction the movement is a little fading. I tested my ETX and the speed of was about 1 hour wrong in 24 hours. Not very good !. Watch it, when buying an ETX. To improve it I modified the electronic circuit. This modification will most certainly void your warranty.!!! Exercise caution. Myself nor Meade can be responsible for any damage that may result from attempting this modification. However this after modification, my ETX is spot on.
By changing the 2,5 Volt reference diode from a fixed LM385 to an adjustable LM336 of 2,5 V, The DC motor voltage and therefore the motor speed can be adjusted. The resistor of 3k9 was changed to 2k2 and across the new reference diode a 22k potentiometer was connected. The arm of the potentiometer was connected to the adjust-gate of the new LM336. With this potentiometer the RA-speed can be adjusted plus or minus 7 %.
The ETX power consumption will hardly rise. From 20 to 20.5 mA. Still enough for about 50 hours. (With alkaline batteries). In the circuit the value of some resistors is decribed as e.g. 3K9 , which means 3,9 Kohm or 3900 Ohm.
After the modification my ETX was good for making pictures of the moon and planets (0-10 seconds) and as a platform for my F/=3.8 , 200 mm zoomlens camera. With my camera on top, I made some nice deep sky pictures (exposure times of 15 minutes). Secondly for visual obeservations, objects stayed much longer within the sight of the ETX telescope. Due to the very high F/ factor (=18) , using the ETX for fo deep sky pictures is not very practical. The exposure times are just too long and in most cases nothing is captured on film. Secondly due to the poor mechanical construction, the ETX is still humming (moving). Sometimes it runs 30 seconds stable and then it moves e.g. one jupiter diameter in 1 or 2 seconds. With my ETX, most pictures of 1 till 10 seconds are ok but a have not succeeded in making any succesful longtime exposures with the ETX.
Subject: re: tracking problem Sent: Wednesday, March 12, 2003 14:44:34 From: email@example.com (Phil Sullivan) It seems very that very fast tracking is not an unusual problem. I also recently bought an ETX90 RA with a drive rate which was very close to twice the sidereal rate. I considered inserting a handbox and button type switch in the circuit which just turned off the motor when the button was pressed. This would then give me approximately equal "slow motion" control rates for both West and East directions. I decided against this because I planned to use the ETX90 as my travel scope, and I wanted to keep it tidy and simple. Instead I found a small 200 ohm potentiometer at the local electronics surplus house and installed it (in series) between the voltage regulator and the motor. I mounted it on the inside with the adjusting shaft coming through a small hole in the side of the mount base. On the first night I was able to adjust the voltage on the motor to give a nice, accurate sidereal rate. I only used the 200 ohm potentiometer because that was easily available at the surplus house. I was actually looking for a 100 ohm potentiometer, but one of 50 ohms would also probably work fine. The current is very small, so a physically tiny potentiometer is all that is required. Tim Gartman's ("Another RA Drive Mod") solution [above] would not have worked in my situation. The amount of adjustment would have been inadequate. I needed to slow my motor down by about 50%. Just adding a small potentiometer was also a lot simpler. Mike, you've got a great site. Thanks, and keep up the great work. Phil Sullivan
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