Latest update 12-9-97

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 modification in the electronic circuit 


How it looks after the modification

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.