Modification of EQ-3 Motordrive to fit NewPolaris

Original motordrives for the old Vixen NewPolaris mount are hard to find and quite expensive if bought new. I decided to go for a motordrive for the EQ-3/CG-4 mount and adapt it to my NP.

Mounting the RA axis steppermotor is easy. It fits perfectly - no surprise since the EQ-3 is more or less a NP copy.

The NP is not designed to take a DEC motordrive. Some hardware modifications will be necessary. I'll start with this when the Mars session 2003 is over.

Adapting worm gear ratio

The RA axis gear wheel of the NP features 144 teeth, the counterpart of the EQ-3 only 130. As a result the motor speed has to be adjusted if the EQ-3 drive is used with the NP, otherwise the tracking is too slow. This can be achieved easily by exchanging the crystal clocking the Atmel microcontroller in the steering-box. The original crystal is a 3.579545 MHz type in a HC18 package. To compensate the ratio of 144/130 a 3,9650344615MHz crystal is needed. Usually 3,93216MHz and 4MHz are available coming closest to the calculated value. I found a 4MHz type at a local dealer which should come close enough.
Replacing the crystal is quite easy if you can tell the hot side and the cold side of your soldering iron apart since the steering is build completely in throughhole mounted technology. The crystal is located on the left side of the lower pcb, wrapped in blue duct tape and bend down to fit under the top pcb.

New crystal soldered in place

After changing the crystal I did some testing and ran the mount against a clock for some hours to verify the tracking speed. Very accurate tracking for 3.5 hours according to the scales indicates the tracking speed is roughly ok.

Heffer is taking care of tests...