ATM Equatorial platform - balance

["Bodor, Murray" <bodor.murray@aquila.com>]

Hi,

Another question for you equatorial platform experts (you were so helpful last time!).

The platform top, north and (single) south bearings are complete, the whole thing moves quite smoothly with very little friction.
Now I am in the process of connecting my 5V 0.9 deg stepper to a 500:1 gearbox output 
to a rubber drive roller. I mounted this arrangement on a piece of .5" x 2" x 4" plywood, and attached 
a spring and turnbuckle to the end to control the tension of the roller pressed against the sector. I 
wanted to avoid making the drive roller a north support bearing.
My circuit supplies power to the half-stepped motor through a 5W resistor. 

When I mount my scope on the platform, snug up the roller and run the circuit, the platform moves 
quite effortlessly for the first 2/3 of the range of motion. Then it starts to labor, with the drive roller starting to slip. 
If I tighten up the tension on the roller more, the motor begins to stall out. I suspect that the balance is off just enough 
so that the motor is trying to lift the scope in the last 4" of travel. I am at 51 degrees N. lat.

What causes this? Is it because the true COG of my scope is too far below where the polar axis intersects? Anyone have any tricks on correcting this?

Thanks in advance,
Murray