Bescor MP-101

At one of our CIRC (Central Illinois Robotics Club) meetings, someone brought in one of these units to see if we could find a way to get it to pan at a slower speed so that it would pan 45 degrees in 5 minutes. A quick disassembly of the controller revealed the speed control to be a simply slide potentiometer. 

 
 

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Description

The Bescor MP-101 is a relatively inexpensive high precision pan tilt mechanism with dual 1/4-20 universal mounts, above and below, for use on the top of a standard tripod. 

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Hardware Datasheets

 

Schematic

This is my first attempt at creating a schematic using Lucidchart. The orientation is looking at the back of the board as that's the view I had of the traces. It is complete other than the wires going to the "limit switch plate". Some of the traces pass under the H-Bridge IC's so I'm unable to guarantee their accuracy.  The switches shown are 5 way sliding switches where the one for power only uses 3 of the available points.


Disassembly

The gallery to left shows the steps to disassemble the center motor housing section (work in progress). The rubber pad and the double sided sticker below it need to be pulled up to expose the three hold down screws. The pictures below show the pan function rotation limit switch setup. The unconnected wires in the schematic above are wired to the contacts shown below with the white wire connected to the steel plate with the contact wiper that completes the circuit.

 
 

The simplest approach we could see to limit the speed of the pan function was to wire an impedance directly into the wires for the motor. This was convent as all of the wire colors are used more than once except for the blue wire going to the pan motor. I soldered two extension wires that I routed outside of the housing by drilling a small divot into the side. This allowed me to swap out different value resistors and diodes without having to take the housing apart. The low value resistors I had on hand when tried just caused the motor to stop functioning. The approached that worked was to install two diodes parallel to each other with opposing polarities to cause a consistent voltage drop of approximately one volt. This only caused panning 340 degrees to take 53 seconds instead of 43 seconds. By itself this wasn't as slow as desired but it was found that the auto feature still is slowed by the speed control on the remote if you leave the remote plugged in. The combination of the remote's speed control and the added diodes slowed the pan function to the desired level.