Variable Speed Modification for the JMI Motofocus Controller

I have two JMI Motofocus units, and I had trouble overshooting focus with both of them.    My quickest stab on the button could get no less than 0.006" of travel.    A little snooping inside a controller, and a little "hacking" showed that adding a 5k pot to the slow speed circuit could reduce the minimum travel to 0.001", and still leave adequate motor torque to handle a 7 lb. load at the vertical.
The only problem was where to locate the pot.    I needed about 0.030" more clearance for the depth of the pot than the original enclosure would allow.
The solution was to relocate two jumpers and one diode, and to insulate the top of the board from the body of the pot - just in case.
This is a relatively easy mod.,  and results in variable-speed slow focusing, with less than 0.001" minimum travel.

Here's how it went:

Motofocus board from the factory

Diode D2 and two bare jumpers in the way

Lift one end of D2, remove the jumpers

Move the jumpers to the back side of the board

New location for D2

0.015" plastic insulation - just in case...

The pot barely fits, but fits

top view of barely...

Lift one end of R10

Green and yellow pot wires to R10 and the board

Everything hooked up

Ready to go back in the case

Ready for bench testing

Bench test - my heaviest rig + 1 lb. - 7 lb. total
    Comparison - Factory vs. Modified
  • High speed - no change to circuit, no change in performance
  • Low speed - knob fully CW - no change to circuit, no change in performance
  • Low speed - knob fully CCW - nice and slow, 0.001" minimum travel, good motor torque
  • Low speed - knob slewing - smooth speed control changes