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Building A Professor Motor Controller For SCX Track

The SCX track system is based on the old Scalextric type track, but they use their own power supply and power base system which allows the cars to travel in either of the two directions on the track.  Curiously Scalextric track systems do not have this capability, even with the brand new "Sport" system.  For our electronic controllers this this ability for dual directions of travel creates a bit of a challenge because of the polarity sensitivity of our electronics.

Some added complexity is needed in the controller to address both polarities of operation (and corresponding directions of travel on the track), where normally most of our controllers are functional in only 1 polarity.  To accommodate the polarity issue, with the 2 wire controllers used with the standard track, a simple switch added can handle the "dual" polarity operation.  With the 3 wire "pro" controller system that uses brakes, a more complex polarity switching system with an isolated brake circuit is needed.  The unit shown above with the 3 wire cord is in effect a replacement for the "pro" controllers since it is fully functional with brakes.  The wire harness used here was salvaged directly from a defunct SCX "pro" controller.

The wire harness of the SCX controllers use the long standard black/white/red color code for the wires.  But, for some unknown reason, the way the use these wires does not coincide with the normal red (brake), black (track) and white (power) color code.  So, if building one of these yourself, be sure to pin out each connection with a multimeter or continuity tester.

The low voltage available from the 12 volt Irwin power supply requires a bit of tuning in the controller to get the right sensitivity.  I wound up shorting out 5 of the 16 normal semiconductors to get the right type of performance curve (the 5 shorts can be removed if a higher voltage supply is ever used) using the PMTR2071 low cost variable sensitivity kit.  The 2 wire SCX controllers can be made directly from the PMTR2061 controller model ($47.95) spliced into the controller plug with the addition of the low cost sensitivity kit ($3.50) added to tune the sensitivity.  The 3 wire type controllers can be custom built from scratch since the specs do not match any of our current models if a "pro" wire harness is provided such as shown above.  Cost each approximately $59.95 for the custom built ones.

Hope these tips are are of interest those with SCX racing systems ....

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