Cab04P Throttle Mechanical Stops.


An easy to install method of making “Mechanical Stops” for the Cab04/06 Engineer Throttles, utilizing the Knob’s two grub screws, to eliminate heavy fisted operators breaking the Potentiometers.


I mostly use Radio Procabs/Power Cabs on my layout and I have two radio Cab04s “lying” around, so I thought I’d investigate how hard it is to fit some “Stops” to my throttles. I haven’t broken any Potentiometers yet. Testing a Cab06 with an Encoder made me appreciate the positive operation of the Knob’s  stop” or Speed Step “0” position of a Potentiometer style of throttle. Hopefully NCE will release a Cab06PR.


The “Stops” modification took about half an hour and made a really worthwhile modification that operates perfectly.






Mark a line around the Knob’s circumference at the 10 o’clock position, as shown in the photo.


This position is the only place that allows using the Knob’s two Grubscrews and some facia material, to hold a “Pin”, without re-applying the Knob’s Top with the Line.


The Grubscrew shown is provides the “0 Speed” Stop with the “line” on the Knob at 7.30 o’clock position.


How good is that and I didn’t even have to try hard – PERFECT.


The Knob rotates 270 degrees, slightly less than what the “original” Knob rotated with the Potentiometer’s internal 300 degrees “Stops”.


NCE must have thought about us, when they made this knob and the facia.








The rotating “Stops” are already in the Knob – the two Grubscrews. The “OFF” or ZERO Speed Grubscrew painted white in the photo and the second Grubscrew (90 degrees CCW) is the MAX Speed Stop and is just visible.


At the corner of the Potentiometer square cut out, mark an arc 4.6 mm inside the previously pencil marked arc of the knobs circumference.


Hopefully this will position the hole aligning the screw right in the middle of the circumferential “Slot” in the Knob. 


Drill a 3.0 mm hole through the Cab facia.






Cut/grind away the facia’s “ribs” to “lower” the screw head for clearance between the Circuit Board and the screw head.


I used a 3.0 mm screw 12.0 mm long.


Trim the “outside” of the 3.0 mm Nut to provide more clearance inside the knob. See the above photo and your installation.


I applied some Insulation Tape over the screw head for some extra protection from contacting the Circuit Board.


I could have used a Self Taper screw but I wanted the extra strength that a screw and nut provide for those heavy fisted “Old Farts” that want a Knob throttle on my layout.


Install the Knob making sure the Potentiometer is fully CCW. If you want, you can turn the Potentiometer a few degrees CW so the mechanical stop is not “right at” the Potentiometer’s Stop.


Check that the Knob is at “0 Speed Step, when fully CCW.


Decoder Pro’s “Show NCE Cabs” feature is good for this.







The completed Throttle Stop with the Knob at the Full Speed.


Laptop screen shows Decoder Pr’s “Show NCE Cabs” and MAX Speed is Speed Step 120. Close enough for a loco’s MAX Speed. I won’t miss the missing 6 Speed Steps & you won’t either.


Adjustment of Stop: Set 128 Speed Step mode in a test loco. Program CV 2 to provide a fast “take off” at Speed Step 1. This provides visual effect that the Potentiometer is at Speed Step 1.


With Knob fully CCW against the mechanical stop, turn Knob slowly CW until the loco jerks forward, that is Speed Step 1.


Make sure there is some “Dead” travel of the Potentiometer.


The 90 degree spacing for the Knob’s Grubscrews provides a perfect MINIMUM and MAXIMUM Throttle Stops.


This is why I said NCE was thinking about us.


You can have those Encoder Cab04E throttles but I need Potentiometers, confirmed by my Aug11 Cab06 test where I hated the “forever twisting” motion to Stop a loco. NCE, please hurry up with those Cab06PRs if you are in fact making them.


The Card clipped into the small holder glued to my Cab04, is an “Operating Card” with all the instructions for a Visitor even regular operators, to run a train on my layout.