The $6 Controller Upgrade

If you thnk you have to pay $150-350 for an electronic controller, think again, With this simple modification of your Parma controller, you can have the range to run any slot car requiring 25-120 ohms, and do it for way less.

One of the great things about our hobby is the willingness of the participants to share knowledge. Greg Katz of the MSCRC showed me how to do this simple upgrade. I changed his design to meet my own needs.

Because I race anything from Mods to Skinny T’s, I would commonly carry 5 controllers in my box. Now I only need to carry two, and one is only for backup. The bonus is more space in my pit box.

Parts List:

1. A Parma Turbo controller is recommended, though Econos can be used. (To reduce costs even further, look for used ones of any ohm)
2. Resistors of appropriate vale
3. HO World (HOW) 120 ohm resistor
4. Radio Shack (RS) mini center off toggle switch (#275-325, $2.75)
5. Heat shrink tubing
6. 2” of 16 gage wire
7. For best results, solder with Tix solder and liquid flux.

I have done this to six controllers, now, and the time it takes is down to 15 minutes. It’s so simple and affective, you’ll wonder why you didn’t do this long ago.

Instructions:

1. Remove the handle
2. Unbolt and remove the old resistor
3. Drill a 15/64 hole for the switch. (1/4” will work, but is loose).
4. Determine the resistance you want to achieve with this formula:
   (R1 X R2) / (R1+R2) = R, where R1 is the HOW resistor, R2 is the RS resistor you will add, and R is the new resistance of your controller. If this sounds like too much work, here are some examples. Starting with an HOW 120 ohm resistor (measure for exact ohm):
   Using 470 ohm result equals 96.7
   Using 330 = 88.5
   Using 220 = 78.1
   Note: Before starting, check resistance of all resistors to get accurate readings, which you will use in the formula.
   
5. Drill a 1/16” hole in the low and full power bands of the HOW resistor, 1/16” below the barrel.
   Note: Use a very sharp drill bit for this, and epxoxy the back of the resistor, first, to avoid blowing the low band apart.
6. Install the resistor, but leave it loose, for now.
7. Mount the switch. First, remove the nuts and washers. Secure it with the 2 jam nuts supplied, firmly.
8. Cut four pieces of heat shrink tubing the same length as the wires coming off the RS resistors. Heat with a lighter till they shrink around the wire. Now, expose 1/8” of wire on each resistor.
9. Put the RS resistors through the hole you drilled in the full power band of the HOW resistor. For best results, solder with Tix solder and liquid flux.
10. Pass the RS resistor wires behind the HOW resistor, and up to the outside terminals of the switch, and solder.
11. Solder the 16 gauge wire to the low band of the HOW resistor, and pass it behind the HOW resistor, to the center post of the switch, and solder.
12. Reconnect the white wire to the full power band of the HOW resistor. Assemble it so it won’t rub, when the controller is in use.
13. Adjust the HOW resistor against the wiper for smoothest operation, and tighten the bolt.
14. Hook an ohm meter to the black and white leads of the controller, and determine the ohm readings from each switch position. Mark the controller with those readings, with a Sharpie pen, for reference.
15. Bolt the handle back into place.
    
    Remember when I said I used one controller for mods, and one for T’s? By adding a second switch, and two more RS resistors you end up with SEVEN settings. But that’s a story for another day…
    

Below/left: Arrow indicates common ground wire, soldered to narrow "low band".