Servo - January 2016

on the white control wire.

Let’s get our hands dirty modifying the HXT900 servos for CR. Literally, you might choose to wear thin rubber gloves since there’s grease aplenty inside. Be careful and vigilant. Work in a clean bright area; don’t get dirt or swarf in the tiny gears which are nice and greasy. Leave as much silicone grease in place as possible and don’t bend the brass pot shaft during disassembly. Don’t break the tiny internal wires on the PCB (printed circuit board) either.

Both mechanical and electronic modifications are required. Mechanical stops on the output gear must be cut away to allow full rotation. Electrically, we replace the small internal potentiometer (pot) with two fixed resistors. We need a 50% voltage divider, so any matching resistors from

5K to 100K will work. We’ll briefly remove the potentiometer to make it rotate 360 degrees, then replace it to mechanically support the output shaft. Be sure and check out my videos at www.youtube.com/ watch?v=wfmM3cuihJI and www.youtube.com/ watch?v=7pxRhxBPSUM.

➤ Ready? Take a Deep Breath and Let’s Roll

1) Disassemble the servo by removing the labels and four long screws. Remove the top of the case. Carefully pull the output gear straight off; it’s a tight fit on the pot’s flatted brass shaft. Remove three white intermediate gears — noting the location — and set the gears aside. Don’t remove any grease.

2) Remove the bottom of the case and carefully pull the PCB out. Cut three wires at the potentiometer. From the top, push the pot down and out to remove it to work on (Figure 2). Be aware that some pots are glued in. I carefully used a vice to press on the shaft.

Do NOT hammer on the brass shaft; it will bend. Trust me.

3) Use a hobby knife and side cutters to remove two plastic tabs on the underside of the output gear. Flush cut to the bottom of the tabs (Figure 3). Completely remove the tabs or binding will result.