Servo Drivers / Testers By James Goss      In the past I have built several servo drivers that would operate or drive a servo throughout it range of travel. I used these devices to halfway check for a bad servo. I say halfway because they would only move the servo as I moved the pot knob on the driver. The drivers on the market today are much better designed than mine and have more features that qualify them to be called a servo tester. But up until now I didn't really see a need for an elaborate servo driver I only used mine every now and again. I purchased one at the Perry show this year for around $39 when I saw it in operation in a demo that the designer was putting on. There are several on these servo drivers on the market today and they go by many different names such as servo exciter, servo exerciser, and so on, but they all do about the same thing.      The one I bought is called a servoxciter and is operated with a common 9-volt transistor battery. It has two servo plug connectors, one for Futaba and HiTec and the other is for Airtronics. The unit is a handheld device that will accurately produce the required signal to drive most major brand servos. It drives the servo through its travel range in 256 discrete steps, and this assures that it doesn't miss a bad spot. To operate the servo through its range simply rotate the control knob and the servo will follow. It also has an automatic sweeping mode that will operate the servo through its range at two pre-selected speeds with a slow and high rate. This allows you to view the servo's operation for any length of time you like without turning the control knob. If you hold the servo in your hand while testing it in the auto mode you can feel a bad set of gear teeth as well as see it.     When at the field you can check your servos and make adjustments to their travel range without turning on your radio. But what I really like the most about this device is that it will locate the servo's electrical center for you without having your receiver or radio transmitter turned on. When you start to install the servos in a new installation you know how aggravating it is sometimes to have to rig all this equipment up just to be able to install a push rod. With this device all you do is plug in the servo and adjust the knob until the bar graph shows you the electrical center, quick and simple. Just this feature alone will save me a lot of time in a servo installation. I have already used this device many times since I got it and it proves itself each time I use it. It is well worth the small price and I would recommend it to all modelers that are serious about this great hobby of ours. Check it out at http://www.vexacontrol.com and see for yourself.