Just to test the possibilities for using stepping motors for tuning purposes, I modified my (brand new) MFJ-989C with a stepping motor for the roller inductor.

I have salvaged lots of different steppingmotors from old printers and plotters and I found out one would fit in nicely in the space occupied by the rollers counting device.

I removed the original mechanical counter. The big gearwheel driving the mechanical counter was perfect for fitting in the motor with a gearwheel I happen to have in store. Now the roller can be run either manually as before or by the motor.  Nice :)
I had to cut out some from the rollers endpiece for the motoraxis as shown in the picture.

This mod was easy and didn't cost anything neighter in time nor money.

I run the stepping motors with microcontrollers as its very easy to make them do whatever I want by changing the programs. I wanted to tune the roller to predefined positions by pushbuttons, one or more for each amateurband. There must also be a tweaking function to modify the predefined positions as at least my antennaimpedance (an oldfashioned windom antenna) is changing with the weather. Also 160m and 80m are so wide I need to tune the roller over the bands.

I had no idea how a positionsoftware should be structured so I did as follows:

I programmed a counter that keeps track of the rollers position all over the 43 turns. In my case it's around 20000 pulses for the motor. I count the pulses given to the motor by the program and trust it will be correct all the time as it will as long the motor is not stopped by any obstacles or too heavy resistance from the rollermechanism

Each pushbutton contains a predefined count of pulses and the program counts the difference between counter and predefined. If the difference is >0 the motor runs one way, if <0 the other way. I program in C so it is easy enough!

I yet have to solve a good way to reset the system. I really need sort of zero-position signal mechanically from the roller and will probably solve this with a microswitch on the roller. When switching on, the roller will move to the end and get the zero position from the microswitch. Then it will automatically go back to most used band.

My program cannot yet use the EEPROM to save the positions so it will lose the tweaked positions and start up with the predefined ones. Soon the EEPROM will be in use.

My controller is still prototyped as on the picture. There are four FETs driving the motor. I use an ATMEL 2313 microcontroller here but I might migrate to a bigger one to get more I/O lines for the pushbuttons.

The motor I use is an unipolar type which is easy to control electronically. If you have a bipolar motor you'll need two H-bridges for the control. I have tried both models without any problems. Electronics is cheap nowadays...:)

Here are two examples of motors which you can find in printers and plotters.

To the left is a great motor with an axis going through. I will use these for my linear amplifier as it mechanically easy to achieve  the tuning of the vacuumcapacitors either by the motor or manually without any additional gearing. To the right is the motor I used here in my MFJ tuner. It's much smaller but strong enough for this purpose. I just had to change the gearwheel to fit the MFJ original big wheel.