Hy-Gain HDR-300A Conversion, pt1

This was is an old Hy-Gain HDR-300A 180° satellite elevation positioner I got from a friend. It was kinda cool looking, and I wondered if I could mod it to use as a conventional 360° azimuth controller for my tower mounted antenna stack and HD-73 antenna rotator. It was kinda beat up, the bottom was damaged from being dropped (it has a large transformer and is kinda heavy), the case was scraped and scratched, and it was pretty dirty.

The most important step was to see if it could be modded for 360 operation, and be interfaced with the 75 ohm feedback pot in my HD-73 rotator assembly. 

Here's a few  pics and video...

I used external pots to simulate my 75 ohm rotator feedback pot. This unit places 5vdc across the rotator feedback pot, and looks for a 0-5vdc return signal to translate to 0-180° of movement. Calibration would allow for a negative position of -90ish degrees, instead of displaying a positive declining value of 359° or less, when rotated below North via West. However, I could change range on the digital converter, and display to over 600° positive. I decided to set the controller up to rotate North to North to manage this issue, as opposed to South to South as in most typical rotator controllers. 

Once I settled on the rotator stop point as being North (0 degrees), I calibrated the digital display to range from 0 to 360 degrees (0 to 5 volts in). I did, however, need a variable 0-300 ohm pot to tweak the 0° reading when the rotator was at North via East (full ccw). Once that was done, it displayed properly and was amazingly linear! Above is my chicken scratch drawing of the electrical mods to the display circuit...

More on the conversion in the next post...