I fixed it !!

Antenna rotor

So last I wrote, the Tecom azimuth / elevation unit had position reporting issues when rotating the azimuth in the counterclockwise direction – it would falsely report +1.3° which prevented the “sector scan” and “raster” modes from operating as intended.

Position feedback synchro transmitter

My first thought was that the synchro transmitter was faulty and therefore reporting an incorrect value when rotating counterclockwise, perhaps as a result in slop in the internal mechanics. Out of curiosity, I contacted a few firms that deal in surplus equipment to get a replacement price .. well, as predicted, the prices came back in the $650 range. Ouch. To replace a single synchro transmitter would mean spending 8 times the purchase price to repair the unit. It is pretty safe to say that replacing a synchro unit is not going to happen.

Instead of replacing what I thought was a faulty synchro transmitter, I decided to swap the units between azimuth and elevation (put A in B). After a bunch of effort to swap transmitters, the problem still persisted! Ah ha! It was in the signal path and NOT the synchro unit! … crap!

Position controller computer

After a bunch of poking around and desperately searching for schematics, I managed to find a service manual for the motor controllers which the position control unit uses. I say busily tracing out all the wires and trying to visualize how the controller cards might integrate the data and work out how to control the motors. Once I drifted away from what turns out to be the brains of the position controller, I saw a couple COTS (commercial, of off the shelf) motor controllers.

Motor controller hardware

I looked up the part number for the motor controllers, and after some intense searching, I found a complete circuit diagram which confirmed my initial suspicion. One of the four brown potentiometers on the control unit sets the position feedback gain for the buffer / amplifier, allowing me to increase the sensitivity as well as set the bias a tiny bit higher, allowing me to effectively “zero” the position.

Drive mechanism

All in all, it has been an extremely interesting tear-down / repair of the antenna rotor / elevation controller. I’ve learned a lot about the mechanical considerations in a high-precision, military spec. antenna positioning system. Show here is the drive mechanism which couples the DC motor to a 40:1 gear system which in turn drives a 8-10″ gear tied to the elevation mechanism. After opening the belt-access panel, it was clear that the mechanical tolerances used on this side were quite odd compared to the rest of the assembly. My suspicion is that this unit has been serviced in the past with parts which are not quite the same spec. – well the large drive gear anyways, since it appears to protrude out of the housing.


Aside from this little oversight which I fixed by using spacers and RTV compound to re-seal the panel, I have been extremely impressed with the build quality of the components.

There are some old-school techniques used in the circuitry which would never pass modern safety standards, but all the same, the unit appears to be well thought out and quite safe to operate. It is fun to see the engineering solutions of days past – in this case, some 20-30 years ago.

After a nice clean-up and resealing the unit, I believe it is almost ready to be pressed into service for my EME needs.

Serial control port

Next will be attempting to interface with the position controller unit via the built-in Serial port. This would in effect allow me to remotely control the antenna position from some distance, which is quite nice because the control cable (while long) is somewhat limited in its length. Oh, and at 40+ pins, it is unlikely that I will bother trying to lengthen it.


Full Tecom antenna positioning system playlist can be found here.


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One Response to I fixed it !!

  1. Oleg says:

    Hi, James!
    What is the fate of the rotor Az/El ?
    You have applied it for a hobby or sold ???

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