EL rotator back in action, and lessons learned

In my first posting, I said that one of my ongoing projects was finishing the elevation rotator (EL rotator) that is part of my amateur satellite setup. Well, it's still not finished, but at least it is back in action after damage (accidentally caused by me) had stopped it elevating for several weeks :-)

I am going to write an article about the design and construction of the EL rotator that, I hope, will be published in Oscar News. I’ll also put the information on my website. But, in brief:

• A satellite TV linear actuator rotates the antenna boom in the vertical plane.

• A potentiometer, housed in a metal box and connected to the boom by a pantograph arrangement, senses the amount of elevation.

• The ARS hardware and software (from EA4TX) in my shack measure the elevation and commands the linear actuator up or down as required.

The EL rotator had worked well for several months, but I had left three small jobs uncompleted:

• Calibrating it more precisely than I had when first getting it going.

• Trying to cure RF interference coming from the actuator motor when it is in motion (noticeable and annoying but not a showstopper).

• Completing the waterproofing by putting some sealant around the lid of the metal box in which the position-sensing potentiometer is housed.

A few weeks ago, I decided to tackle the third job, using RTV silicone sealant. I put a bead around the joint between the box and its lid, and ticked the job off as done.

A week or so later, I noticed that the EL rotator was stuck at an elevation of about 20 degrees, even though I had used the ARS software to park it horizontally. It quickly became clear that the elevation reading as measured by the EA4TX hardware was erratic below about 40 degrees and dropped to zero even when the rotator was still elevated.

More tests showed that the position-sensing potentiometer was at fault. So I had two questions to answer: why had it failed, and how could I fix it?

I concluded that the only thing that could have happened to cause such a sudden failure was acetic acid fumes from the sealant somehow affecting the carbon track or the wiper of the potentiometer. When I opened the metal box to test the potentiometer, there was a very strong vinegar smell despite the small drain holes in the box and the sealant being fully set. (Without knowledge of chemistry, this is really a guess rather than a proven theory).

With hindsight, a number of options were available for fixing it:

• Replacing the potentiometer.

• Rotating the potentiometer so that an undamaged portion of the track is used to measure the 90 degree swing of the elevator.

• Using PCB cleaning spray to flood the carbon track in the potentiometer to see if this cleared whatever was causing the problem.

• Blowing a strong jet of air from an aerosol into the body of the potentiometer to achieve the same thing.

My design should have let me just loosen some grub screws on the shaft that links the potentiometer to the pantograph arm, and then easily remove and replace the potentiometer. So, at the time, I only thought of the first two options, and chose the first as being the most certain of success.

However, when I started the repair, I discovered that I could not shift the grub screws, due to a combination of corrosion and the Loctite applied to them when installed. In fact, I stripped the slots on the screw heads trying to free them.

So removing the potentiometer then involved:

• Drastic surgery (using a grinding wheel on a Dremel) to separate the potentiometer assembly from the pantograph arm.

• Soldering a new socket onto the pantograph arm.

• Cutting and fitting a new shaft extension for the potentiometer.

• Fitting the new potentiometer itself.

This all took time but went pretty smoothly. I waterproofed the grub screws (using self-amalgamating and PVC tape) to avoid a repeat of that problem. And I have not used sealant on the metal box, relying instead on what is a fairly snug mechanical joint.

I still have to sort out the other two jobs (final calibration and dealing with the RFI), but at least I am back in action.