Tuesday, August 26, 2014

How Not to Wire a Rotator

My tri-band yagi has arrived and is in temporary storage until I'm ready for it. That primarily entails installing the mast bearing, rotator and mast. Of these the only significant item is the rotator. For that I have unpacked the rotator that I have had in storage for over 20 years: a Ham-M series 5.

The rotator dates from the 1970s and I bought it used in 1985. Although it has the original small brown control box I upgraded both the control unit and motor unit back then to be equivalent to the Ham-III. So it's sufficient to the task. That is if it still works.

I cleaned up both units and then unburied the long roll of 8-conductor cable from a back corner of the basement. Deciding to get right to it I connected everything together and powered up. Nothing blew up or apart. The brake solenoid had a loud and healthy sound to it. That, too, was a good sign. My luck ended there.

The motor wouldn't turn and the direction indicator didn't indicate. The first problem was expected since the rotator relies on an non-polarized electrolytic (start/run) capacitor to shift the relative phase between motor windings. The capacitor is an original so it is well past its best-before date.

I cut the leads and visited a electric motor repair shop. I've done this in years since it's typically faster and cheaper than ordering an exact replacement part from the US.

It was an interesting experience. The folks at the shop I randomly selected were eager to help me but somewhat puzzled by my quest. First, they don't sell motor run capacitors. They stock them only for repairs. Second, the fellow helping me was very particular about the numbers on the part being replaced. I couldn't satisfy him by being helpfully flexible. I knew for certain that the voltage rating is pretty much irrelevant (these shops rarely carry anything rated less than 115 VAC) and the capacitance tolerance is wide. His insistence on getting an exact voltage match turned out to be his concern that the replacement part wouldn't fit inside my motor.

I assured him that size wasn't an issue and 115 is very favourable in comparison to 30. He remained skeptical. The closest part he had in stock was 145-175 μF. The rotator requires a 108-155 μF capacitor. I shrugged and told him I'd take it. The poor guy looked flabbergasted that I'd be so cavalier about the business. I assured him that the part would work out until I could order an exact replacement from the US. Giving up on me he invented a price for the capacitor, which I paid, and left him standing there still looking confused.

The larger capacitor does not fit inside the cramped Ham-M control box. I tried various methods to stuff it in there. I gave up and wired it to the outside of the unit. Some hams wire the capacitor to the motor unit which not only works but increases torque and requires two less wires in the cable. I did it the easy but ugly way by placing the capacitor on the shelf behind the control unit. I'll buy a proper replacement later or a used, later-generation control unit. In the picture you can see it clip-leaded to the control unit, with the old one alongside for comparison.

The motor still wouldn't turn. It seems that more can go wrong after long-term storage. So I started troubleshooting the control unit and motor unit. Both checked out okay. Even the direction indicator circuits looked fine.

There is a quick-release connector installed at the motor end of the cable. I opened these connector terminations. One side was in pristine condition, while the other was heavily corroded and had broken wires and pins. Those still intact could not withstand the wiggle test.

The corrosion must have been present when my antenna system was dismantled in 1992. Perhaps water got in and pooled on only the one termination. I did have protection on the connectors, though it must not have been very effective. It takes only a small flaw in the seal, and time, to destroy cables and connectors.

Luckily I have spares of the same type in my junk box so I warmed up my pencil iron and replaced the faulty connector. I also resolved to pay closer attention to sealing the connectors this time.

Twisting the connectors together showed that I had resolved the motor issue: it now turned nicely in both directions. Yet one more problem remained: the direction indicator still would not work.

The meter needle quivered a bit as the rotator turned but otherwise stayed fixed in the southeast direction. There was continuity through the connector and the control unit seemed fine so I opened the motor. I expected to find a light coat of oxidation on the direction potentiometer wiper or windings.

I cleaned the pot and checked it over its range. It was fine. The direction indicator still would not work. Now, finally, I had the idea of testing the long length of 8-conductor cable. That's where I found the problem. So after an inspection of the now-open motor unit (mechanically sound and grease in good condition) I closed it and dealt with the cable.

What I seem to have forgotten is that my rotator cable is not one continuous run. It is in 3 sections that are spliced together. My memory of this is vague at best. I do hope I paid a good price for it back in 1985! I could tell as I uncovered the splices that this was definitely my work, so there was nowhere else to place the blame. Expedient solutions that seem like such great ideas at the time often come back to haunt us. I was now being haunted by my former incompetence.

There was light corrosion on the wires at two of the splices and heavy corrosion at the third. It was there that I found a failed splice, and several that were on the verge. The proximate cause of the indicator failure was in the white wire (at bottom of the picture, bridged with green clip leads. I repaired all 8 wire joints in that splice. The rotator then worked as it should.

From the dirt on the outer jacket adjacent to the splice this section would have been buried. In my former station all cables were buried under the 10 meters of lawn from the tower base to a row of shrubs. The coax, at least, was burial grade. That splice in the rotator cable, even encased in silicone, did not keep out the water. Consider that a lesson. Over the years just a small trickle of moisture will do serious damage.

This is a problem that not only happens to amateurs. Earlier this month my landline phone and DSL were out of service for several days. The telephone repair tech discovered that, over 20 years of service, the entrance box and cables were soaked and badly corroded. She had to replace the lot.

Another thing I discovered was that none of my three multimeters is in good condition. Oxidation of internal switches is usually the culprit in the case of inconsistent or failed ohmmeter function. If you can believe it the old Radio Shack device pictured above performed best. Modern auto-ranging devices are so cheap today that I plan to buy new rather than bother with repairs.

At last I mounted the rotator on the tower and ran the cable back into the shack. It works great. Except, of course, that it is turning nothing. That item is now at the top of my priority list.

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