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Friday, December 14, 2018

Rebuilding a Ham Rotor


A bit of patience and some time... 

Following up on my article on rebuilding/refinishing a tower (https://t.co/WY4VtD82kT) I promised to also write an article on rebuilding an antenna rotator. The rotator described here is a typical model marketed for many years by Hy-Gain. The information also applies to many CDE and MFJ rotators. 

For many years I used a light duty TV rotator to turn a 5 element VHF Yagi. With my greater commitment to HF and contesting came the desire to improve on my wire antenna and graduate to a beam. Doing so required a beefier support structure and, because of the Yagi antenna’s directional nature, a means to rotate it in the direction that I wanted to work.



For anyone contemplating a similar upgrade, I should add that I do not find that I can get significantly more stations than I did with my Carolina Windom off-centre fed wire antenna but, the weak-signal noise that comes with distant stations is much less now and therefore they are easier to copy. Both antennas happily coexist on my tower and I find one or the other most useful depending on the band and my purpose.

I was fortunate to receive the donation of a Ham II rotator and control box, although it was in pretty rough shape after many years atop a tower. Birds had coloured it multiple shades of white and it was caked on pretty heavily.  My first task therefore was to scrape and wash off as much of the debris as I could. Once that was done I decided my next move should be to hook it up to the controller to determine if it was usable - it wasn’t.

The 8-terminal connection plate was so badly rusted that 6 of the screws could not be loosened with any amount of penetrating oil, rust remover, heat or any other means at my disposal. The previous owner had clipped the control cable so there were a few centimetres of wire available that I could connect clip-leads to. Using this method I was able to connect it to the controller and I switched on the power… nothing! I tried the rotation control in both directions with no luck. The illumination bulb on the controller lit but that was it.

Twisting the top of the rotator produced some motion however it soon became clear from the rust debris that fell from between the two rotator sections that internal work was going to be required. I did manage to find some documentation on the Internet that provided some insight on how these rotators are assembled along with warnings that, if you do take them apart, you had better mark your steps so that it goes back together the same way. The Ham series of rotators goes back a long way and have progressed through several models and versions. I also found out that it is not always clear which version you may have as labels have long since disappeared in the elements of the outdoors.
I decided that I had nothing to lose so I mounted the rotator on a section of pipe on my shop vise and examined the 4 bolts holding the halves of the external shell together. Needless to say they were also rusted badly. The shell itself is pot metal so very fragile, something to keep in mind when starting this task. Over the next several days I soaked the bolts with every chemical I could think of. I was eventually able to loosen and remove 3 of the 4 bolts. I drilled out the last bolt, eventually re-threading the hole to accept a replacement. It was finally apart!

As I mentioned, there are plenty of warnings about separating the shell. I dutifully made a mating mark on the halves and started to lift the bottom (it is mounted upside down for service). Despite my care, several ball bearings rolled onto the floor of my workshop and under those places that made retrieval difficult. Note to anyone attempting this process… place a shallow cardboard tray under the rotator. I was expecting rust but I wasn’t quite prepared for the revelation that a number of the ball bearings were no longer recognizable as such. At this point I contemplated putting the unit in the recycling bin.

Not one to be deterred by a challenge I decided to have a go at clean-up. Over the next few days I carefully cleaned out as much rust as I could. I removed the rusted terminal plate and soldered on a pigtail that would later accept a waterproof connector. New terminal plates were available by mail order (about US $25 with shipping) but I figured a new one would rust again just as quickly and as I was well into the innards a better means was available.

The motor mechanism was easily disassembled but I took special care to take lots of photos as I went along and I was happy I did so afterwards as it made re-assembly that much easier. The gears were not rusted but did have grease and other debris on them. A solvent bath cleaned things up nicely. The manufacturer recommends that very little grease be used internally as it only gathers more dirt.



I paid special attention to the rheostat on top of the motor. This is the most vulnerable part of the rotator, and the most expensive to replace. it consists of a circular core wound with nichrome wire. As the rotator turns a brass wiper travels across this winding thereby varying the resistance. This resistance is shown on the meter movement that shows the bearing of the antenna N-E-W-S. The wire is very fine. Two things are most common when things go wrong. The rotator is forced past its limit of travel at either end, bending the brass wiper. This can be caused on early models without a mechanical brake and even on later models as the brake fails to activate in time due to rotational forces of the antenna. Third party add-on kits can prevent this from happening [link]. The second common cause of failure is a break in the nichrome winding of the rheostat either by a mechanical mishap or burn-out through improper connection to the control box. The winding of my rotator showed a burn mark confirming the latter. Fortunately it was at the extreme end of rotation and I was able to make a repair. The wiper had a minuscule void worn through where it had travelled over the wire. This was repaired with a small drop of solder on the outside surface. Resistance readings confirmed that it operated smoothly over its travel from end to end. I was pleased with myself for not having to spend the money (and waiting time) to order this as a replacement part.

I found replacement ball bearings were readily and cheaply available in bulk on eBay. I needed 100 but ordered 250 as the price was almost the same. I paid US $15 for the lot. Contrast this to a local supplier who wanted 25 cents per ball bearing. Again the caution from the manufacturer not to use too much grease. A thimbleful of white lithium grease is all that is required for all gearing and the 100 ball bearings to operate smoothly.

Now came the time to put it all together again. Fortunately this was not as difficult as I imagined while doing the disassembling. As long as you put the ring gear in the right way around, read ‘ooops’ here…, then move the rotator (wiper) fully counterclockwise, and manage to slip the motor assembly into the toothed ring gear with an angled downward movement, all the while mindful of the markings you made earlier, all should go well.
I used stainless steel hardware to reassemble the shell halves. I have read nothing to indicate that this is improper and I figure it will be much easier should I ever have to open it again for servicing.





I acquired a waterproof 8-lead connector pair that will make it much easier to disconnect and service the unit in future should it have to come off the tower.
This is a do-able project for most of us with basic skills if you go step-by-step, take lots of photos and mark your work pieces as you disassemble.
In this article I refrained from listing each assembly/disassembly step, as the factory manual is readily available on-line at http://bama.edebris.com/download/cde/ham-ii/ham2.pdf

I used it as my guide and was able to figure it out without too much difficulty. This guide also provides nominal resistance readings so you can troubleshoot the rotator before taking it off the tower. Should you need to order parts, the most quoted supplier is Norm’s Rotor Service http://www.rotorservice.com/

~ John VE7TI





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