One problem came to the fore though. If the rotator needed changing as well as the yagi then the riggers would have a much bigger job to do. My task for the weekend was to work out how to reattach the cable dangling down the tower to the rotator control box and see if the antenna would turn.
The control box had a bit of paper on the underside allegedly showing the connections to the main cable run, so on Saturday I popped over to GP and wired the whole thing up from outside the tower compound. The antenna turned but the indicator needle didn’t move at all. I suspected that pot in the rotator head had been damaged, but took the rotator controller home to make sure it was working properly.
Whilst the control box was in bits I discovered a flat spot on the positioning pot in the rotator control box, but luckily I had a similar pot in my junk box so was able to swap it out. I had hoped this was the cause of the non moving needle and it was good to fix but certainly wasn't the core problem!
I went back to GP on Sunday morning to test my handiwork but as per Saturday the antenna was still moving but the indicator needle was remained static. Time for some trouble shooting.
Foolishly, I had used the diagram taped to the base of the control box showing the pin out of the control box, the colours of an intermediate jumper cable and how these both linked to the colours of the cables on the tower to the rotator motor and when things weren't working I realised I shouldn't have been so trusting. I decided that I had to work out role of each cable from scratch. This was made easier as the motor cables were clearly working and only 5 wires were required overall. That said, there were eight wires in the cable to the rotator head! On a couple of control box outputs the thin cables in the multi way jumper cable were being run in parallel to increase the current handling capability, but when I traced these through to the control box it became evident that several poles were being cross wired by the jumper. I thought I had found the problem but when I paired wires up correctly this didn't sort things out. So where now?
Out came the DVM and I started to identify the role of the cables
running down the tower. Once I had eliminated the motor power cables it was quite easy to discover which were the positioning pot in the rotator head. It was a great relief when I saw the resistance changing in this pair as I turned the antenna - the pot in the head was OK, meaning the problem was at ground level and therefore fixable. Phew.
A quick comparison with the erroneous wiring diagram suggested that the original installer had actually wired the jumper cable to the main run like for like i.e. red to red, white to white etc, except of one pair where there had to be a mismatch. As I had already identified the motor pair and two of the control leads I knew that the all the remaining wires on the tower were either unused or the remaining "lost" connection. So, I wired them all in parallel and hey ho the rotator indicator started to turn with the antenna. It was a simple process of elimination to find out which of my parallel set was the real deal.
So overall we have a result as shown in the video below. The antenna turns and the control box follows. I took a look at the main co-ax run to the antenna and the copper is bright - as new. This means we probably don't have to change that run unless I can find enough heliax. More good news!
I will try to get back to GP soon with my soldering iron and VNA. If I put a PL259 on the remaining co-ax and sweep the antenna we will have an idea of performance. It may be that a 2.5 element yagi is usable and I think that this will be a bonus for JOTA in a few weeks time.