Friday, 9 October 2020

New extension cable for my Tennamast.

WARNING: If you do this please observe safety precautions. Follow the Tennamast instructions and contact them if in doubt. What follows is what I did freestyle myself. The Tennamast instructions have arrived and are shown at the bottom of this posting. Note that they suggest doing things in a different order. Stay safe whatever you do.

GM4FVM Tennamast - succesfully re-cabled and fully working again (vertical in the foreground).

I had a plan to refurbish my Tennamast. It has been up for over ten years and I was aware that it could do with some attention. The plan (plan - well plan is a putting it a bit strong) was to strip it down, check the cables and pulleys, replace where necessary, clean the winch parts, take back the winch bodies to bare metal and repaint, lubricate the mast and rebuild.

During the Covid period I cannot summon any of the local like-minded amateurs to help me with the heavy lifting. Not that there are any locals who care about VHF-DX anyway.  A plan like that needs good weather too, as otherwise the whole thing needs taken apart and brought into the dry to work on it. 

It is possible for me to lift the mast. I picked it up and carried it when putting it up. I supposed I could take the car out of the garage for a while and work on it in there. If I took the antennas off the top pole that would break their waterproofing seals and they would need to go into the garage during such a long job anyway. So I was not about to do it right now. Next Spring seemed like a good time.

I started to notice trouble recently when I stabbed myself on the extension cable. I do not suffer well, so what really happened was that I pricked my finger on a frayed wire which had sprung out from the cable. Not quite a stab, but it hurt just the same. 

This did not particularly bother me much at the time, but the cable then frayed further and started to unravel in both directions. The overall strength of the cable was probably not much reduced, but this type of thing is the start of a chain of events which ends eventually with the cable giving way. So I would need to take action sooner than I had envisaged.

It has to be said that even if the cable did eventually give way, as it the extension cable and not the tilt over luffing cable all that would happen would be that the two sections would telescope together. If it did happen that would probably not be sudden. Most likely the cable would unravel, stretch, and lengthen, which would be obvious as it would no longer extend properly. But you have to be safe and if there is any weakness I would rather change the cable. Anyway, there would be less chance to stab myself with a new cable. 

Strands of wire peeling off the extension cable (seen after removal)
Stuff happens. Ten years is not bad for a cable. I raise and lower this mast a lot. As it is normally down by default, it is usually extended and lowered twice a day. I have read that some people get twenty years out of a cable, but they seem to leave their mast either up or down most of the time (like my CUG mast).

October isn't the best time to do this and in the event the original plan never happened. As usual, something else happens. I decided to move my refurbishment forward and start now, but it turned out to be a bit less comprehensive.

I ordered a new cable and pulleys from Tennamast. I did not need to buy the Tennamast replacement cable as I could have bought similar cables and pulleys more cheaply at the ship chandlers in nearby Eyemouth. I bought the Tennamast ones for convenience. The cable has a loop neatly made at one end and the correct size replacements bolts and spring washers. I did not think that I needed the bolt and washers, but it turned out that I did. I could not see how the top pulley had fared and it probably needed replacement. I did not want to be stuck in the middle of the job unable to source the right sized pulley.

Anyway, due to a slight mix up at the far end, the cable arrived without the pulleys. Also late were the instructions for changing the cable. I found a set of instructions on the internet from GD3YEO (here) but that was for a three section mast. Having looked at that I realised that the two section mast might be lower but it is much simpler to work on.

In any case, GD3YEO's experience encouraged me. At this stage I only had the cable and I could wait for the other bits. However, it was a nice day, so why waste the weather? I decided, at 13:00, to spend the afternoon starting the long refurbishment process by taking down the antennas, masthead amplifiers and rotator, storing them in the garage in preparation and sealing the coax ends against the rain. Then, when the other bits arrived, I would be ready to start my task. The downside was that as the year draws on the chance to put the thing back up would get less, and I might get stuck with it in the garage.

It wasn't to be like that. When I got outside I hatched another plan. I wondered if I could take the antennas and preamps off as one piece and not remove the coax and seals. Would that be too heavy for me alone to lift off in one go? No, as it turned out. Then I took off the rotator and left it connected. 

Given that I hoped to have an hour or two of good weather, why not try to change just the cable itself? I reckoned I would have to work alone because Mrs FVM has injured her rib, and anyway she was working from home to make some money to pay the shack electricity bill. She said she was "busy".

I was a bit surprised to find, once I removed the top and bottom mast sections from the rotator, that the SPID rotator bolts had cut their way into the thick wall tube I was using. I had been having a problem with the SPID losing alignment (more on that some other time, but I have solved it). During that period I tightened the bolts very tight to avoid any turning in the wind. 

The SPID locator bolts had cut into the alloy tubes
Although this extra tightening proved unnecessary, because of it the bolts were cutting their way into the alloy tube. This is a better outcome than distorting the tube, which was what I had feared might happen. Anyway, it looks OK to leave it as it is. Oddly enough I had considered making one or two holes in the tube to securely locate the bolts, but that seems to be happening without me using the drill.

Tennamast recommend steel tube but I go for lightness ...

Without the instructions I was flying blind, but why not try to press on? If anything resulted in me getting stuck I could just take off the coax, park the bits in the garage, and wait for the other parts. 

By various dodges I was able to partly extend the mast, tip it over, loosened (but did not fully remove) the stop bolt, undid the upper pulley bolt enough to remove the top pulley and then pulled the top section out of the bottom section.

Upper mast section removed, revealing corroded cable bolt

It was now clear that the bolt and nut holding the cable on to the upper section were corroded and slightly loose. The spring washer had gone rather limp. The replacement cable had a new bolt and nut, plus a new spring washer.
Fitting the new cable calls for three hands, including one for the camera

GD3YEO's report suggests this an even harder task on the three section mast.

The photo shows the stop heel on the bottom of the upper mast section. The stop bolt is mounted in the middle of the lower section, and thus passes the cable nut along the centre line of the mast as you crank it up to maximum. The significance of this became apparent later. Anyway, there is only one way to put the mast back together as the heel only goes in one way if it is to fit past the pulley, never mind catch on the stop bolt.

At this stage I pulled the new cable through with the old cable. This turned out to be important, as it meant that the cable was in place before putting the pulley back. It was clear to me which way the cable went over the pulley and down the lower section. Later I found out that the Tennamast instructions suggest doing it differently.

New cable simply passed through old cable loop, bound up and pulled through the mast.

As the new top pulley had not arrived yet I took the old one and washed it in soapy water to remove oily deposits. Tenna use nylon pulleys and it looked reasonably OK after ten years. I will replace both the pulleys at some other stage.

It gets a bit fiddly now and I may have got something slightly wrong which I had to correct later.

I now pushed the upper section back into the bottom section far enough to leave a loop of wire at the top to thread round the top pulley. I then put back the pulley on its bolt and the stop bolt. I would probably have been better to leave the stop bolt working but a bit loose until the cable was through the pulley and pulled really taut.

Once the cable was over the pulley I pushed everything together, checked that I could not pull the top section out of the bottom section, and turned it vertical for tests. Everything was working but it seemed a bit stiff. When fully up the top section jammed slightly, but with no weight on the top this was not a complete surprise.

When I turned the mast vertical I heard something sliding down the inside of the tubes. This was complete red-herring, and probably a stone or something irrelevant. I panicked, thinking there might be some sort of bush between the top pulley and the bolt, as there was a lot of play in that (though I had checked before and I could see the top pulley turning). 

So I phoned Tenna who reassured me on the top pulley issue. However, they said that the only likely issue I might have would be if the cable had become trapped on the "wrong side" of the stop bolt. It should run directly up from the bottom pulley to the top pulley, passing the side of the heel on the top section as it went, over the pulley and down straight to the fixing point. D'oh, of course. They advised me to get someone to push the top section in and then loosen the stop bolt to allow the cable to fall into its proper path under tension.

I decided that this was serious enough to call Mrs FVM away from her work to help. She found a ten minute gap and struggled against her injury into the garden to help. With only one side of her body usable and with great pain, she pushed the mast down while I loosened the bolt. I thought she could have been a bit more cheerful about it, but there was a "twanging" sound as I undid the stop bolt so it seemed to work. Anyway, I let her go back to get on with whatever it is she does.

After that it seemed to run very freely. I use a battery powered drill to work the winches, though it was not fully charged as I was not expecting to do this job right now. I had done so much work that both the drill batteries were back on charge and so I ended up using a mini electric screwdriver to tilt the mast back and forth and raise and lower it. Even though there was no weight on the top this was a good sign. If a puny electric screwdriver can wind a mast up and down it must be running freely. 

I lubricated the mast sections and did various tests. It was clear that there was no danger of the top pulling out of the bottom section (it is fail safe, even if the top bolt failed the heel would catch on the pulley). Anyway, the cable could not pull itself higher than the pulley which is inside the bottom section. Better to check everything anyway. 

It was great to see the nice new cable, even in my now rusty and dowdy winch. The refurbishment still needs to be done later, but everything has been checked, cleaned and lubricated, and nothing amiss was found.

Nice new cable in my rusty old winch, and still with original bottom pulley.

After that I slotted the rotator back, then the antennas and preamps, which were still on the top pole. After a few quick tests it was clear that everything was working.

I would estimate that the whole job took 90 minutes for the mast part. Taking the antennas and rotator off and then putting them back on took over an hour because I left them connected and they were heavy for one person working alone. However, doing that avoided resealing the coax.

It saved time, but keeping the antennas and so forth on their pole made the lifts very heavy for me alone. I had a sore shoulder for a couple of days. Nevertheless, the job was done almost entirely single handedly.

I did not set out intending to get the whole cable job done in one afternoon. That idea just emerged as I worked. Doing it this way means that I still need to do some work on the winches and replace the pulleys at some later stage, probably next year. But I have removed the weakened cable and it taught me the detail of how the mast works. I would be happy to work on it again.

Tenna were very helpful. The tip about checking that the cable was on the right side of the stop bolt was really important.

The two pulleys and the instructions have now arrived. The instructions show a different order for doing things and are posted below. As usual click to enlarge any of the images.

My Tenna mast is sturdy and well made. I rather enjoyed taking it apart and putting it back together again, even if the job is not complete yet. Now I know that the old frayed cable is not going to stick into my hand and "stab" me.




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