Repairing the Brake and Motor on my Yaesu G400 rotator

The brake in my G-400 rotator bell fell off and got jammed in the gears, this is not the first time this has happened and previously nothing was damaged, other than the brake disk, and I managed to get new parts from Yaesu UK.  The new brake disk, part number S8000136, was only £0.66 the pad, part number S8000135, was only £0.33 each, you need 2 of these.

The pad is left and disk is right.


I spoke to Yaesu UK again but unfortunately they were out of stock with little chance of getting more, bodging time.

I had a few plastic washers about the same diameter which I filed out the middle to match the elongated guide as seen in the metal disk above.  I then needed a brake pad, the original is a cork disk but after testing a few materials I found the leftover surround from stick-on rubber project feet had relatively high friction.

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For the new pad I first cut out the centre circle to allow it to fit on the motor then stuck it down onto the plastic disk.

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This worked well and had quite a bit of friction, I would even say more then the original cork which I was able to test with what was left over from the original cork pad.

The way the disk/pad works is quite simple.  When the motor is not spinning a spring in the motor pulls the motor spindle the disk and pad are attached to down which pulls the brake disk and pad to the pad stuck to the motor assembly as shown below.  It’s quite impressive, due to the gearing, how much force these little brake pads actually cause in stopping an antenna turning freely in the wind.

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When the motor spins centrifugal force, and possibly the position of the magnets in the motor, forces the motor spindle to rise which separates the 2 pads (one on the disk and the other on the motor assembly) so it turns freely;

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Motor dead

However this time not just the disk failed.  As the rotator was being controlled by a computer interface, which was tracking the ISS, the controller commanded the rotator to move the antenna and I hadn’t noticed it was stuck ( the disk stuck in the gears) and therefore the motor burnt out, and quite obviously once opened;

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I thought I had a spare motor, well I did, it looked fine but when I tested it some of the coils on that seem shorted.  Each motor coil, there are 4, are approx. 1.8 Ohms end to end, the motor should measure 7.2 Ohms in total.  The spare motor had 2 coils which measured 0.82 Ohms and 2 which measured 1.8 Ohms – when tried in circuit it didn’t spin.

At this point I couldn’t find another motor, not from Yaesu, not from eBay not from a few other options suggested to me on Twitter.  While searching I came across a Blog post from KB5WIA who had fixed the same motor by re-winding the coils.

This was going to take some time but I had little choice. I won’t reproduce David, KB5WIA page but will show some of various steps I followed.

First was to dismantle the motor, this takes about 2 hours to remove each plate surrounding the motor one by one.  There are no small parts and once you get the method of removing the disks it becomes easy.

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Next was to remove the wire off the formers, there is quite a lot of it, about 15m worth, so to replace all 4 coils you need around 60m of wire. Get a 100m spool.

The wire is SWG #25 but I only had SWG #28 in enough quantity so used that.  Like KB5WIA I created a jig to mount the former in a drill using an M6 bolt, washer and nuts.  An M6 nut fitted perfectly inside the former to stop it spinning.

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This was then mounted in a drill, left, and 160 turns of #28 wire wound then cut. Each coil took about 3 minutes to wind.

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After all 4 were done they were taped up and put back onto the inner former ready to have each outer disk put back on.

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Putting each outer disk back back on was the most time consuming, it took around 3-4 hours. Again once you get the technique right it’s much easier, I also put disks on from both ends, starting with half from one side before the other half from the other side.

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Once all back together it looks almost like new.

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After wiring it up all worked, but not after a hiccup where I missed soldering one of the coils to the PCB board inside the motor. After that it was fine.

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