Tuesday, 21 February 2017

Getting inside the Horus

The Horus is a sweet piece of kit - when it's fully operational. Unfortunately my own unit arrived with a gimbal that was double-centring. To FrSky's credit, it didn't take long to receive a replacement stick unit, and today I cracked open the transmitter to install it. It also offered an opportunity to inspect the Horus internals. Was it all good news? Read on...

Opening the case

Some transmitters are easy to open up, for example my MPX Profi 4000 is a model of accessibility - just spring open a couple of latches and bingo, all is revealed. In contrast, the Horus isn't quite so easy - it's held together with six self tapping screws, and the front and back halves are criss crossed by cables.

Separation

The first task was to undo the six cross-head screws from the back of the case. Even with the screws removed, the two halves were a tight fit, and some wiggling was needed to tease them apart. (Tip: don't remove the textured side cheeks - it's not necessary and they are a PITA to reassemble.) 

Care was needed to avoid straining the cables which still spanned the two halves. Fortunately there was just enough room to get my fingers in to disconnect the battery.

The next job was to release the connector for the external antenna. Access is via a flap on the rear moulding. The retaining nut came off easily with the help of a socket head spanner, however the locking washer needed a bit more persuasion. I now had more room to play with, and was able to unplug cables for the charger port and sliders.

Showing disconnection points for access

All that remained now was the RF cable, one end of which plugs into the module via a UFL connector, the other end goes to the antenna on the back. I didn't want to touch this, as the connector on the main board was sealed (and in any case they are intolerant of repeated disconnection). Fortunately the solution turned out to be simple: with the other cables out of the way, I was able to flip the rear moulding over the top of the unit, without disturbing the RF cable. All the components were now easily accessible.

RF cable should not be unplugged

I used a non slip mat to prevent the two halves from sliding around on the workbench. Next task - swapping the gimbal!

Replacing the gimbal

The gimbal assembly is held in place by four self tapping cross-head machine screws, accessible from within the case. The screws are quite tight, and was necessary to use a screwdriver with a substantial grip. Underneath the gimbal sits the bezel, a simple aluminium ring which is keyed to prevent rotation.

The gimbal itself is a neat unit with Hall sensors mounted on small circuit boards in place of the usual potentiometers.

Gimbal MC-12

Compared to the complex mouldings of the Taranis sticks, the mechanical design of the Horus gimbal is rather elegant. However, I do have some concerns about the wiring: the cable for the aileron sensor was far too short for comfort and not very flexible. I had to bend it into a tightish 'U' in order for it to reach the main board, and even then it was straining at extremes of movement. Furthermore, at extremes of movement, the loom was brushing slightly against the rocker arm.

As a precaution against fracture, I would recommend applying a little hot glue where the wires exit the sensor boards.

Loom arrangement

[Aside: in contrast to the production unit, my prototype Horus the boards are oriented vertically instead of horizontally - this is a better arrangement for the RH gimbal, though worse for the LH unit. It seems that the manufacturer has compromised in using the same design for both gimbals.]

With everything reassembled, I switched on and calibrated the sticks, and was able to verify that the new gimbal was working correctly.

All in all, working on the internals isn't difficult, but it's worth planning each move, and taking it slowly. In the meantime, FrSky really need to take another look at the gimbal wiring.

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