Tuesday, 21 February 2017

Getting inside the Horus

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

Opening the case

Some transmitters are easy to open up, for example my MPX Profi 4000 uses a couple of spring latches, no screwdriver required. In contrast, the Horus is held together with six self tapping screws, and the front and back halves are criss crossed by cables.


The first task is to undo six cross-head screws from the back of the case. With newer units, you may also need to undo four black screws between the module cover and the handle (these are blocked off on my unit as can be seen in the photo below).

Then gently split the front and rear halves, by tugging on the handle. Some wiggling may be needed to tease them apart. Take care not to strain any cables spanning the two halves. Also, don't peel off the textured side cheeks - it's not necessary and they are a PITA to reassemble.

Next, squeeze your fingers in (or use long nosed pliers), and unplug the battery connector from the main board.

Next release the external antenna connector. This is secured by a nut accessible via a flap at the rear. A socket head spanner will help here. You'll also need to remove a locking washer (mine was stubbornly trapped on the connector and required a bit of teasing with a screwdriver).

Next, unplug the cables for the charger port and the rear sliders.

Showing disconnection points for access

All that remains now is the cable to the GPS module. Don't attempt to unplug it (in any case the UFL connector is sealed to the board). Just flip the rear moulding over the top of the unit, without disturbing the RF cable (see photo below).

RF cable should not be unplugged

All the components will now be easily accessible. Use 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 will required 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 on my unit 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, the loom was brushing slightly against the rocker arm.

This is all a little dissappointing, and not something one would expect to see on a high end set. As a precaution against fracture, I would recommend applying some hot glue at the cable clamp.

Loom arrangement

With everything reassembled, switch on and re-calibrate the sticks. You may also want to look at the Analog Inputs menu to check the centres and end points values.

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. [Edit: sets manufactured from May 2017 have  flexible silicon cables.]

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