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MULTIPLEX Royal Pro Modification

Since the early days of my conscious passion for a remote controlled models I was a big fan of a Multiplex radio equipment. It's not because of the feature set or some exceptional quality, they are surely both present, but differ not so dramatically from the rest of a top-level competitors. It's all about different approach to the overall design and ergonomics, which I value really a lot. Multiplex is "European" in its roots, compared to the whole bunch of so-called "asian radios".

After a long period without flights I finally decided to get back to practice and suddenly find out that my radio is damaged: one of the three-position switches was seriously bent! Usually I store it in a bag with other field equipment such as charger, batteries and screwdrivers, that can find itself in unpredictable places across the apartments. It seems like someone unfortunately kicked the bag when he was walking by :(.

I had to disassemble the radio to replace the switch and decided that this is a good opportunity to tell the world about three little upgrades that I made to my Royal Pro transmitter.

#1 Internal Antenna

Initially I've purchased my radio, equipped with a 35MHz PCM transmitter module (old style one-way communication channel). It came with an anachronistic 1.5m rigid telescopic antenna, which I've immediately replaced with a short rubber spiral one. I knew that, in theory, spiral antennas have slightly worser characteristics, but it doesn't made a sense to me because I wasn't into a long distance glider flying anyway…

Later, when I changed a plane in favour of helicopters I came to the conclusion I have to switch to the modern Multiplex M-LINK system, which is actually a 2.4GHz FHSS (frequency-hopping spread spectrum) bidirectional system with optional telemetry. All these stuff is vital in helis, because of required level of reliability (you really don't want to hit somebody with a blade of an uncontrolled helicopter rotor spinning at 2500RPM) and convenience (for example, it's good to know how much charge is left in the batteries).

When I first saw the antenna supplied with a newer 2.4GHz module, the aesthetic part of me began to complain that my favourite TX will now begin to look silly with this short piece of a L-shaped nonsense. However, I've installed it in place and used the radio till the antenna accidentally got broken a couple of months later. This was the time to make a little experiment. I always wanted a transmitter without an external antenna at all, so I decided to mod my Royal Pro to suit my needs.

As far as you know, 2.4GHz antennas used in RC radios (as well as in a consumer Wi-Fi equipment) are just a primitive ½ wave dipoles. Under the plastic casing there is a thin coaxial cable with a metal thingie in the middle of it.

I took the stock Multiplex M-LINK antenna and carefully removed all the plastics. Then, I've reinforced the coax cable with heat shrink tube, so it be able to maintain the strait line form. The main problem was where to place it inside the radio body. I've used a spare servo horn from a mini servo and a superglue to make a seat for the HF antenna connector right below the display. The antenna was placed perpendicularly to the vector that goes from the pilot to the model during the flight, so the most radiation is emitted to the model (see 2.4GHz dipole antenna radiation diagram).

As long as the old antenna base at top of the radio (the hemisphere with a figure cut) was no longer in use I decided to cut it of the transmitter "head" to achieve a cleaner design and simpler form. I've used a Dremel-like tool with an abrasive cutting wheel to roughly cut-off most of the excess material and finished it with a broach file. The hole that formed in place of a hemisphere was covered from inside with a circular piece of plastic cut from an iPhone 4 box. It's white, opposed to the surrounding blacks, but I don't care.

#2 Additional Switches

On Royal Pro 9 there are two small black stoppers near the letters "P" and "K" which cover the holes for an optional switches that you can install yourself if you feel a need. Multiplex sells these switches that already have cables and plugs soldered to them, but they just cost too much. You can easily build them yourself in no time for dollar or two if you familiar with a soldering iron instead of paying $20-30 for a complete solution.

When it comes to expansion of the radio you have a choice what type of switches to install. According to the manual, the following combinations are possible: 2 × two-pos switches or 1 two-pos switch with 1 three-pos on the other side. There is no way to plug two three-position switches at once!

I wanted 2 two-position switches and luckily had a couple of small soviet military-grade-quality switches in my storeroom. Despite of my efforts I was unable to find the compatible plugs, so I simply soldered the cables down to the PCB. Not too elegant, but it works! The rest of installation is trivial: took off the rotary encoder wheel (be careful with a small washer on the axle! Make sure you place it back during the assembly. If not placed, you be unable to click on the wheel. Been there, done that.), unscrew the four screws that fix the "corner module" in place, pop the stub that is glued to the radio case with a drop of a hot-glue and fix the new switch in place with a nut and washer. Assemble in a reverse order. Repeat for the opposite side of the radio.

For better TX handling and switch selectivity during the flight I put pieces of a fuel tube on some of them (mostly critical, like autorotation mode switch). This way you can easily differentiate between the switches even they have the same stick length.

#3 Metal Stick Ends

MULTIPLEX does a great job by including three joystick handle sizes in their transmitter set: the long handles for "thumb plus index finger" flyers, short sticks for "thumb-only" pilots and also a medium size for those who are uncertain about their style.

I've used a medium length sticks until I became a heli-guy and now the shortest ones are only the option for me. Stock sticks are made of plastic and during intensive flight they sometimes could slip out of your fingers. It's very unpleasant and even dangerous, that's why I've found a nice guy at the RC Groups forums who made a custom, adjustable aluminium handles for me. They are top-notch quality (built with CNC) and I recommend them for everyone who adopts "thumb"-piloting with Royal Evo/Pro.