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Crossed Wires | Back to Article Index

By Stan Yeo

Recently I was asked to finish fitting out a four servo wing sports soarer and program the transmitter. No problem I thought, three or four hours toil and the job would be finished. Wrong. I ended up spending nearly fifteen hours on the job. A number of things went wrong prompting me to write this article and make a mental note not to take over a part completed task in the future!

The problems I encountered ranged from poor servo installation, non-conventional wiring and programming an unfamiliar transmitter. Servo installation is an article by itself and is frequently covered in kit instructions, reviews and plans etc. Programming the Tx. Well just because the word computer is used in the product description is no excuse to adopt the computer industry's habit of writing instruction manuals to baffle all and sundry. To add insult to injury they are in my experience getting worse rather than better. To give credit where credit is due the technical departments of the importers are generally very good at answering your questions when they are contacted.

This leaves one problem, that of wiring etc. I am aware that not all radio control systems sing from the same hymn sheet when it comes to wiring, the colours used are frequently different as is sometimes the connection lead wiring order. Being aware of these differences etc. should have protected me from cross wiring mistakes. Unfortunately when I carried out polarity checks on a pre-wired wing I was working on I failed to spot the red and black wires were reversed. Result four blown servo amplifiers! I am not the first to do this and undoubtably will not be the last.

This article is therefore a discussion of various the wiring tasks associated with today's radio control systems.


An increasing number of modellers, particularly those with computer sets are replacing their original transmitter batteries with higher capacity ones to give longer Tx. usage times. Unfortunately the set chargers are seldom able to deliver the higher charge currents required to charge the batteries properly so a higher capacity charger is needed. A word of warning though. Most transmitters have a diode (electrical non-return valve) in the charging circuit to prevent the Tx battery being discharged through the charging socket. This means that cycling Tx batteries in-situ is not possible and there is a danger when fast charging of 'blowing' this diode if the charging current is too high.

All the main UK imported R/C systems (Futaba, JR, Hitec Sanwa) use a similar DC power socket for charging but, there are two sizes and two ways of wiring them. One uses the centre pin as negative whilst on the others the centre pin is positive. Likewise there is a difference in size of the centre pin, one is 2.1mm diameter and the other 2.5mm. The plugs are also available in two lengths so always buy the longer one.

Tx Charging Socket Polarity Table

Transmitter Hole Diameter Centre Pin Polarity
JR 2.1mm Negative
Hitec 2.1mm Positive
Futaba 2.5mm Positive
Sanwa 2.5mm Positive

Some Tx batteries have pressure or push fit connectors whilst others have a polarised plug and socket arrangement. When buying replacement packs from other than the equipment importer it is not always possible to buy a pack with the correct plug fitted. However most plugs can be made to fit but this usually means the plug is not polarised so be very very careful. Check and double check that the wiring is the same as the original when you plug the battery in otherwise you could have an expensive repair bill on your hands.

Airborne Wiring

Fortunately all the current production R/C systems mentioned polarise their airborne equipment the same way i.e. Negative, Positive and Signal (but be careful if you have any old Sanwa black plug equipment as the positive and negative are reversed!). Unfortunately there are two plug and socket systems and four different colour codes! The main difference is Futaba has a polarising tab whilst the others are polarised with bevelled corners on the top face of the plug. A tip if mixing plugs and sockets and they are tight, scrape the sides of the female with a scalpel blade as opposed to trimming or filing. This reduces the grabbing effect of a slightly rough surface finish.

Airborne Equipment Colour Codes.

Make Negative Positive Signal Plug
Futaba Black Red White Futaba (tab)
JR Brown Red Orange 'Uni-Plug'
Hitec Black Red Yellow 'Uni-Plug'
Sanwa Black Red Blue 'Uni-Plug'
Multiplex Black Red Yellow 'Uni-Plug'

The Plea

Sometimes it is not always possible for a number of reasons to use matching colour coded servo wire when manufacturing wiring looms. If this is the case please do not make up your own colour code but use the colours indicated above for the different polarities i.e. Negative should be either Black or brown. This will help you and subsequent owners should the wiring require modification or repairs.

Servo Reversing (Does NOT apply to digital servos)

Even with the advent of built in transmitter servo reversing and computer Txs it is still sometimes not possible to get one servo of a pair to operate in the required direction. In this situation you have two choices, 1. To install a servo reverser on one of the servos or 2. To electronically reverse the servo. The most common instance of this problem is when using two servos to operate the flaps via a 'Y' lead. Sometimes the problem can be overcome by turning the servo 'over' but this is not always possible. The problem is not always solved using a computerised transmitter and using two channels to operate the flaps because the servo reversing on some computerised Txs is carried out AFTER mixing rather than before i.e. JR388. Tip. If no matter what you do one servo is always operating in the wrong sense when using flapperons, V tail mixing or elevons swap the channels the servos are plugged into. Also if it is not a permanent connection tag the plug and socket you take apart.

Diagram of wing with flap servos

The principles of servo reversing are very straightforward. Basically the servo motor terminals must be reversed along with the two outer terminals of the feedback potentiometer (pot). The problems arise mainly from two sources 1. the servo amplifier is hard-wired to the motor and pot i.e. soldered directly onto them i.e. not via fly-leads, and 2. Getting access to the servo pot. My advice is remove the servo pot. Do NOT attempt to reverse the leads on the amplifier as apart from being extremely fiddly there is a danger of damaging the surface mount components on the amplifier PCB (printed circuit board). Another piece of advice is make a sketch of the wiring before you start. If after completing the task the servo runs full travel it is likely that only one of the operations has been carried out.

Diagram of servo amplifier wiring

Aileron Extension Leads

When the aileron servo lead needs to be extended to connect the servo to the Rx please do not cut the servo lead and insert an extra length of cabling. Try and use a propriety servo extension lead. This way the servo is easier to replace should it fail or get damaged and it does not invalidate the warranty. To pass the extension lead through the wing remove the plastic holder from the Rx end of the lead. The connectors are held in the holder either by a plastic tongue on the holder or a metal tongue on the connector. Either way disengage the tongue using the point of a scalpel knife and remove the connector. After passing the extension lead through the wing re-assemble the plug ensuring all tongues are properly engaged and the wires are in the correct positions.

Where the aileron servo lead, including any extensions, exceed 500mm it is possible for the lead to act as an aerial and pick local transmitter signals and feed them back into the receiver thereby increasing the risk of interference. One way to minimise this risk is to fit Toroidal Chokes to the servo lead as close to the receiver as possible. These chokes look like lamb's tail docking rings i.e. small fat rubber bands. The value of the impedance (alternating current resistance) produced is a function of the square of the number of turns so the more turns the more efficient they are. To fit them dismantle the servo plug as above and wrap the lead around the choke as per the photograph.

Photograph of choke fitted to servo lead.


Finally a brief note about battery charging as most early life battery problems can be attributed to inappropriate charging. A standard charge is a charging current of 10% of the capacity of the battery i.e. a 600mAhr battery would be charged at 60mA. A fast charge is one where the charging current is typically the capacity of the battery i.e. for the battery above 600mA. Some batteries can be fast charged at 2 times the rated battery capacity but this is not recommended as it shortens the life of the battery and the charge is not as complete as it would be if charged at a lower current. Basically the faster you put it in the less you get out. Likewise the faster you take it out the less you get out! (batteries are rated at their 5 hour discharge current). Also it is recommended that the first few charges are standard charges to condition the battery. If charging at currents greater than 10% of the capacity of the battery then it is recommended that a peak detect charger is used to avoid overcharging and damage to the battery.


I hope the above has been useful. If you do carry out any of the above tasks please take care, be methodical, produce wiring diagrams before you start, follow the colour code conventions, use a small pointed soldering iron, thin multi-cored fluxed solder and heatshrink sleeving to insulate solder joints. Do not use insulating tape, it looks untidy and offers no support to the solder joint. Also be aware that modifying you R/C equipment will in all probability invalidate your guarantee.

Finally my apologies to the professionals. I am aware of your views re modifying equipment etc. but a lot of modellers are going to do it no matter what advice to the contrary they are given so in my view the safer option is provide assistance in order to reduce the number of mistakes made. For other associated model flying articles please visit my website





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