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REMOTE CONTROL BY BOB YOUNG Servicing your R/C transmitter Modern R/C equipment has dramatically improved in quality & reli­ability in the past few years but still responds well to routine maintenance. This month, we will look at some of the basic servicing procedures. So your favourite toy is ailing? Range is down, one of the servos is chattering away around neutral and all in all you feel it is unwise to venture out to the flying field, race track or pond. You desperately need a relaxation fix. What to do? From the outset I must state that the best place for ailing R/C equipment is back with father (ie, the manufacturer). However in Australia 1993, father usually resides overseas. Thus, the next best is factory appointed agents. These agents usually have trained technicians, circuits, good test equipment and the cor­ rect range of spares, a vital point in equipment that is subject routinely to 100G+ de­celer­ations. Having decided to waive the above options, you are about to embark on the great adventure – finding out how your set works. Test equipment This ancient unit is an absorption wavemeter that has served the author for many years. 82  Silicon Chip For AM systems, the test equipment required is very basic and for those fortunate enough to possess an oscilloscope, even the modulation pattern is plainly visible. For FM systems, the requirements in regards to test equipment are more stringent and thus more expensive. PCM (pulse code modulation) adds a new dimension, with software analysis on top of FM to be taken into account, and is outside the scope of this article. The really basic elements for AM servicing are the usual assortment of handtools, a toothbrush, a can of CRC.226 spray cleaner and a multimeter. To this, in descending order of impor­tance, may be added the following: cycling battery charger, oscilloscope (preferably 15MHz bandwidth or better), absorption wave­meter, servo analyser and signal generator. For FM sets, you can add a modulation meter and frequency counter to the list. Finally, for tuning a modern transmitter, a spectrum ana­lyser is a must, because part of the tuning procedure involves the suppression of harmonics. The transmitter Fig.1 is the schematic of a typical Tx and recourse to the actual circuit diagrams for your make and model of set will be a great help. Fig.2 gives the typical PPM pulse train. The great difficulty with modern R/C equipment is the in-house integrated circuit. In the old days of discrete components, circuits could be traced, components were clearly labelled and substitutes could often be purchased at the local electronics store. These days, the encoder and decod­ er are usually in a single IC labelled with a house number and available only from the manufacturer’s agent. Fortunately, the RF section is usually still discrete and thus can be serviced. However, I must point out here that the most probable causes of trouble are battery or mechanical. The electronics rarely fails, so there is much that can be done by the handy modeller to keep his or her gear in good condition. One of the problems with R/C transmitters as far as testing is concerned is the measurement of power. As the ANTENNA ENCODER/ MULTIPLEXER/ MICRO MASTER CLOCK AM MODULATOR CONTROL POTS FM RF BUFFER AMPLIFIER PA RF OSCILLATOR RF METER Fig.1: block diagram of a typical radio-control transmitter. The encoding circuitry will be contained in a single IC but the RF section is usually discrete & thus can be serviced. 1-2ms 350us 50us 20ms Fig.2: typical PPM pulse train from a radio-controlled transmitter. If you have a CRO, you can check that this waveform appears at the output from the modulator. antennas are built in and do not use coax connections, it is difficult to hook up test equipment. This type of equipment is also expensive and not readily available to the average modeller. Thus, one of the most helpful instruments for transmitter testing is the absorption wavemeter. They can be built by the home constructor and provide a useful guide to transmitter output. One of the photos accompanying this article shows my origi­nal wave­ meter, much admired over the years by customers but sadly now showing its age. Built in 1955, this meter has done Trojan service. Standing in the one spot at Riverwood for 22 years, it has provided me with an instant guide to the relative field strength of all transmitters. Because it contains no batteries, it provides a stable and thus reliable indication of transmitter output. In open air, it will provide a reading from a typical Tx up to 10 metres. When using a wavemeter, it is important to remember that long extension leads or large masses of metal placed in the vicinity of the wavemeter or transmitter will influence the meter reading. Thus, the Tx test area must be kept clear of these items. While there is very little in the circuitry of an absorp­tion wavemeter, its mechanical construction can be a little tricky although the photos of my treasured unit may not demonstrate this. If possible and if parts are available, I may be able to describe the construction of an absorption wave­meter in a future issue. Battery checks To begin your analysis of your R/C system, take the back off the Tx and go straight for the batteries. Statistically, this is number one on the list of suspects. Modern rechargeable AA cells have a useful life in excess of five years if treated with respect and some of the SAFT AA cells in Silvertone sets are still working after 10 years. Personally, I recommend replacing battery packs in transmitters every three to five years and airborne packs in the same time corrosion. When the cells vent, they give off corrosive gases which can eat the legs clean off components and devour PC board tracks. “Black wire” usually appears in the black or negative bat­tery lead and is only associated with nicad batteries. This curious corrosion completely removes all traces of copper from the conductor and replaces it with some sort of black garbage. The wire then becomes dark or black in appearance, very brittle and incapable of carrying any current. Electronic problems usual­ly associated with a lack of earth will then begin to appear and ultimately the set will fail completely. It is more dangerous in the airborne battery because of the amount of current drawn by the servos. A complicating factor is the high level of engine vibration which may eventually snap the wire as it becomes more brittle as the corrosion progresses. Tin plating the conductors slows the process considerably and unplat­ ed copper conductors should not be used as battery leads. The corrosion can cross soldered joints but usually stops at the switch. So all wiring associated with the battery, switch and charging circuits should be examined regularly. This may mean removing covers or cutting off heatshrink sleeving on cables. Please do not be put off by this for the results may be well worth it. Model aircraft in particular demand preventative maintenance and even if the batteries come out of the inspection squeaky clean, you will at least have no concerns in this area. The batteries and leads should be examined once every two years and One of the most useful instruments for transmitter testing is the absorption wavemeter. They are very easily built by the home constructor & provide a useful guide to transmitter output. frame or after physical damage from a crash. Inspect the batteries for any signs of corrosion and, in particular, examine the battery leads very closely for signs of “black wire syndrome”. You should also examine the components and the PC board area above the battery for once salting of the terminals begins to appear, every six months after that. CRC-226 sprayed onto the battery termi­nals, charge socket and switch from new will slow down the black wire problem considerably. Repeat this procedure every 12 months or so. Since I last wrote about “black wire September 1993  83 freezing and vibration testing failed to produce the slightest shift in neutral at my factory but as soon as the customer took it home, the neutrals would shift. This went on for several weeks. You can imagine the havoc created in the service department. Tempers were fraying and reputations were in tatters. The owner of this particular set lived in a small flat and did all of his work on his models on the kitchen table after tea. In other words, after he had cooked his evening meal. Thus, we eventually reasoned, the kitchen would be full of steam and cooking smells. In desperation, I blew on the PC board through a piece of heatshrink sleeving which localised the airstream to a small segment of the PC board. The tube provided a venturi effect, chilling the air and leaving moisture on the PC board. Bingo! The neutrals shifted immediately I blew on the PC board just above the negative battery terminal and by quite a considerable amount. The same test on a new transmitter of the same brand and model yielded no result. The servo neutrals remained normal. The set’s history While there is very little circuitry inside an absorp­tion wavemeter, its mechanical construction can be a little tricky. A wavemeter contains no batteries & provides a reliable indication of transmitter output. syndrome” in the February 1990 issue, I still have found no clear explanation of the cause and I am more mystified than ever about this problem. I have even found several cases of “black wire” in signal leads and one in the positive lead. The red lead in question was in a portable telephone and the corrosion had eaten the tracks off the PC board. The black lead was perfectly OK, something that I have never encountered before in any nicad-powered system. Board contamination The above problem raises the spectre 84  Silicon Chip of the most serious outcome of battery corrosion – contamination of the PC board and surrounding electronics. We have a tendency at Silver­tone Electronics to call all problems by pet names and by far the most baffling service problem I have ever encountered was the “kitchen table syndrome”. The problem manifested itself in a shift of servo neutrals, something quite extraordinary in PPM systems. There was no sign of corrosion in the encoder components or PC board tracks. This shift appeared at random intervals and all attempts to pin down the cause were fruitless. Heating, An examination of the history of the transmitter revealed that the problem appeared after the customer had the original battery replaced, because it had split during charging. The original was a button cell battery pack and these were quite prone to this problem once they had aged. It appeared that the battery chemicals had vented onto the PC board and formed a substrate which, when overlaid with cook­ ing fumes and steam, provided a leakage path sufficient to alter the pulse width of the one-shot generators. Scrubbing the PC board with solvents and spraying on a liberal coating of lacquer completely eliminated the problem and the set soldiered on to a respectable retirement. As always, this problem was simple once solved. We now do the “blow test” as routine on all transmitters over a few years old. In addition, PC boards are always cleaned and lacquered after battery replacement. Modern sets incorporate the lessons learned in dealing with these problems and some transmitters now have the battery in a semi-sealed com- partment to minimise the incursion of vented battery gases into the areas containing electronics. Some gas may still find its way up into the electronics however, so always be alert for signs of corrosion, particularly where the battery wires join onto the PC board. Charging the batteries This now brings us to the problems of battery charging. No more vexing a problem exists for modellers than fighting their way through the maze of argument and counter argument surrounding the care and charging of nicad batteries. I feel that much of the above damage is the result of poor charging techniques. Yet modern nicad batteries have many built-in safeguards to prevent damage caused by overcharging and figures quoted by SAFT, for example, give a safe overcharge of 20,000 hours at the c/10 rate. Why then, does “black wire” occur, what can be done to prevent it and what is the actual chemical process involved? The battery literature main- How do you come to grips with a foe as slippery as this? (Editor’s note: the electrolyte in nickel cadmium and alka­line manganese cells is based on potassium hydroxide (ie, caustic potash) and this is released if these cells vent or leak. The vent for nicad cells is at the positive end. If the cells are leaking, the electrolyte can travel under the heat­shrink sleeving of the case and then up the battery leads by capillary action and ultimately migrate to the tracks of the PC board. Thus, it would seem that the “black wire syndrome” is essentially a product of corrosion between copper and potassium hydroxide). Storing nicads Originally, common wisdom for the storage of nicads was to fully discharge each cell and store it in the discharged state with a strap shorting out each cell. I have seen nothing since that has altered my view that this is the correct method for storing nicads. It is, however, almost impossible to do with a set of stacked cells that have been sealed in a plastic housing. Nicads are now the number one killer of model aircraft. It is safe to say that all sets fitted with nicads will be subject to corrosion to a greater or lesser degree at some stage of their lifetime. tains a stony silence on all of the above. In the absence of any official, definitive data, I can only offer the following subjective advice based on 40 years of practical experience with nicad batteries. Firstly, nothing is as it seems. Above I stated that I feel the damage is caused by overcharging yet I can quote several cases of sets which were purchased from new, charged once or twice and never used again; a very common problem in modelling. These sets some years later exhibited severe black wire corro­sion. Again, I call this problem the “black wire syndrome” be­cause I first encountered it in the black or negative battery lead and yet, as stated above, I have also encountered black wire syndrome in the signal and positive battery leads. Therefore, I recommend that after each operating session, you should use a cycling battery charger. Discharge the batteries to their safe endpoint (1V per cell) and leave them in this state until the night before the next session. At Silvertone, I use a chart recorder to trace the voltage curve on all sets we service. This uses a fixed load current of 270 milliamps (which is the industry standard for the simulation of a 4-servo system) and gives a trace of about two hours for a good set of nicads – equivalent to 8-10 15- minute flights. If the set is not used for a period in excess of six months, run a couple of discharge/charge cycles to keep the chemicals circulat­ing inside the battery. As before, it’s best to leave the cells in a discharged state. Avoid overcharging and high rate charging. If you do not agree with leaving the batteries flat, then cycle them every time before you go flying. If you do not have a cycling charger, then use a battery discharger and your regular charger. SILICON CHIP has published details of these devices, as noted at the end of this article. The No.1 killer I have spent a considerable amount of time on nicads in this issue because they are now the number one killer of model aircraft and a great source of vexation for all modellers and indeed all users of nicads. It is safe to say that all sets fitted with nicads will be subject to corrosion to a greater or lesser degree at some stage of their lifetime. Some of the latest transmitters fitted with sealed batter­ies which are housed in a moulded compartment inside the trans­mitter case may be the exception. These batteries slide into their compartment and the clips make contact with nickel plated leaf springs. Thus, there is a solid nickel barrier between the batteries and the transmitter interwiring. This type of trans­mit­ter is a pain to repair because once the back comes off, all contact is lost with the battery. However, they do represent the most logical approach to preventing battery corrosion. The principles above apply to all nicad-powered devices. They are problems we will all become more familiar with in time. This is not to say that nicads have become more unreliable. Rather quite the opposite, for they have become much more robust and reliable over the past few years, particularly in the AA cell configuration. However, the reliability of the electronics has far outstripped that of nicads and left them in the low spot on the totem pole. Next month, we will look at some of the electronic and me­chanical maintenance procedures. References (1). How to Get the Most Out of Nicad Batteries, by Garry Cratt. SILICON CHIP, August 1988. (2). Nicad Battery Discharger, SILICON CHIP, July 1992. (3). Automatic Nicad Battery Discharger, SILICON CHIP, November 1992. (4). Single Cell Nicad Discharger, SILISC CON CHIP, May 1993. September 1993  85