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REMOTE CONTROL BY BOB YOUNG Servicing your R/C receiver This month, we will look at the technical aspects of servicing the modern R/C receiver. Apart from using your eyes, the equipment required is a toothbrush, a can of CRC-226, a voltmeter, an oscillo­scope & a signal generator. To begin, one must keep in mind at all times that the receiver has lived out its entire operational life in an extreme­ly harsh environment; usually subjected to high levels of vibra­tion, high “G” forces, crash damage, plus possible water and dust ingress. As if this were not enough, receivers which fail in flight due to a simple component failure then have to undergo the trauma of a crash, before the wretched thing can be lobbed up on the bench of some poor serviceman, usually waiting with baited breath for the next horror story. Servicing model equipment adds backed into the propeller of the model behind me. Its prop chewed the leads off my servos and receiver and sliced the battery pack in half. In the crash which followed, the engine bearer smashed through the receiver case and broke some components. Do you think it will cost much to repair?” Or “I had my hydro howling and it hit a submerged log. It did a triple somersault and sank in 50 metres of salt water. It was only under the water for about two days before I could get it out. Do you think it is repairable? PS: you will notice the Rx case is a funny copper colour and another thing, I “I had my hydro howling & it hit a submerged log. It did a triple somersault & sank in 50 metres of salt water. It was only under the water for two days before I could get it out”. a new dimension to “Mondayi­ tis”. Every Saturday and Sunday, the weekend warriors are out there doing their thing, flying and crashing, racing and sinking, lead-footing and rolling. Every Monday the phones run hot with their horror stories. “I was flying along, minding my own business, when this tree jumped in front of my model...” Or “I was in this pylon race and I accidentally 42  Silicon Chip cannot seem to see any tracks on the PC board. I am sure they were there the last time I drained the water out of the receiver, after I collided with me mate’s Deep V.” Don’t laugh, I have had all of the above and more happen to me personally. In one horrific period, I lost six models in six consecutive flights. It was the closest I ever came to giving up flying completely. Oddly enough, they were mostly due to pro­pellers breaking in flight. The models quite literally explode in mid air when a prop sheds one blade. At the time, I was reworking big motors for my speed runs and using nylon props. In the end, I tried virtually every brand of nylon prop on the market and was finally forced to use wooden props. These break easily on rough fields and the cost is very high. However, the cost of a lost model is even higher so I had to grit my teeth and persevere with wooden props. Nylon props have improved a lot since then but always make sure they are correctly balanced. Crash hazards The last crash in the series was the most galling, however. At the time I was training for the World Aerobatics Championships in Pennsylvania, USA (1971) and used to get up at 5am and drive to “Bedrock” for a session before work. I did this every day for three months. Now “Bedrock” (Heath­cote Road, Sydney) was a dirt strip which used to grow a new crop of rocks overnight. I used to take a broom and sweep the strip every morning and every morning there would be a new crop of rocks. This particular morning as usual, I stood at the side of the runway, midway along the strip, and taxied to the end of the runway to take off. I opened the throttle and began the take-off and as the model drew level with me, I spotted a new rock right in front of my prop. Too late – the prop shattered, the nose of the model disintegrated and the whole mess fell in a heap at my feet. It never even left the ground! I was hopping mad and one model short with six weeks to go to the championships. CAP FLEXIBLE TUBE 300mm LONG Fig.1 (above): the end of the antenna is often glued to an aluminium chassis in a zig-zag pattern, as shown here. This is undesirable, as it allows noise pickup & cancel­lation in the folded sections, resulting in detuning of the front-end. Fig.2 at right shows the correct way to deal with the antenna. Such is the pressure on the dedicated contest flyer. You never knew when the next blow would be struck. Somehow you always made it to the contest but the amount of midnight oil used in the effort leaves one exhausted. However, the real point of this story is that the explosive vibration levels experienced by models throwing a prop blade can be transmitted to the receiver and servos. Also, the electronics must survive the fall to earth. This must be kept in mind at all times when servicing model equipment. With this background, we can now move on to the servicing of receivers. If the receiver is your own then you know its exact history and the probable cause of the problem. If the receiver belongs to a friend or customer, then suspect everything! If the receiver comes from a model aircraft, then suspect everything, including the aircraft. Non-electronic faults To clarify this last point, another story is in order. By far the worst receiver repair I ever had to deal with belonged to a friend of mine and was one of my first Mark VII production receivers. Because of this, I had to be particularly careful about establishing the cause of the problem. Despite all the care lavished on prototypes and early production units, faults can easily slip through. Anyway, the complaint took the form of a loss of control at the top of a loop. My friend swore blind that the fault had only begun to manifest itself in the last few weeks. Prior to that, the receiver had worked flawlessly. To compound the problem for me, in the earlier receivers we had an antenna phasing problem which caused a TO RECEIVER similar result. I was sure the Mark VII receiver was free of this fault but one could never be sure. After exhaustive testing, I began to suspect detuning due to engine vibration or simple component degradation and I went through that receiver with a fine tooth comb. I could not find a fault of any kind. Week after week this went on until finally, in exasperation, I went flying with him to see the problem for myself. Now this business of going with a customer is a real pain for it usually entails a 100km round trip in heavy traffic and blows away at least half a day. But sometimes it is unavoidable. To cut a long story short, The model was a very fast swept wing semi-scale F-86 Sabre. As soon as I saw the model I knew what came next. Sure enough the loop was performed and the model did a perfect flick roll off the top of the loop. It wasn’t loss of control in the true sense but a genuine flick-roll. As it turned out, Bill had moved the centre of gravity (CG) back the week before the problem began. The whole thing was an aerodynamic problem. Moving the CG forward cured the problem and I heard no more complaints about that particular receiver. I have spent a considerable amount of time on the foregoing because these sorts of problems caused me endless trouble until I had gained sufficient experience to recognise this sort of fault. Filling out the complaint sheet correctly is a vital part of servicing in model work and the serviceman must stay alert to any external factors causing what appears to be a purely electronic problem. Antenna installation Antenna installation is a classic 25mm problem often encountered in model work. Most model receivers come with one metre of hookup wire for an antenna. The problem is that this is too long for many models, particularly model cars. Most cars come with a flexible tube about 30cm long into which the last 30cm of the Rx antenna is slid. What do you do with the excess 70cm? You dare not cut the antenna short, for it will detune the receiver front-end badly. Now the ingenuity used by some modellers in devising the worst possible use for this excess antenna often leaves me speechless. Usually it is wound up in a ball and left lying under the servos or some such electrically noisy device. Often it is glued to the aluminium chassis in a zig-zag pattern, as shown in Fig.1. This is undesirable, as it allows noise pickup and cancel­lation of the folded sections, resulting in detuning of the Rx front-end. The correct way is to make a small bobbin and drill two holes in each end about 25mm apart. Thread the end of the antenna through one hole and wind the excess into a coil on the bobbin. Thread the 30cm to be inserted into the flexible tube through the other hole and Bob’s your uncle. It makes a neat little antenna (Fig.2) which should be mounted well clear of servos, battery packs and inter­wiring. In electric models, keep it well clear of the speed controller and motor batteries as well. All of these devices generate electrical noise and will interfere with the receiver. I cannot stress too strongly that the best reception is achieved with the maximum length of antenna, in the clearest location possible. On aircraft, a 90 degree change in direction is December 1993  43 REMOTE CONTROL – CTD permissible (from cockpit to fin and down to the tip of the tailplane) but do not fold the antenna back upon itself more than about 5cm. The maximum which can be cut off most receiver anten­ nas without serious receiver detuning is about 10cm. This often occurs during a crash and is a commonly asked question. Grilling the modeller Therefore, from my point of view, the first step in receiv­er servicing is to grill the customer on the symptoms and estab­lish the nature of the fault. Be sure to ask if there have been any changes to the model prior to the fault appearing. Changes in antenna location, CG of the model or new servos can all introduce problems. Battery problems Always be alert to battery problems as they are very high on the list of probable causes, although not as high as in the transmitter. Crash damage and engine vibration radically alters the statistical analysis of fault probabilities. Make sure you get the battery pack used during the flight when the A fault occurring at the end of eight 15 minute flights often indicates that a battery has gone flat. Check the capacity of the battery with a cycling charger or a graph. I routinely graph all batteries sent in for servicing. Do not forget to ask if the battery charged correctly and fully the night before. Batteries charged and left to stand for a week will self-dis­charge and this will influence flight duration. A slope soarer (with two servos) flown gently will last about 3.5 hours on a 500mAh battery pack. A 4-servo pattern ship flown vigorously will last about two hours and a helicopter with four servos about 45 minutes. The corollary to this is that the servos in each of these models will be subject to different rates of wear. In addition, helicopters are subject to a lot of vibra­tion. Be sure to find out what type of model your customer is flying. If the model comes in as well, I look at the antenna and advise the customer on the correctness of the installation. In this regard, I always routinely replace the receiver antenna on the grounds that it gets severely stressed “Always be alert to battery problems as they are very high on the list of probable causes. And make sure you get the battery pack used during the flight when the fault oc­curred”. fault oc­curred. Often, the model battery pack is difficult to remove and the modeller will send in a spare pack. If the fault is in the battery, you can spend a lot of time on a wild goose chase. I will not give a warranty on a repair unless I can examine the actual batteries to be used. Always be aware that the indus­try standard for average battery consumption for a flight battery is 270mA, as against approximately 120-150mA for the Tx. This means that the receiver battery is the shortest duration member in the Tx/Rx battery pair. Also be aware that this will vary depending upon the number of servos and the style of flying of the operator. 44  Silicon Chip in a crash; I often find the conductor broken inside the insulation. Receiver checks Finally, it is time to move onto an examination of the receiver. Regardless of the nature of the complaint lodged by the customer, begin with a very close physical examination of the receiver case, its PC board and components. Often, old crash damage has been missed or has just become obvious. Look for impact marks on the case. Plastic cases often return to shape after an impact, leaving almost no trace of the object which distorted the case. Aluminium cases were better in this regard, as they were stronger. They also protected against electrical noise and left marks if a sharp object impacted with the case. The real danger here is that delicate components which are unsupported inside their housings, such as crystals and IF coils, can be cracked internally and it is difficult to check on this. A vibration table is a great help and will often show up this type of fault, while freezer cans can be of help too. Next, move to the servo leads or pins and give them a visual inspection and a good scrub with a toothbrush and CRC-226. Remove any dust or dirt and inspect the PC board for corrosion. Often, receivers are purchased secondhand and whilst the current owner may be using it in an aircraft, the previous owner could have had it in a submarine, with no waterproofing! Be suspicious of everything is my motto. Somebody bringing back a model and radio that you have just serviced – in a bucket – is no joke. I once had an alcoholic customer who did just that. Some modellers really take their modelling seriously and faults in the radios or servicing are never forgiven. Next, cut open the receiver battery pack and examine the cells individually for signs of corrosion or physical damage. If there are signs of damage, discard the pack. Damaged cells will often let go in flight under engine vibration with disastrous results. Short circuited cells are a common problem in airborne battery packs. Do not replace a single cell as this will often result in unequal stress on that cell and premature failure of the pack. Pull back the insulation on the battery leads and examine them for “black wire” corrosion. Examine the battery connector for signs of damage or corrosion. Water on the connectors will result in electrolysis and damage to the pins. Always be on the alert for this type of problem. Finally, charge the pack and check each cell voltage. They should be within 0.07V of each other. Any cell showing a greater deviation than this is suspect. The actual terminal voltage will depend upon the internal chemical composition of the cell. This will vary from brand to brand but most will come off the charger at about 1.35-1.4V per cell. That’s about it for this month. Be sure to keep those nicads cycled. SC