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REMOTE CONTROL BY BOB YOUNG Maintaining your R/C transmitter; Pt.2 Last month, we left off in the middle of a discussion on the modern approach to battery housings. This month, we continue with hints on maintaining transmitter reliability. One curious fact that I forgot to mention last month, in regard to the “black wire” problem, is that the black or negative wire is attached to the non vented end of the battery. If “black wire” is caused by chemicals leaking from the vent, they should attack the red or positive lead which is attached to the vented terminal. This is rarely the case and in spite of the Editor’s note seeking to explain the mystery, I remain unconvinced. My guess is that the process involves some sort of electrolysis. I also failed to stress that the PVC insulation covering the wires must be stretched back to reveal the con- can do to prevent the problems of old age in this area. Firstly, the batteries are mounted in many different ways in modern transmitters but the most satisfactory way, from a day-to-day operational point of view, is for the batteries to be in a welded pack and hard wired into the transmitter. This is the only 100% foolproof method of ensuring battery continuity. We have already discussed (last month) the very valid rea­sons for welded packs in self-contained hous­ings, which make contact with nickel plated slide-in contacts. This arrangement, as good as it is, does leave the battery pack vulnerable to mishandling. In I cannot recommend cycling chargers too highly, for all sorts of reasons. Preventative maintenance is an absolute must in model flying &, for that matter, in all modelling. ductors. The wires should be bright silver or copper. If “black wire” is present, the wire will appear dark grey to gloss black. The wire will also probably come away in your hand with the slightest tug. Do not attempt to re-solder it for it will not solder properly and will also contaminate your soldering iron tip. Before leaving the batteries, there are several things the handy modeller 86  Silicon Chip time, with continual use and the odd removal and re-insertion for examination, the slide-in contacts can be com­pressed and become intermittent. So keep these clean and correc­ tly tensioned. CRC-226 sprayed on the battery pack ends and con­tacts will help prevent corrosion forming in this very vulnerable area. A very common method of inserting batteries into R/C trans­mitters is to clip nicad AA cells into dry battery holders. Here we have a potential catastrophe just waiting to happen. Most AA-cell holders that I have encountered appear to be made of green cheese and in time, due to constant pressure from the terminal springs, the plastic at the ends bends away from the batteries and contact pressure is lost. Hard wire the batteries My advice here is to dump the battery box and hard wire the batteries into the transmitter. If this is too hard, then examine the battery box closely for signs of distortion at the ends. If this is occurring, dump that battery box and look for one made of rigid plastic and with adequate webbing to support the ends. Make sure the battery ends, springs and terminals are free from corrosion and that the springs are correctly tensioned. Finally, spray the battery pack ends and terminals with CRC-226. One word of caution in regard to battery boxes: the trans­ mitter is a portable unit and is subject to bumps and knocks. Some of these jolts are severe enough to flick a battery from the box and then power is lost completely. Make sure that the batter­ies are locked into place by wrapping some insulation tape or elastic bands around each set of four batteries. This applies to the battery box in the model as well but the 8-cell boxes are the worst as the cells tend to spring up into a “V” if knocked. In fact, I do not recommend battery boxes at all in the model, due to the effects of engine vibration. Soldering cells together And now I should comment on soldering to nicad cells. The manu- facturers do not recommend soldering to cells direct and they warn that cells can explode or at least be damaged by the heat. You would have to apply an awful lot of heat for one to explode but they are relatively easily damaged during soldering. For this reason, welded tabs are vir­ tually a must on cells intended to be soldered together. However if you do wish to solder cells with no tabs or replace a tab that has come adrift, then here is the procedure. File both ends of the battery until the area to be soldered is quite clean. This is an absolute must! – use a very hot soldering iron, with a good thermal mass. A large Scope iron is quite good for this job. Tin the cell ends first by simultaneously applying the solder (resin cored 60-40) and the iron to the terminal points. A quick dab is all that should be necessary. If the iron is hot enough, the solder will flow immediately with minimum heat transfer to the battery internals. However, if the iron is too cold or the thermal mass insuf­ficient, you will need to hold the iron in contact with the battery for an extended period. This will result in a build up of heat to the battery internals and almost certain permanent damage to the cell. Now tin the wire ends and, with a quick dab of the hot iron, solder the lead to the cell. Always remember that perfectly clean contacts, a good hot iron and quick dabs are all that are needed. Do not leave the iron in contact with the battery for any extended period. Now we come to the problems associated with ordinary (non-rechargeable) AA-cell batteries. By definition, these need to be replaced often and most of the above comments apply to this type of battery. Lock the cells into place with tape or elastic bands and keep the contacts clean and tight. They will also corrode the terminals, particularly if they are left to go flat, so keep up the CRC-226. The process of generating the electrical energy in a dry cell battery calls for the zinc case to be consumed. In time then, the case will begin to leak as the internal chemicals eat their way through the case. For this reason, it is important to remove the cells if they are flat or are to be left standing for any period of time. We are all too familiar with the mess that develops inside a battery-powered device in which the batteries have been left too long. Manufacturers these days put a second case of steel or cardboard around the zinc case to help contain this corrosion. This is only a help, not a cure, so take those old cells out. Again, a similar process of electrolysis takes place and the terminals will begin to corrode before the batteries show visible signs of leakage. Constant inspection and lubrication with CRC226 is the only answer to the problems of corroded terminals. If you must solder to dry cells, exactly the same procedure must be followed as above, with one extra precaution. The nega­tive end on some cells is not actually the end of the battery, but is a pressed metal disc, held in place by the rolled over ends of the outer casing (see Fig.1). This disc relies almost entirely upon the terminal CUT HERE ROLLED END ZINC BATTERY CASE METAL DISC ROLLED END Fig.1: the nega­tive end on some cells is a pressed metal disc, held in place by the rolled over ends of the outer casing. Soldering a wire to this disc can result in the negative terminal going open-circuit. spring pressure to force it against the bottom (negative) casing. Thus, if a wire is soldered to this disc, there is a very real risk of an open circuit on the nega­tive terminal. The cure is to remove this disc with a sharp knife and solder directly onto the zinc casing. Simply cut down through the outer casing about 2mm behind the end cap. This only applies to cells with a cardboard casing. A steel casing cannot be cut and such batteries should be used in a battery box. The positive terminal needs no attention other than filing. Many years back, I lost several good models before I dis­covered this trick. Modern transmitters run on 9.6V (eight cells). Using the voltmeter, check to see that the battery pack comes up to approx­imately 1.25V per cell when it comes off charge. Many of the new breed of transmitters have an inbuilt voltmeter with a liquid crystal display, so this is a routine matter. Always check this voltage with the transmitter switched on. Most transmitters will work with one or even two cells short circuited but range will be down. When one is flying and the model is 600 metres away, it’s not the ideal time to discover that your Tx pack is down one or two cells. If you are using a cycling battery charger, then a shorted cell will show up as an extraordinary reduction in time to discharge. I cannot recommend cycling chargers too highly, for all sorts of reasons. Preventative maintenance is an absolute must in model flying and, for that matter, in all modelling. Ponds are cold places to enter in winter, while car tracks are very busy and nicely built and painted cars soon look very second­hand after a few collisions. I have spent a considerable amount of time on the battery packs for good reason. It is the area where butchery abounds. I get transmitters in for repair with batteries soldered with blow torches, acid flux, and with brands of nicads mixed together, a very poor practice. I get “black wire”, batteries that look like a salt cellar in the rainy season, and in these I also get holes eaten clean through the aluminium transmitter case by the battery chemicals. I get battery boxes that look as if they have never made contact in their life and dry cell batteries that are flat out lifting the needle off the voltmeter stops. I get PC boards that are green and black and with the copper tracks eaten clean off the substrate. I also get components growing whiskers and with legs corroded completely through. All of these faults were easily preventable yet most had resulted in crashed models. Most transmitters will run for their entire lives with no electrical faults. However, if you keep the transmitter in service long enough, you will encounter battery problems. From here, it is a short step to damaged components and PC boards. For this reason, I strongly recommend routine replacement of the nicads once every five years. My own transmitter was built in October 1993  87 1974 and apart from the replacement of nicads, is still original. It is interesting that even though I built later models than this transmitter, it was my favourite model so I just hung on to it. I have never felt the need for FM, PCM or bells and whistles; just a simple to operate, reliable transmitter. The receiver nicads call for the same attention but here I also recommend that the receiver pack be replaced after any physical damage, even if it appears to be working satisfactorily. Meter circuits The meter circuit in some of the older sets is often a source of mystery to many modellers. There are several reasons for this. Basically, there are two sorts of metering circuits, “battery test” and “RF indication”. RF indication is the most useful but it can be confusing for it seems to give a different reading every day and is an endless source of complaint and enquiry. For this reason, most manufacturers these days fit the more simple and predictable “battery indication” meter. To use it correctly, extend the antenna fully and hold the transmitter in both hands with the antenna vertical and the meter at eye level. The meter will now indicate RF power and battery condition very predictably, provided the same routine is carried out each time. If the needle falls out of the normal range under these conditions, then do not fly until you have checked out why. I find battery indication meters a real pain. In testing, I am forever swapping the transmitter crystal from the transmitter to my signal generator. If I forget to put the crystal back in the transmitter, the battery meter indicates action but the receiver does not agree. On the other hand, RF indication tells me straight away to “put the crystal back in dodo”. An RF indication meter can even be used as a field strength meter if another transmitter is brought close to the antenna, with your transmitter switched off. I have often confirmed transmitter failures on the field with my own transmitter using this technique. An RF indication meter is very reliable & is much more indicative of transmitter health if used correc­tly. It can even be used as a field strength meter. The problem with RF indication is that it draws a small amount of RF energy from the base of the antenna, rectifies it and uses the derived DC voltage to drive a meter. The problem is that the voltage available at the base of the antenna varies if the antenna is collapsed or extended, the user’s hands are on the transmitter or off, or even if the transmitter is lying on its back on the ground. All of these will give a different meter reading which often throws the uninitiated into a complete spin. Visions of intermittent transmitter operation are imme­diately conjured up in the mind of the modeller and the poor manufacturer or distributor is bombarded with questions for a week thereafter. In fact, an RF indication meter is very reliable and is much more indicative of transmitter health if used correc­tly. 88  Silicon Chip Finally, check all wiring for frayed or otherwise suspect appearance. If a lead comes off one of the control pots, results will vary from the pulses disappearing from that pot to the pulse returning to neutral. Rarely, if ever, have I seen a wiring fault in well-built transmitters. Transmitter checks If you have access to an oscilloscope, then look for the output of the modulator and check that all of the pulses are jitter free and move smoothly with each pot. A noisy pot will show up as extra pulses appearing in the pulse train or a sudden jump in pulse width on one channel. A similar effect will show up on the old half-shot encoders if an earth fault is present. Included in earth faults is “black wire” syndrome and if extra pulses are present, check all earth wiring for this problem. If a noisy pot is encountered, sometimes a spray of CRC-226 on the pot shaft will eventually work its way down onto the pot element and clean it. If not, replace the pot. There should be one more pulse in the pulse train than the number of channels. Thus, a 4-channel set will show five pulses, a 7-channel set will show eight pulses, and so on. As stated last month, RF tuning is rarely necessary but if it is, a spectrum analyser is a must. Some of the output coils are wave traps for harmonics and should be treated as such. To check the modulation on an AM transmitter, clip the earth lead to the input probe, thus making a loop. Place this in close proximi­ty to the fully extended transmitter antenna and set the scope’s vertical gain to maximum. A solid green band, blocked off by the modulation, should appear on the screen, the amplitude of which will depend on scope’s bandwidth. Check that all of the pulses are present, with no extras, and that they vary smoothly when the control sticks are moved. Check that the green band is an even colour. If there is fading from the centre out, then there is distortion in the RF output, often an indication of parasitic oscillation. If you do not know how to fix this, then send it off for service as you may be causing problems for other modellers on the field. For FM sets, the problem of viewing the modulation is a little more difficult. A modulation meter is virtually a must. The receiver is the next best thing and the pulse checks men­tioned above can be carried out with the scope connected to the receiver’s demodulator. You can check the RF output with the loop to see if the RF output is free of parasitics. Check also that collapsing the antenna does not introduce parasitics. Often a badly-tuned transmitter will break into oscillation when the antenna is fully or partially collapsed. Again, this may cause trouble to others on the field. This one is particularly troublesome at times, as a lot of operating takes place in the pits with collapsed antennas. Whilst on this point, do not run transmitters for any extended time with the antenna collapsed as it may result in overheating of the output transis­tor. That’s it for this month. Next month, I will discuss the care and mainteSC nance of receivers.