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REMOTE CONTROL BY BOB YOUNG More on servicing your R/C receiver Last month, we looked at the mechanical aspects of re­ceiver servicing. This month, we will be looking at the electron­ic aspects, with an emphasis on AM & FM sets. To begin, a circuit diagram of the receiver under discus­sion is a great help (as if you need to be told) and a component overlay is almost as important. There was a time not so long ago (before the popularity of FM) when a good knowledge of one re­ceiver was all you needed to service almost any receiver on the market. Technology has changed all that. FM brought into vogue the single IC receiver and PCM the in-house microprocessor. Surface mount technology has added a new complication in that most surface mount components are not marked with values (except resistors) and most components are hardly PERIOD OF NO CARRIER All receivers from PCM onwards are outside the scope of this article and must be left to the factory-trained and support­ed technician. I will be confining this discussion to AM and FM receivers using conventional components and some surface mount. Check the transmitter Servicing an AM receiver is a fairly straightforward busi­ness and, as usual, involves a strict discipline for the most efficient results. Adequate test equipment is a must and the home serviceman is at a disadvantage if his kit does not include an oscilloscope. I will attempt to include some tips for those with little equipment but you CARRIER MODULATED RF ENVELOPE Fig.1: a typical modulation pattern from an AM transmitter. Note that the modulation is completely blocked off for 350µs at regular intervals & this will result in an erroneous reading if you try to measure the carrier frequency using a DFM. recognisable. The new through-hole components will be even more horrendous in that they go into vias (ie, plated through holes in the board) and will be invisible. To cap this, knowing designers as I do, they will probably put them in vias which are located under ICs, so we are about to enter the true throwaway era. 70  Silicon Chip really are facing an uphill battle. To start with, and this applies for both AM and FM systems, check that the transmitter is working. For those with little equipment, placing the transmitter close to a TV receiver will usually result in a series of bars on the TV screen. This indi­cates that the transmitter modulation is OK and that the trans­mitter is radiating. Moving the controls will often result in a change in the bar pattern. A better test, is of course, to use a second receiver which can also be used for voltage comparisons. For those with an oscilloscope, testing an AM transmitter is fairly easy. First, clip the ground lead to the probe tip to make a sniffer loop, then hold the loop near the Tx antenna or, better still, slip it over the antenna. Now, turn up the scope’s sensitivity until a modulation pattern begins to appear on the screen. This will appear as a thick green line, blocked off into 100% modulated blocks – see Fig.1. The effectiveness of this test will depend upon many fac­tors, the prime one being the frequency response of the oscillo­scope. Often, even a poor scope will show some low level RF on the screen – enough to determine that the transmitter is working correctly. The FM transmitter presents more of a problem. The TV test may work and the second receiver certainly will. For those with test equipment, a scanning receiver, an RF test set or a modula­ tion meter will suffice to establish that RF and modulation are present and that the transmitter is working to some degree. Note that, for this series of articles, I am going to ignore the spectrum analyser on two counts. First, so few people have one of these devices that they may be discounted as far as most readers are concerned. And second, anyone with one of these devices probably has little need of instruction in how to use it. Having established that the transmitter is radiating, the next step is to establish the operating frequency. Most sets have plug-in crystals or modules these days and the number Fig.2: the circuit diagram of a typical FM receiver. Note the provision of a tuning point to aid the alignment process. +4.8V 120  3 2 .001 5.6M 36k 11 1 0.1 2 4 1 13 12 10 11 4 8 3 7 IC1 SO42P 14 2 3 5 0.1 27pF XTAL1 33pF RFC1 4.7uH 27pF 4 3 C1 3.3pF 33pF 2 1 L2 1 2 L1 ANTENNA is the correct one. Be aware that some modellers accidentally put the receiver crystal in the transmitter and vice-versa. This may result in a loss of range or a complete loss of signal if the receiver has another correct receiver crystal in it. A tricky problem here is that occasionally I have found crystals which have either gone off frequency or were incorrectly marked during manufacture. If in doubt, heavy some friend or acquaintance, or even the local serviceman, into checking the crystals for you. At Silvertone, I have a dedicated RF generator which we built many years ago. This is fitted with a 100dB stepped atten­ uator, switched crystals for all model bands, a crystal socket, an inbuilt 8-channel pulse width encoder complete with pot for operating the channel one servo, and a BNC connector and modula­tion kill switch for checking the crystal frequency. The output stage is fitted with a signal level meter which doubles as a crystal activity checker. Thus, for us the testing of an AM set begins with a voltage and field strength test on the transmitter, frequency count on the Tx crystal, modulation and purity checks of the RF sinewave on a 50MHz scope, T1 1 4101 4 5 47 CF1 270  “The number of times I have received transmitters with the wrong crystal in the socket is beyond my recall”. 2.2k 0.1 2 13 14 .01 47 270  3 If you have no equipment, then you are on your own and all I can suggest is that you check that the crystal 4 IC2 SO41P .0033 Check the crystal 5 8 12 6 7 9 1k 10 56pF 56pF TUNING POINT 100 36k 0.1 2.2 T2 4102 VR1 1M 1 IC3 LM111 VR2 6 10k 4.7k 5 4 8 7 10k S TB1 D1 1N4004 of times I have received transmitters with the wrong crystal in the socket is beyond my recall. Here, AM presents a real problem and FM is the easy one. Any frequency counter will just simply read off the carrier frequency if a sniffer probe is held in close proximity to the transmitter antenna. Be careful, though – move the transmitter just close enough to the test equipment to give a reliable read­ing. You can overload input stages and damage them if you stick the Tx antenna right down the poor thing’s throat. Because the carrier in an AM transmitter is blocked off for 350µs every 1-2ms, a frequency meter will give the incorrect frequency. The actual variation will depend on the counting period and the point at which the count started. Thus, unless the frequency counter can start and stop in less than 1ms, the chanc­es are that you will get an incorrect count. Counters such as this are not easy to come by and I have finally located one just recently after years of searching. So unless you have such a frequency counter, the best bet is a scanning receiver which shows the carrier frequency on the dis­play. If you have a scanning receiver, just tune for maximum RSSI (received signal strength indication) – or noise if your receiver has no signal strength meter – and read off the frequency from the display. January 1994  71 REMOTE CONTROL – Checking The Receiver and harmonic content checks on a spectrum analyser. The Tx crystal is then tested for activity if the Tx output appears to be on the low side. We then move onto a full visual inspection and more detailed work if required. For FM sets, a modulation meter is added to the above tests. This will give the frequency deviation and the demodulated audio waveform. I always check to see that the AM content of the modu­lation is within reasonable limits. Some FM sets have a very high AM content in their modulation. Receiver checks At long last we are ready to move on to receiver testing. Begin with the mechanical inspection and testing as outlined in the last two issues. Do a physical examination of the receiver battery and check the terminal voltage of each cell. All should be approximately equal. Next, test the receiver battery at the socket for no-load voltage. This should be about 5V. Some car sets are now running anything up to 7.2V for the higher-powered servos so be aware of this variation. Now plug the which gives approx­imately a 2-hour trace for a normal battery. A cycling battery charger is very handy for this type of testing and will give a very good indication of battery capacity. If you have only a voltmeter, get your friend to wriggle all of the transmitter controls briskly while you check the load voltage on the battery. The cells should not drop below 1.1V each under full load. Now measure the voltage at the point where the battery supply comes into the PC board. If you have voltage there and still no servo operation, then you really do have a problem. Fig.2 shows the circuit diagram of a typical FM receiver. At this point, I usually check the activity of the receiver crystal and its frequency, as it is easily and often broken in a crash and it is easy to remove and test. If this is OK, I then move on to a full voltage test on the PC board using the scope. Starting at the crystal oscillator, check that the oscilla­tor is running and giving a reasonable level of RF output. Next, check to see that all of the RF and IF coils are continuous, by using “I usually check the activity of the receiver crystal and its frequency, as it is easily and often broken in a crash. It is easy to remove and test”. whole system together and switch on. Check the battery voltage again under load – this should not be below about 1.1V per cell. If it is lower than this, then recharge the battery. If there are cells which are below 1.25V after charging, then dump the pack. The load should be all servos plugged in, Tx and Rx switched on and no servos operating. With all servos operating, the voltage may drop as low as 1.1V depending on a range of factors, including the servo current, number of servos and inter­nal condition of the batteries. At Silvertone, we use a cycling graphic analysis system and the batteries are placed under a 270mA load 72  Silicon Chip a voltage test where DC is applied or a continuity test where there is no DC as in the front-end RF coils. Coils are often broken in a crash and go open circuit. One point here is that when re-tuning the receiver, stay alert for signs of internal damage to coils and crystals. A large shift in the position of the slug in any tuning coil often indicates a broken coil. Replace the coil as a precaution. Remember always that the key element in servicing model aircraft equipment is prevention and any suspicion should be acted upon. If you have worked your way through the receiver to the detector and you finally have audio, you are past most of the fragile bits. From here on, it is generally routine servicing and the fault is usually visible crash damage. I have not gone into the complexity of every type of cir­cuit, as there are too many for the space allowed. Instead, I have briefly covered the spec­ial­­ised areas which are peculiar to R/C servicing. Receiver tuning Finally, just a word or two on tuning the receiver. Before doing this, unplug all servos and, if you are using the transmit­ter, remove its antenna (warning: do not let the Tx run for too long in this condition as the output transistor may overheat and suffer damage). In an AM Rx, there are two main types of detec­tors: (1) the simple diode detector; and (2) the transistorised version of the old “anode bend” detector. When tuning receivers with a diode detector, connect the negative lead of a voltmeter to the diode output and the positive lead of the meter to ground. When power is applied, the meter will read a small reverse voltage until the transmitter is turned on, at which point it will rise to about 0.6V or 0.7V, depending on the signal level and tuning. Reduce the signal level by moving the transmitter away or reducing the signal generator output until the voltmeter reads approximately 0V. In other words, tune at the lowest signal level you can read on the meter. Starting at the antenna coil, tune for maximum voltage and progress along the chain. When you get to the oscillator coil, this will tune to a peak and drop off slowly on one side and abruptly on the other. Tune into the abrupt side until the oscil­lation stops, then back out to the peak. When the oscillator starts again, continue in the peak direction for a full turn. That is the final setting. There is one problem with tuning the receiver this way, due to the fact that some receivers have a wave-trap in the input stage to suppress unwanted input signals. Unless you know the tuning specification and set-up procedure, there is little that can be done about tuning this wave-trap correctly. The main thing is to be aware of the situation. Tune the IF coils in the normal manner, firstly for peak voltage, then if a scope is available, for wave shape. SILICON CHIP BINDERS BUY A SUBSCRIPTION & GET A DISCOUNT ON THE BINDER (Aust. Only) Active detector Tuning an active detector is quite the opposite. In this case, the meter is hooked up with its negative lead to ground, while its positive lead goes to the tuning point (ie, the collec­tor of the detector transistor). In a well thought out receiver, such as Silver­tone, Futaba and some other Japanese receivers, the detector is clearly identified and a specially shaped resistor is provided to hook the meter or scope probe onto. In a really civilized receiver, the third pin on the bat­ tery connector will be arranged as the detector tuning point. Unfortunately, in the majority of cases, there is no thought given to the tuning and it is almost impossible to hang a lead on some receivers. When the receiver is switched on and the Tx is off, the active detector will read about 4V. This will drop to about 1.5-2V when the Tx is subsequently switched on. Tune for the maximum dip in voltage and trim the IF for wave shape (always at the lowest level of signal). FM receiver tuning FM receivers usually use a quad­ rature coil or ceramic filter as an audio detector. Hook the scope to a suitable point and tune for maximum audio, again trimming the IF coil(s) for wave shape. Again, keep that RF signal level to a minimum. One interesting point with an FM receiver is that if the quadrature coil is tuned to the wrong side of the carrier, the audio will appear in an inverted form. This is the reason why it is difficult to change some overseas FM sets which come in on 27-29MHz. The overseas 27MHz sets use a low side receiver crystal while in Australia, we use a high side receiver crystal. Thus, using a standard Australian crystal pair will invert the audio and the set will not work. The answer is an especially cut re­ceiver crystal on the low side of the carrier. That’s it for this month. Next month, SC we’ll look at servos. These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers and are made from a dis­ tinctive 2-tone green vinyl that will look great on your bookshelf. ★ High quality. ★ Hold up to 14 issues (12 issues plus catalogs) ★ 80mm internal width. ★ SILICON CHIP logo printed in gold-coloured lettering on the    spine & cover. Yes! Please send me ________ SILICON CHIP binder(s) at $A14.95 each (incl. postage in Australia). NZ & PNG orders please add $5 each for postage. Not available elsewhere. Enclosed is my cheque/money order for $­__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Signature­­­­­­­­­­­­_________________________ Card expiry date______/______ Name _____________________________________________________ Street _____________________________________________________ Suburb/town __________________________ Postcode______________ SILICON CHIP PUBLICATIONS PO Box 139, Collaroy, NSW 2097, Australia. Phone (02) 979 5644 Fax: (02) 979 6503. ✂ Be sure to keep the signal level at the lowest level possible at all times, by constantly reducing the signal generator output or moving the Tx further away. Run through the complete set of coils again once the Rx is tuned to ensure that there is no interaction between coils. January 1994  73