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BUILD THIS AM RADIO TRAINER; PT.2 In this second & last article on the AM Radio Trainer, we show you how to assemble & align it for best performance. You won’t need an RF signal generator for this task, as we describe a simple alignment oscillator at the end of this article. By MARQUE CROZMAN & LEO SIMPSON The big attraction of the AM Radio Trainer, apart from giving you the opportunity to build a classic circuit, is the fact that the PC board is over-printed with the circuit diagram. This is instead of the more usual component overlay diagram and should enable the novice to better come to grips with the func­tions of the various components. There are also a number of test points on the circuit board and these can be used for voltage measurements or to provide waveforms which can be displayed on an oscilloscope. We will feature some typical waveforms in this article, so you will know what to expect. Another point to note about the board is the large area of copper in the pattern. Most of this copper is all connected to the 0V rail from the battery and forms a “ground plane” for the circuit. This helps isolate the various sections of the circuit from each other and thereby ensures a good level of performance. Before you start assembly of the board, there are a number of checks you should do. First of all, check that there are no shorts between tracks or breaks in tracks. These should be re­paired before you go any further. Second, make sure that the board is suitably drilled for all the components. In particular, make sure that the IF transformers can be inserted and that there are holes drilled for the volume control potentiometer, for the mounting screws and shaft of the tuning gang, the 3.5mm headphone socket, the power switch and the battery holder. There should also be a pattern of small holes in the large circular region where the loudspeaker is to be mounted – otherwise the sound will be muffled. The resistors should be inserted first. You can check the colour code for each resistor value by referring to the table of resistor values accompanying this article. However, whether or not you are familiar with the resistor colour code, we strongly suggest that you check each resistor value with a digital multi­meter (switched to the appropriate “Ohms” ranges) before it is inserted and soldered into place. The resistors can be inserted either way into the board but it is a good idea to install them so that their colour codes all run in the same direction. This makes it so much easier to check their values later on. Besides, it looks better. Trimpot VR2 for the audio amplifier output biasing can also be installed at this stage. Note that its value should be 100Ω, not 200Ω as specified on the circuit last month. July 1993  53 This close-up view shows the mounting details for the on/off switch, the headphone socket, the loudspeaker & the volume control. The loudspeaker is secured using three small solder lugs which are soldered to the groundplane. Next, you can install all the capacitors with a value under 10µF, which means all the non-electrolytic capacitors. These are specified as monolithic or ceramic disc types. In practice, you are most likely to be supplied with small rectangular capacitors which have leads 5mm apart, to match the hole spacing on the board. These will have their capacitance marked in one of two possible codes, EIA or IEC, as shown in the capacitor code table accompanying this article. Having inserted the ceramic capacitors, the electrolytics are next. These have a black stripe down one side to indicate the negative lead. The electrolytic capacitors must be installed the correct way around otherwise they will be reverse-polarised and they will become leaky (in the electrical sense). Next, install diodes D1 and D2. Don’t swap them around otherwise the circuit won’t work well at all. The OA91 germanium diode (D1) will have a larger glass body than the 1N4148 silicon diode (D2). Diodes are also polarised so be sure that the co­loured band for the cathode is at the right end. Note: on the circuit, the cathode end of the diode is the end to which the arrow is pointing. The arrow also indicates the direction in which current can flow. Normally, diode symbols on our circuits are marked with A and K to designate the anode and cathode. 54  Silicon Chip Both diodes should be installed with a stress relief loop at one end so that they are less likely to be fractured if the board is stressed; ie, flexed or bent. A trap for young players The transistors go in next. Be sure to check that you get them around the right way. All the transistors specified come in plastic TO-92 encapsulation and the three leads from the under­side are in a triangle configuration. This is shown on the pinout diagram on the circuit. But there is a big trap for young (and old) players in assembling this board. Because we have printed the circuit on top of the board and arranged the circuit pattern to match it, it has been necessary to take liberties with the leads of most of the transistors. For Q1, Q2, Q3, Q4 and Q6, it is necessary to push the base lead between the emitter and collector leads, so that their leads match the circuit. If you don’t do this, the circuit won’t work. And make sure you put the correct transistor in each position. IF transformers Now you can install the oscillator coil and IF transform­ ers. These all look the same except for the colour of the slug at the top. The colours are as follows: oscillator coil (L2), red; 1st IF transformer (T1), yellow; 2nd IF transformer (T2), white; third IF transformer (T3), black (ie, no colour). One point we did not cover in last month’s circuit descrip­tion concerns the capacitors which are connected in parallel with the primary winding of each of these transformers and the oscil­lator coil. Have a look now and note these capacitors. However, if you have a look on the PC board, you will find that there is no place to put the capacitors. That is because the capacitor for each unit is actually inside the can and is wired internally. So you don’t have to worry about it. Having capacitors inside the cans of resonant coils is common practice in radios, transceivers and TV sets. It ensures manufacturing consistency, minimises wiring and saves board space. By the way, you should resist the temptation to twiddle the slugs of the IF transformers and oscillator coil by using a small screwdriver. Don’t do it. You should buy a set of plastic align­ment tools and use one which has a blade with a neat fit in the slot of the slug. If you can’t purchase a suitable alignment tool, you can make one out of a plastic styling comb. Cut off the long thin portion of the handle of the comb and then shape one end so that it is like a small screwdriver blade. You can easily do this with a sharp utility knife. There are several reasons not to use a small screwdriver to adjust the slugs. First, it is all too easy to damage the slots in the slugs. Second, the blades of screwdrivers are often mag­netised and this can affect the magnetic characteristics of the slugs. Third, when you are going through the actual alignment of the radio, the steel blade of the screwdriver will badly affect the resonance of the coil and you will get quite misleading results. Ferrite rod antenna When installing the ferrite rod antenna, you will need to solder the coil connections first and then secure the ferrite rod itself in place with a small plastic cable tie through the board. This is a temporary mounting method and there is a particular reason for doing it this way at this stage. The coil has four coloured cotton-covered wires and these should not be shortened back since they are already pre-tinned. The circuit board holes for the antenna connections are labelled with the Rear view of the assembled project. Bend the tags of the volume control & tuning capacitor so that they touch their respective pads on the board & solder them in place. The on/off switch, loudspeaker & headphone socket are connected to the PC board via wire links. colours; ie, white (WHT), black (BLK), red (RED) and green (GRN). The plastic dielectric tuning capacitor is secured to the PC board by two small countersunk screws. After these are insert­ed and tightened, the three tags need to be bent at right angles to make contact with the relevant pads on the PC pattern; they are then soldered. Secure the volume control potent­ iometer to the board with its washer and nut. Bend the tags so that they touch the pads on the board and solder them in place. The battery holder and on/off switch are next to be mount­ed. The battery holder is mounted on the component side of the board and is held in place with two 8BA screws and nuts. Use short lengths of hook-up wire to connect its terminals to the relevant spots on the PC board. The on/off switch is mounted through the board and secured with a nut and washer. The termi­nals are then connected to the board with short lengths of wire. Speaker mounting Three small solder lugs hold the speaker in place, as shown in the photo. The lugs are soldered to the ground plane, equally spaced around the rim of the speaker. Mount the headphone socket next to the on/off switch. The tab closest to the board is soldered to the ground plane. The other two connections must be made in such a way that when the headphone (or earphone) jack is in- serted, it disconnects the speaker and connects the headphone. This means that the tag which makes contact with the tip of the jack when it is inserted must connect to the negative side of the 100µF 16VW capacitor. The other tag is connected to one side of the speaker. You can check the switching operation of the socket by using your multimeter. The other terminal of the speaker is connected to the ground plane of the board via a short length of hookup wire. To finish off the construction, four 25mm tapped metal spacers are secured to the board with machine screws, one in each corner. This allows the board to sit on a flat surface and provides clearance for the volume pot, tuning gang and loudspeak­er. Now check all your work very carefully and you will be ready for the next stage which is alignment. Aligning your radio The major difference between this project and any other that you may assemble from the pages of this magazine is the need for alignment. Even if you have assembled the radio precisely as we have described so far, there is little chance that it will work satisfactorily when you first turn it on. This is because all the slugs in the IF transformers need to be adjusted to give the best gain. At the same time, you will need to adjust the slug in the oscillator coil and the trimmer capacitors associated with the tuning gang to give the best “tracking”. These latter adjustments ensure that the resonant circuit of the oscillator coil “tracks” with the input resonant circuit across the whole of the broadcast band. If this is not done, the sensitivity will vary quite mark­edly across the broadcast band. Before you start the alignment process though, rotate trimpot VR2 fully anticlockwise. This will set the quiescent current in the output stage transistors, Q6 and Q7, to zero. Rotate the volume control pot fully anticlockwise and the tuning knob fully clockwise or anticlockwise. This done, connect a 9V battery or DC power supply set to 9V and then measure voltages around the circuit. Connect the negative probe of your multimeter to a point on the ground plane and then measure the following voltages: Emitter of Q1 .......................... +0.95V Emitter of Q2 ............................ +0.5V Emitter of Q3 ...............................+1.1 Emitter of Q4 ............................ +4.7V Base of Q7 ................................. +4.0V TP8 ............................................ +4.6V In each case, the voltage should be within about ±10% of the value noted above. It will depend on the precise value of the supply voltage, the resistor tolerances and the individual gains of the transistors. If the voltages are quite different from the values listed above, then you should investigate why. By the way, these voltages are “no signal” voltages, be­cause little or no signal should be picked up by the input stage and the volume control is turned down so that there is no signal going through the amplifier stages. Naturally, the presence of signals will alter the voltages, although not greatly. July 1993  55 Note that if you take the trouble to calculate the expected base bias for each transistor and then subtract 0.65V to get the emitter voltage, you will find an odd result for the base bias voltage of Q2. This is because a major factor in its bias condi­tion is the detector diode D1. This has a static forward voltage of 0.2V and this effectively “loads down” the voltage at the emitter to about 0.5V. You can also measure the current drain now. This can be done by connecting your multimeter (switch­­­­ed to a low current range) across the on/ off switch. If your multimeter has automatic polarity switching, you don’t have to worry about how this connection is done. If your meter doesn’t have auto polari­ ty, connect the positive probe to the battery side of the switch and the negative probe to the other side. With the switch set to OFF, the current through the meter should be less than 10 mil­liamps. If the current is substantially more, you probably have a fault. Note that there is a risk in this procedure of connecting your multimeter across the on/off switch. If one side of the multimeter shorts to the groundplane, you could damage your meter or, at the very least, blow its internal fuse. A safer way of monitoring the current drain is to connect a 1Ω resistor in series with the positive lead to the battery holder. This done, use your multimeter to monitor the voltage Fig.3: this diagram shows the locations of the antenna & oscillator trimmer adjustments on the tuning gang. across the resistor. For example, if the voltage reading is 9mV, (9 millivolts) then by Ohm’s Law, the current will be 9mA (9 milliamps) Aligning the IF stages requires the injection of a 455kHz signal into the front end of the circuit. Connect an RF oscilla­tor, set to 455kHz, through a .001µF ceramic capacitor to test point TP1. If you build the test oscillator described later in this article, you will not need the .001µF capacitor. Ideally, you should disable the local oscillator by connecting a short lead between the collector of Q1 and test point TP2 but in prac­tice, it doesn’t seem to matter. Connect your multimeter (set to read DC volts) between test point TP3 and ground. Set the generator to give an RF signal output of about 1mV. Now the idea is to adjust each of the slugs in the IF transformers in turn for a minimum voltage on test point TP3. What happens is that as you adjust the slugs, the gain of the IF stages improves and the signal fed to the detector diode (D1) increases. The detector diode rectifies the IF signal and so as the signal increases, the negative voltage produced by the detector increases. Hence, the voltage at test point TP3 decreases. If you want to look at it another way, you will be adjust­ing the slugs for a null voltage at TP3. If you have an analog multimeter, you will find it more suitable for this task than a digital meter since you can judge the centre of the null more easily by the way the pointer swings back and forth as you tweak each slug. Oscilloscope method If you have access to an oscilloscope, you can connect it to TP5 and observe the IF signal directly. Now, as you adjust the slugs, you will see the CAPACITOR CODES ❏ ❏ ❏ ❏ Value IEC Code .022µF   22n .01µF   10n .0047µF   4n7 EIA Code 223 103 472 RESISTOR COLOUR CODE ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 1 1 1 1 1 2 2 2 1 2 56  Silicon Chip Value 1.2MΩ 1MΩ 820kΩ 56kΩ 47kΩ 39kΩ 27kΩ 12kΩ 10kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1kΩ 470Ω 100Ω 4-Band Code (1%) brown red green brown brown black green brown grey red yellow brown green blue orange brown yellow violet orange brown orange white orange brown red violet orange brown brown red orange brown brown black orange brown yellow violet red brown orange orange red brown red red red brown brown black red brown yellow violet brown brown brown black brown brown 5-Band Code (1%) brown red black yellow brown brown black black yellow brown grey red black orange brown green blue black red brown yellow violet black red brown orange white black red brown red violet black red brown brown red black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black brown brown yellow violet black black brown brown black black black brown Setting the tuning range without an RF generator In the accompanying procedure for setting oscillator and antenna tracking we assumed that you had access to an RF signal genera­tor. For many constructors, this won’t be the case and they will have to rely on broadcast signals at the top and bottom of the broadcast band. However, this poses something of a “chicken & egg” situation. How do you do the tracking adjustments if you cannot receive the signals? In most cases, you should be able to readily receive a signal at or near the bottom of the broadcast band, especially at night. However, picking up a signal at the top end of the band might not be anywhere near as easy. A solution to this problem is available if you have another AM radio. How’s that again? Well, as you now know, every superhet radio has a signal increase or decrease. Adjust the slugs for the best possible signal amplitude. Note that if there is a tendency for clipping of the signal at TP5, just reduce the signal input from your RF oscillator. local oscillator and for an AM broadcast receiver this oscillator will be 455kHz above the tuned frequency. There­ fore, you can use the local oscillator in your other AM radio to set the tracking adjustments at the top of the band. The method to follow is this: place the ferrite rod of the AM Radio Trainer near the antenna rod of your other AM radio (this will usually be at the top of the case). Rotate the tuning knob of the AM Radio Trainer fully clockwise to tune to the top of the band. Tune your other AM radio to 1165kHz or as close to this figure as you can. As you do so, you should be able to hear a faint heterodyne whistle from the speaker of the AM radio. Now proceed to peak the antenna and oscillator circuits as described in the article. These adjustments ensure that the RF input circuit and the local oscillator cover the correct range of frequencies so that you can tune over the broadcast band. Ideally, you need an RF signal generator to do this task. If you don’t have access to one, you will have to rely on tuning stations at the top and bottom of the band. In Australia, the broadcast band is specified as 531-1602kHz, so to be sure of covering this band, it is normal to make a radio tune slightly more, say 525-1620kHz. Let’s first proceed on the basis that you have an RF signal generator. Set it to 525kHz and rotate the tuning knob fully anticlockwise. This sets the plates of the tuning gang “in mesh” which is the maximum capacitance condition, for the low frequency end of the band. Now adjust the slug in the oscil­lator coil for maximum loudness of the signal via the speaker, or for maximum signal amplitude at TP5, if you have an oscilloscope. Fig.4: this is the waveform that will appear at test point TP5 during alignment if you are using a signal generator modulated at 400Hz. Fig.5: this 1kHz sinewave shows the crossover distortion nicks which will be present when the quiescent current in the audio output stage is zero. Tracking adjustments Now rotate the tuning knob so that it is fully clockwise. Set your RF signal generator to 1620kHz. Tune the adjustment screw on the back of the tuning gang labelled “oscillator trimmer” (see Fig.3) for maximum signal amplitude, as before. Rotate the tuning knob fully anticlockwise and redo the oscillator coil slug adjustment again at 525kHz. This done, go back to the top of the band at 1620kHz and adjust the oscillator trimmer again. These adjustments need to be done a number of times as the top adjustment affects the bottom adjustment and vice versa. You have now adjusted the oscillator range so that the broadcast band can be tuned in. As a point of interest, the oscillator will now be tuned over the range 980-2075kHz. Now you need to adjust the ferrite rod coil and antenna trimmer (on the back of the tuning gang). Set the tuning knob fully anticlockwise and set the RF signal generator to 525kHz, then move the coil along on the ferrite rod until the signal amplitude is at a peak. Now set the RF generator to 1620kHz and turn the adjustment screw on the back of the tuning gang labelled “antenna trimmer” (see Fig.3) until you peak the incoming signal again. You should now repeat these adjustments for the optimum response. When this is done, the ferrite rod coil should be fixed in place by melting a little candle wax over one end. That completes the alignment of the AM Radio Trainer. Quiescent current All that remains to be done is to set the quiescent current by means of trimpot VR2. By selecting a value Fig.6: this is the waveform from the calibration oscillator shown in Fig.7. The hash on the waveform is the residual 3.58MHz harmonic content. July 1993  57 A Crystal Controlled IF Generator 4.7k 1.5k 4.7M +9V If you can’t lay your hands 0.1 on an RF signal generator to 0.1 do the alignment for your 16 AM Radio Trainer, then you 4011 14 12 .001 can build this crystal con4.7k 4.7k IC2 6 4.7k 11 10 5 8 1 3 4 10 4040 IC1d trolled IF generator board. It IC1a IC1b IC1c ö8 13 OUTPUT 6 9 2 is bas­ed on a standard Amer470pF 470pF 470pF 7 1.5k 11 8 ican 3.579545MHz colour 4.7M burst crystal. When divid­ed by 8, you end up with a freX1 3.579MHz quency of 447.4kHz. This is within 2% of 455kHz and is 22pF 22pF probably more accurate than CALIBRATION OSCILLATOR you would obtain by setting a typical RF generator to Fig.7: the circuit divides the output from a 3.58MHz crystal oscillator by 455kHz. eight & then filters it to provide a sinewave at 447.4kHz. Three CMOS gates of a 4011 OUTPUT quad gate package are connected in series and the 3.58MHz crystal 0.1 470pF .001 0.1 9V GND connected between input and IC2 IC1 output via a 1.5kΩ resistor. The 4040 4011 gates are biased into the linear re1 1 22pF 4.7k gion with the 4.7MΩ resistor and 470pF X1 the output is a square wave. This 22pF 4.7k 470pF is buffered by the fourth gate of the 4011 which then drives IC2, a 4040 12-stage binary counter. The Fig.8: the parts are all mounted on a small PC board coded 06107931. divide-by-8 pin of the 4040 is then used as the output. strip. The final output is a sinewave followed by the capacitors and ICs. A third order low-pass RC filter with an amplitude of about 35mV Next mount the crystal and the then removes the harmonics and peak-to-peak into a 10kΩ load. PC stakes. Lastly, the bat­tery clip reduces the amplitude to a level leads can be soldered in. Construction suitable for injecting into the IF In operation, this oscillator needs Check the board carefully for to run from a fresh 9V battery, as shorts and breaks in the tracks. it drops in frequency below about This done, install the resistors first, 8.5V or so. PARTS LIST 1 PC board, code 06107931, 88 x 30mm 1 9V battery clip 1 9V alkaline battery 2 alligator clips Semiconductors 1 4011 quad 2-input NAND gate (IC1) 1 4040 12-stage binary ripple counter (IC2) Capacitors 2 0.1µF 63VW metallised polyesters 1 .001µF ceramic 3 470pF ceramic 2 22pF ceramic Resistors (0.25W, 1%) 1 4.7MΩ 1 1.5kΩ 3 4.7kΩ 58  Silicon Chip This view shows the fully-assembled alignment oscillator. Note that it should be powered from a fresh 9V battery, as it drops in frequency below about 8.5V. Connections to the AM radio are made via alligator clips. Acknowledgement: our thanks to Bob Barnes of RCS Radio Pty Ltd for producing the prototype screen printed boards. RCS Radio can supply the board in two versions: a standard phenolic board with the circuit screen-printed in black on the topside, or the deluxe board which is screen printed in two colours (white cir­ cuit on a deep blue background). The code number is 06106931. The standard board is available for $19.90 and the deluxe board is $24.90. Post & packing is $2.00. Contact RCS Radio Pty Ltd, at 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. The sensitivity of the receiver can be improved by mounting the ferrite rod up off the board using a plastic bracket. The reason for doing this is that the copper pattern on the board substantially de-sensitises the antenna. of 100Ω for this trimpot, we have deliberately restricted the range of adjustment. This has been done for safety’s sake because if the range of adjustment was larger, it would be possible to destroy one or both of the output transistors, because of excessive quiescent current. The best way to adjust the quiescent current is to feed a sinewave modulated signal into the front end of the radio from an RF signal generator. Connect an oscilloscope to the output at test point TP8 and adjust the volume control for a signal ampli­tude across the speaker of about 2V or 3V peak to peak. At this stage, VR2 should still be fully anticlockwise If you now have a look at the signal on the scope screen, you will see the classic sinewave with crossover distortion – notches in the waveform at the crossover point (see Fig.5). Now if you rotate VR2 you will see the cross­over nicks disappear from the waveform and, at the same time, the sound will become cleaner. Rotating VR2 to reduce the crossover distortion will not increase the current by much, by no more than a milliamp, but it will make a big difference to the sound quality. By the way, you should measure the current drain of the radio while you are adjusting the quiescent current with trimpot VR2. Typically, the current drain of the radio at 9V should be less than 10 milliamps when the volume control is at minimum setting (ie, no signal through the audio amplifier stages). With the volume control well advanced to make the radio quite loud, the current drain may be 40 milliamps or more. You can also easily measure the current drain of the radio without the audio stage. Just plug an open-circuit 3.5mm jack into the headphone socket. This disconnects the loudspeaker and causes the amplifier to latch up and thus draw negligible cur­rent. Under Fig.9: this is the full-size PC pattern for the calibration oscillator. these conditions, the rest of the radio circuit will draw around 4mA or less. Mounting the ferrite rod By now, you will have tried out the radio and possibly found that its performance leaves something to be desired, even though you should be able to tune in stations right across the broadcast band. You will find lots more stations at night, pro­vided you are not attempting to listen to your radio close to a TV set or computer. Both cause loud whistles across the dial. There is a further step you should take to get the best out of your radio and that is to mount the ferrite rod antenna up off the board by at about 25mm. The reason for doing this is that the copper pattern of the PC board substantially de-sensitises the antenna – in fact, any metal will do this. To mount the antenna rod off the board by the requisite amount, we made up a rightangle bracket out of scrap plastic. This was secured to the PC board with two screws and nuts, while the rod was secured to the brack­ et with two small plastic cable ties. Mounting the antenna rod in this way will make a substan­tial difference to the sensitivity. You should repeat the peaking procedure for the ferrite rod coil and antenna trimmer. Notes & Errata The trimpot specified for VR2 in the audio amplifier output stage should be 100Ω, not 200Ω as specified in the SC first article. July 1993  59