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BUILD AN FM RADIO TRAINER; PT.2 This second article on the FM Radio Trainer describes the construction & alignment. You do not need any special equip­ment for the job – just a soldering iron, multimeter, trimming tool & a simple alignment oscillator which you build yourself. By JOHN CLARKE The construction of the FM Radio Trainer includes the PC board assembly plus some coil winding. There is also a small amount of wiring to be installed on the underside of the PC board for the loudspeaker and volume control. To simplify construction, the board has a screen printed overlay to show 58  Silicon Chip the positions for the components – see photo. All you have to do is install the parts according to this overlay or you can follow Fig.8(a) which is a conventional parts layout diagram. As well as the usual parts, there are also four shield pieces soldered to PC stakes on the top of the board. These pro­ vide shielding for the tuned RF amplifier and mixer stages (Q1 & Q2). These shields are made from 19mmwide blank PC board materi­al. In addition, two baffles made from 25mm-wide PC board mate­rial are soldered to the underside of the board, adjacent to the loudspeaker. These enclose the loudspeaker along the edges of the board to improve the bass response. PC board assembly Before starting construction, it is a good idea to check the PC board for any shorts or breaks in the copper tracks. Repair any faults that you do find (generally, there will be none), then check the hole sizes as set out below. First, check that a pattern of 0.6mm holes has been drilled for the loud- SPEAKER 9V BATTERY HOLDER OFF 470uF SPEAKER +9V 10  .01 TP3 .01 +9V TP2 5.6k 5.6k ON S1 TP GND 100pF D2 1uF A VR1 B .0068 C 1 470uF IC4 1k 390pF 100uF 8.2k 68  47k 56pF .01 Q4 75  +9V 100  100  IC3 +9V TP GND .01 1 T1 100k 1uF .01 VC5 10pF .01 VC4 .01 Q3 3.9pF +9V +9V .01 10k VC3 39pF VC1 1k 47pF L1 TUNING SHIELD 27k 75  ANTENNA ANTENNA TP1 4.7pF L3 VC2 56pF 220pF .01 Q1 10k 47  330  RF1 18k L2 560  +9V 10k 470k .01 47W RF2 10k SHIELD 82pF .01 Q2 330  +9V 47k .01 SHIELD 68pF .01 47  +9V 330pF +9V 1 18k SHIELD IC1 .01 .01 T2 XF1 T3 .01 IC2 .01 .01 +9V 100  +9V SILICON CHIP FM RADIO TRAINER 100k .01 .01 270k +9V .01 330  Fig.7 (right): this is the pattern that’s screened onto the top of the PC board.The dotted lines connected to two of the terminals on the tuning gang (VC1/VC3) are actually tracks on the PC board. They are shown here so that you can see how the tuning gang is connected to the rest of the circuit. Note: pattern shown 67% of actual size – the full size pattern measures 563 x 115mm. VOLUME 47  .01 VC6 +9V 1 T4 TP4 1k 390pF 10uF 10  D1 .047 speaker and that 6.5mm holes have been drilled for the volume and tuning controls. The spindle hole for the VC1/VC3 tuning gang should be 7mm in diameter, the corner mounting screw holes should be 3mm in diameter, and the holes for the antenna PAL socket, switch S1 and the battery holder should be 2.5mm in diameter. Finally, check that 1.5mm holes have been drilled to accept the base and can pins of T1 and T4. You can do this by test fitting these two components. In most cases, the holes will all be correct but, if not, enlarge any undersize holes as necessary using the appropriate size drill bit. Once you are satisfied that all the hole sizes are correct, begin the assembly by installing PC stakes at the six shield mounting points, at test points TP1-TP4 & TP GND, and for the antenna socket earth – see Fig.8(a). This done, install the resistors at the locations indicated. Table 1 shows the resistor colour codes but it is also a good idea to check each resistor using a multi­meter, to make sure that you have the correct value. Diodes D1 and D2 can now be installed, followed by varicap diode VC1 and ceramic filter XF1. Make sure that the diodes are all correctly oriented (XF1 can go in either way around). VC1 has a similar appearance to D1 and D2, so be careful not to get them mixed up. Note also that D1 and D2 are specified May 1995  59 60  Silicon Chip TUNING VC3 VC1 39pF L1 1k RF1 27k 18k Q3 10k 560  .01 .01 47  L3 VC4 82pF .01 47  .01 VC5 1 T1 1uF .01 47k 330pF .01 68pF 10pF 330  Q2 SHIELD 10k RF2 4.7pF 10k SHIELD 470k TP1 VC2 3.9pF 330  .01 220pF Q1 L2 56pF .01 .01 47  18k TP GND 100k .01 1 .01 IC1 1 00  SHIELD 1 .01 T3 1 .01 1 IC2 .01 .01 1 .01 IC3 100  SILICON CHIP FM RADIO TRAINER T2 2 2 XF1 100  .01 .01 47  VOLUME VC6 Q4 56pF .01 75  330  .01 270k 100k 68  1 T4 .0068 GND Fig.8(b): this is the etching pattern for the PC board, shown here 67% of actual size. Check your finished board carefully against this pattern for possible etching defects before installing any of the parts. 1uF D2 8.2k TP4 TP 47k D1 100pF 100uF Fig.8(a): follow this parts layout diagram in conjunction with the layout pattern on the PC board when installing the various parts. Take care to ensure that all polarised parts are correctly oriented & keep all component leads as short as possible. The volume control (VR1) connects to points A, B & C to the left of IC4. 75  ANTENNA 47pF ANTENNA SHIELD 10k 5.6k .047 TP3 .01 .01 TP2 SPEAKER 470uF 10  10uF 5.6k 470uF 1k IC4 B C 1 10  VR1 A 390pF 390pF 1k ON S1 OFF SPEAKER 9V BATTERY HOLDER 5T 10T installed from the copper side of the PC board. Short lengths of tinned copper wire are then used to connect the tuning gang terminals to the PC board. Tuning dial 9mm By now, the PC board assembly will be substantially L1, L2, L3 1 : 2 com­ pleted, with only the 4.5T 0.8mm DIA ENCW T2 AND T3 coils, transformers and a few ON 5mm DIA MANDREL 0.25mm DIA ENCU WIRE ON sundry items to be installed. PHILIPS 4313 020 40031 BALUN CORE Before going further though, the tuning dial label should be fitted to the tuning wheel. To do this, first cut the label out in a neat circle CT using a pair of scissors, then 1 3mm 6 cut a neat central hole with a sharp utility knife. This 2mm done, remove the backing 2 5 sheet and fit the label over 56 5 the large spindle of the dial 1 with the numbers facing 4 3 1 the PC board (do not stick it down yet). 2 Next, secure the tuning BASE DIAGRAM TOP VIEW wheel to the tuning capacitor shaft with its screw and rotate it fully clockwise. The label should now be carefully 3 4 3 4 aligned so that “88” lines T1 T4 WINDINGS: PINS 4-5, 4T, WINDINGS: PINS 1-2, 30T, up opposite the edge of the 0.25mm DIA ENCU WIRE 0.125mm DIA ENCU WIRE. board. Now rotate the dial PINS 1-3, 30T, 0.125mm PINS 5-6, 10T BIFILAR, 0.25mm DIA ENCU WIRE fully in the opposite direcENCU WIRE. PIN 4-CT, 6T, 0.25mm DIA ENCU tion and check that “108” is WIRE WOUND OVER WINDING 1-2 now indicated. Fig.9: follow these winding diagrams & the instructions in the text carefully when If everything lines up making up the coils & IF transformers. In particular, make sure that all windings are correctly, stick the label to wound in the direction shown. the tuning wheel at the edges, then remove the tuning in the parts list as 1N4148s but you overdo it, otherwise you could dam- wheel and complete the job. Finally, can also use 1N914s. age the lead connections inside the re-attach the tuning wheel to the shaft. The four ICs can now be installed transistor. Coils on the board. These must be installed The remaining transistor (Q3 – with the notched ends oriented ex- BF199) is installed with the flat side Fig.9 shows the coil winding details. actly as shown on the overlay. Make of its body towards the tuning gang. The three air-cored coils, L1-L3, are sure that IC4 is the LM386 audio Splay its base lead so that it will fit identical and are made by winding amplifier. comfortably into the holes provided 4.5 turns of 0.8mm diameter enamThe next step is to fit the capacitors. and, as with the Mosfets, push it down elled copper wire onto a 5mm drill Table 2 lists the codes for the low-val- as far as it will comfortably go before bit. Before winding, stretch the wire ue polyester and ceramic types. The soldering its leads. slightly by clamping one end in a vyce electro­ lytic types are all polarised The two RF chokes, RF1 and RF2, and pulling the other end with a pair and must be oriented as shown on each consist of a short length of tinned of pliers. the overlay. copper wire which passes through the Be sure to wind these coils in the Once the capacitors are all in, you middle of a ferrite bead. Install these direction shown. If the coils are wound can install the three Mosfets (Q1, Q2 & now, then fit the three trimmer capac- in the wrong direction, they will not Q4). Each of these devices is installed itors (VC2, VC4 & VC6). VC2 and VC4 fit the holes in the PC board. with its metal tab towards the battery will have the same body colour, while After winding, install the coils on holder. Push each device down onto VC6 will be the odd one out. the board (push them all the way the board as far as it will comfortably The VC1/VC3 tuning gang is down) and trim their leads so that they go before soldering its leads but don’t mounted using two screws which are protrude through the board by about May 1995  61 This close-up view shows how the shield pieces are installed on the top of the PC board, around the RF amplifier & mixer stages. These shield pieces are supported by soldering them to PC stakes. 2mm. Note that the enamel insulation on the leads must be removed before they can be soldered. This can be done by applying heat from your soldering iron until the enamel melts, after which the leads can be soldered in the normal manner. Transformers T2 and T3 are wound onto the balun formers. These formers are oval in cross section and contain two holes. Begin by tightly winding 5 turns of 0.25mm ENCW on one former, making sure that both ends exit from the same side. Mark this side with a “1” using a lead pencil, then wind 10 turns of 0.25mm ENCW from the other side of the balun and mark this side with a “2”. The other balun is wound in identical fashion. After that, the two baluns can be installed on the PC board with the correct 1:2 and 2:1 ratios, as shown on Fig.8(a). Transformer T1 is made by winding two coils onto a minia­ture Neosid former. To wind this coil, first push the former into its 6-pin baseplate, then solder one end of the 0.125mm ENCW to pin 3 (see Fig.9). Now, starting from the bottom of the former, wind on 30 turns in the direction shown, with each turn adjacent to the previous turn (ie, the coil is close-wound). This done, terminate the free end of the winding on pin 1. The top winding is positioned 3mm above the bottom winding and consists of 4 turns of 0.25mm ENCW. Start by terminating one end of the wire to pin 5, then wind on four turns in the direc­tion shown and terminate the free end on pin 4. Finally, install the F29 screw core in the former. Transformer T4 is somewhat more complicated to wind than T1, so we’ll go through the procedure step-by-step. Begin by close-winding 30 turns of 0.125mm ENCW between pins 2 and 1 (note: this winding goes in the same direction as the 30-turn winding on T1). This done, connect a length of 0.25mm ENCW to pin 4 and wind 6 turns over the previous 30-turn winding as shown. Place some insulation tape over this winding to prevent it from unravelling. The top winding on T4 is bifilar wound; ie, it is wound using two lengths of wire that have been twisted together. To do this, first cut two 150mm lengths of 0.25mm ENCW, place them next to each other and clamp one end of each wire in a vyce. Secure the other ends of the wires in Table 1: RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  2 ❏  2 ❏  1 ❏  2 ❏  4 ❏  1 ❏  2 ❏  3 ❏  1 ❏  3 ❏  3 ❏  1 ❏  4 ❏  2 62  Silicon Chip Value 470kΩ 270kΩ 100kΩ 47kΩ 27kΩ 18kΩ 10kΩ 8.2kΩ 5.6kΩ 1kΩ 560Ω 330Ω 100Ω 75Ω 47Ω 10Ω 4-Band Code (1%) yellow violet yellow brown red violet yellow brown brown black yellow brown yellow violet orange brown red violet orange brown brown grey orange brown brown black orange brown grey red red brown green blue red brown brown black red brown green blue brown brown orange orange brown brown brown black brown brown violet green black brown yellow violet black brown brown black black brown 5-Band Code (1%) yellow violet black orange brown red violet black orange brown brown black black orange brown yellow violet black red brown red violet black red brown brown grey black red brown brown black black red brown grey red black brown brown green blue black brown brown brown black black brown brown green blue black black brown orange orange black black brown brown black black black brown violet green black gold brown yellow violet black gold brown brown black black gold brown the chuck of a hand drill, then wind the drill until there is about one twist every 2mm along the entire length. Next, solder one end of one lead to pin 5 of the base – see Fig.9. The other lead at this end goes to the centre-tap and can be stripped of the enamel insulation, so that it is ready to make the connection. This done, wind 10 turns onto the former in the direction shown, leaving a gap of about 2mm to the top of the previous winding. Once these turns have been wound on, you need to determine which free end goes to pin 6 of the former. This is done using a multimeter. Select a low “ohms” range, connect one probe to the already bared CT wire and measure the resistance to the free lead ends at the top of the coil. The end that gives a reading of zero ohms goes to pin 6, while the remaining lead goes to the centre-tap. Complete the winding by soldering all three CT leads to­gether, as shown in Fig.9. Trim off any excessive lead lengths here and push the CT connection as close to the former as possi­ ble, to ensure that it doesn’t end up shorting to the aluminium shield can. Cover the connection with insulation tape if neces­sary. Finally, complete the assembly by fitting the F29 screw core. T1 and T4 can be installed on the PC board, after which their shield cans can be installed. Make sure that the shield cans are centrally located over the formers and that they are pushed all the way down onto the PC board before soldering their pins. The loudspeaker is held in position using two copper wire straps which solder to the adjacent earth pattern on the PC board. Note the two baffle pieces. Installing the hardware Now that the coils are in place, the major hardware items can all be installed. Begin by mounting the external PAL antenna socket and note that its earth lug is soldered to an adjacent PC stake. The tuning control shaft is obtained by removing the cover, wiper and resistance sections from an old potentiometer. What remains is the pot shaft and its threaded bush. Cut the shaft to a length to suit the control knob, then install it from the underside of the PC board and secure it using a nut and starwasher. A round screw-on rubber foot is now pushed onto the underside of the pot shaft to provide a 3:1 reduction drive for the tuning wheel. The volume control potentiometer (VR1) is also installed from the under­ The reduction drive for the tuning wheel is made up using a control shaft & bush salvaged from an old potentiometer, together with a rubber foot which is simply pushed over the control shaft. side of the PC board (after first trimming its shaft). Fit the control knobs to the tuning and volume controls when they have been mounted. The volume control terminals are wired to points A, B & C on the PC board via a short length of 3-way rainbow cable. A short length of tinned copper wire should also be connected between the volume pot body and ground (the surrounding large copper area). This measure is necessary to prevent hum from being introduced into the amplifier when your hand is brought near the pot. The battery holder is secured in place using three 2mm screws and nuts. Do not forget to solder the battery terminals to the board. Switch S1 can then be mounted in position. Depending on the particular switch you are supplied with, you may need to bend its two outside terminals inwards slightly so that they line-up with the holes in the board. The loudspeaker is secured to the underside of the PC board (beneath its grille) using two thick (0.8mm) May 1995  63 Building The IF Alignment Oscillator This FM IF Oscillator generates a 10.7MHz square-wave signal & is used for aligning the IF stages of the FM Radio Trainer. It’s built on a small PC board, requires no adjustments & can be assembled in a few minutes. The circuit for the IF oscillator is based on a single high-speed (HC) CMOS NAND gate and a 10.7MHz ceramic filter. Fig.10 shows the details. IC1 is wired in a fairly standard oscillator configuration. One of its inputs, pin 9, is wired to the positive supply and so IC1 behaves as an inverter. It is biased in the linear mode using a 1MΩ feedback resistor between its pin 8 output and its remaining input (pin 10). the speci­fied 330Ω resistive load for XF1 because of the nominal 60Ω output impedance of IC1. The resulting 10.7MHz waveform at pin 8 is filtered using a 270Ω resistor and a 330pF capacitor to produce a reasonable sinewave signal. This is then fed to the output via level control VR1 and a .01µF capacitor. Although the nominal output frequency is 10.7MHz, it is in fact closer to 10.8MHz because of the phase characteristics of the ceramic filter. This 100kHz difference is of no consequence since the 10.7MHz ceramic filter used in the FM Radio Trainer has a bandwidth of 280kHz. Power for the circuit is derived from a 9V battery (eg, from the battery used in FM Radio Trainer or from a separate 9V battery). The 9V The 10.7MHz ceramic filter (XF1) is wired in parallel with the feedback resistor, along with a 15pF capacitor which provides the correct amount of capacitive loading. The associated 330Ω and 270Ω resistors provide the correct resistive loading for XF1. Note that the 330Ω resistor is AC-coupled to ground via a .01µF capacitor to prevent loading the DC voltages on pin 10. In addition, the 270Ω resistor at pin 8 is smaller than 15pF 56  1M 14 XF1 SFE10.7ML IC1 0.1 7 ZD1 4.7V 400mW +9V 0V 270  10 +4.7V 330  Fig.10 (right): this is the circuit for the IF alignment oscillator. It’s based on a single high-speed NAND gate IC (IC1) & a 10.7MHz ceramic filter. VR1 sets the output level, while ZD1 provides a regulated 4.7V supply to power the circuit. The completed PC board assembly is shown in the above photo. enamelled copper wire straps. These straps are soldered to the heavy earth track on the PC board (see photo). After mounting, connect the speaker terminals to the speaker pads on the PC board using a short length of figure-8 cable. Shield installation Refer now to the overlay diagram for the locations of the four blank PC board shield pieces. Three of these pieces are identical and measure 19 x 64  Silicon Chip 9 IC1 74HC00 8 270  330pF LEVEL VR1 1k .01 10.7MHz OUTPUT .01 70mm. Solder these to the PC stakes on the top of the board, taking care to ensure that their ends are all aligned about 2mm short of the edge of the main board. The 19 x 90mm shield piece is then soldered on the ends of these three shields. The two loudspeaker baffles are made from 25 x 90mm blank PC board material and are soldered to the underside the PC board as shown in one of the photos. This done, secure the six 25mm stand­offs to the underside of FM IF OSCILLATOR the PC board using 6mm-long x 3mm screws. Antenna Ideally, an external antenna should be used for best recep­tion and this can be plugged directly into the PAL socket. Alter­natively, if portability is a requirement, you can fit a tele­scopic antenna. The telescopic antenna used with the prototype has a hori­zontal hole drilled through the base of the unit. 15pF 0V 270  330  1M +9V 56  ZD1 XF1 .01 0.1 IC1 74HC00 270  330pF .01 VR1 OUTPUT GND 1 Fig.11(a): install the parts on the FM IF Oscillator board as shown here. Take care to ensure that IC1 is correctly oriented & insert PC stakes at the four external wiring points. The ceramic filter (XF1) can be installed either way around. Fig.11(b) at right shows the full-size etching pattern for the PC board. battery voltage is regulated to 4.7V using zener diode ZD1 and a 56Ω current-limiting resistor. Construction The FM IF Oscillator is built on a PC board coded 06304951 and measuring 76 x 39mm. Fig.11(a) shows the parts layout. Install the parts as shown, taking care to ensure that IC1 and ZD1 are both correctly oriented. PC stakes should be in­ stalled at the power supply and output wiring points, so that the unit can be easily connected to the receiver. To test the assembly, connect a 9V supply and check that the voltage across ZD1 is about 4.7V. If this is OK, PARTS LIST check that pin 8 is sitting at about 2.3V, as measured by a multimeter set to DC volts (note: this voltage represents the average level of the 10.7MHz signal from IC1). Alternatively, if a frequency meter is available, then the output can be directly checked for a 10.7MHz signal to ensure that the circuit is working correctly. CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ Value 0.1µF 0.01µF 330pF 15pF IEC 100n 10n 330p (n33) 15p EIA 104 103 331 15 1 PC board, code 06304951, 76 x 39mm 4 PC stakes 1 Murata SFE10.7ML ceramic filter (XF1) 1 1kΩ horizontal trimpot (VR1) Semiconductors 1 74HC00 high speed CMOS quad NAND gate (IC1) 1 4.7V 400mW zener diode (ZD1) Capacitors 1 0.1µF MKT polyester 2 0.01µF ceramic 1 330pF ceramic 1 15pF ceramic Resistors (0.25W, 1%) 1 1MΩ 2 270Ω 1 330Ω 1 56Ω 0.5W, 5% RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  2 ❏  1 Value 1MΩ 330Ω 270Ω 56Ω This is fitted with a 20 x 2mm screw and nut. Solder the nut in place to prevent it from coming loose, then bend the screw at right angles and feed it down the centre pin of the PAL plug (you will have to disassemble the PAL plug first). The antenna screw can now be clamped in position by doing up the plug’s anchor screw. Finally, reassemble the PAL plug, leaving out the top metal cable clamp section (this would short the antenna to the socket earth). 4-Band Code (1%) brown black green brown orange orange brown brown red violet brown brown green blue black brown 5-Band Code (1%) brown black black yellow brown orange orange black black brown red violet black black brown green blue black gold brown Another possibility is to leave the PAL socket out and use a telescopic antenna that can be bolted directly to the PC board. Initial tests Before installing the battery, check the assembly carefully to ensure that all parts are in their correct locations and are correctly oriented. The underside of the board should also be checked for missed solder joints and for shorts. Assuming that everything is correct, connect the negative lead of a multimeter set to 20V DC to test point TP1 and connect the positive lead to the positive battery terminal. If the bat­tery voltage drops by more than 1V when power is applied, switch off immediately and check the board for shorts or incor­ rect component orientation. Locate the source of the problem before switching on again. If nothing dramatic happens, you May 1995  65 The alignment oscillator is connected to the antenna input on the tuner board via two short wire links. Power for the oscillator is derived directly from the tuner board, although the actual connections (to the two PC stakes at top right) are not shown here. can proceed to make a series of voltage checks, as set out in Table 3. Note that these voltages are for guidance only and assume a 9V supply. They were measured on the prototype using a digital multimeter. If any measured voltages differ by more than 20% from the prototype, then there is probably an incorrectly placed component on the board. is 0.5mm below the top of can (use the trimming tool); (9). Adjust the core in T4 so that it is 4mm above the can; (10). Set VR1 fully anticlockwise for minimum volume. IF alignment The alignment procedure involves using the IF Alignment Oscillator de- Initial setup To minimise alignment adjustments, the circuit should ini­tially be set up according to the following procedure. Note that all adjustments to the trimmer capacitors and to the ferrite slugs in the coils must be carried out using a proper trimming tool. Do not use a screwdriver in the ferrite slugs, as this can easily crack them. Here is the initial setup: (1). Stretch coil L1 to 10mm; (2). Squeeze L2 to 8.5mm; (3). Squeeze L3 to 7mm; (4). Set VC2 to half mesh (ie, plates half open); (5). Set VC3 fully open (ie, plates fully out of mesh); (6). Set the trimmer capacitors on tuning gang VC1\VC3 to fully open; (7). Set VC6 to half mesh; (8). Adjust the core in T1 so that it 66  Silicon Chip TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value .047µF .01µF .0068µF 390pF 330pF 220pF 100pF 82pF 68pF 56pF 47pF 39pF 10pF 4.7pF 3.9pF IEC 47n 10n 6n8 390p (n39) 330p (n33) 220p (n22) 100p (n10) 82p 68p 56p 47p 39p 10p 4p7 3p9 EIA 473 103 682 391 331 221 101 82 68 56 47 39 10 4.7 3.9 scribed in the accompanying panel. Its output is fed directly into the antenna input to the left of coil L1 on the FM receiver board. Don’t forget to connect the GND terminals of the two boards together as well (see photo). Power for the IF Alignment Oscillator can be derived di­rectly from the radio’s supply via a suitable length of hook-up wire. Alternatively, you can power the alignment oscillator from a separate 9V battery. Take care with the supply polarity. The step-by-step alignment procedure for the IF circuitry is as follows: (1). Apply power and connect a multimeter set to a low DC volts range between test points TP2 and TP3 (near the battery holder). Adjust trimpot VR1 on the IF oscillator for a multimeter reading of 1-3V. (2). Adjust the slug in T1 for a maximum reading, then adjust VC6 for a maximum reading. Note: at all times, make sure that the voltage does not go above 3V. Readjust VR1 if necessary. (3). Connect the multimeter between TP4 and TP GND (near the volume control) and adjust the slug in T4 for a 0V reading. (4). Reconnect the meter between TP2 and TP3 and readjust T1 and VC6. This done, reconnect the meter between TP4 and TP GND and readjust T4 for a 0V reading. (5). Remove the FM IF Oscillator board and attach the telescopic antenna to the PAL socket. That completes the alignment of the IF stages. The local oscillator and RF amplifier stages now require alignment. Only a few parts are mounted on the copper side of the PC board: the tuning assembly, volume control & loudspeaker. Note the short link that’s used to connect the body of the volume control pot to the adjacent earth track. TABLE 3: VOLTAGE CHART Device Expected Voltages Q1 G1 = 0V; G2 = 6.6V; S = 1.3V; D = 8.8V Q2 G1 = 0V; G2 = 5.1V; S = 1.3V; D = 8.8V Q3 C = 8.6V; E = 4.2V; B = 4.3V IC1, IC2, IC3 Pins 1 & 8 = 6.1V; Pin 2 = 0.9V; Pins 3, 4, 5 & 6 = 0V; Pin 7 = 2.8V Q4 G1 = 0V; G2 = 6.5V; S = 1.3V; D = 8.8V IC4 Pins 2, 3 & 4 = 0V; Pin 5 = 4.6V; Pin 6 = 8.9V VC5 Anode = 0V; Cathode = 1.4V (1). Connect a multimeter between TP2 and TP3 and adjust L2 for a maximum reading. If necessary, readjust L3 after each change to L2 (preferably using a frequency meter – see previous section) so that the local oscillator runs at the correct frequency. Note that the antenna should be shortened to reduce signal pickup if the reading on the multimeter goes above 3V. (2). Tune to a station around 104108MHz and adjust VC4 until the received frequency matches the indicated frequency. (3). Adjust VC2 for a maximum reading on the multimeter, again making sure again that the reading does not exceed 3V. Readjust the antenna length if necessary. (4). Repeat the three preceding steps (this is necessary, since adjustments at 6 . 108 90 88 10 3 95 92 101 100 97 Two methods are available for tuning the local oscillator, which is adjusted so that it runs 10.7MHz below the tuned signal. If you have access to a frequency meter, then follow this method: (1). Connect a 10:1 probe to TP1 (near coil L3) and connect the ground lead of the probe to TP GND. Set the tuning dial so that it shows 88MHz, then adjust L3 so that the frequency meter shows 77.3MHz. Note: squeeze the coil slightly (so that the turns are closer together) to lower the frequency, or stretch it to raise the frequency. (2). Set the tuning dial to 108MHz, then adjust VC4 for a reading of 97.3MHz on the frequency meter. Now return to the 88MHz tuning dial position and readjust L3 for 77.3MHz. This done, return to the 108MHz position and readjust VC4 for 97.3MHz. If you don’t have access to a frequency meter, then a com­mercial FM radio should be used for setting L3 as follows: (1). Tune in a station at about 95MHz on the commercial radio and make a note of the exact frequency. (2). Switch the commercial radio off and tune in the same station on the FM Radio Trainer. Note that it will probably not be any­where near the indicated dial frequency, since the local oscilla­tor has not yet been adjusted. (3). If the indicated frequency is too high, squeeze L3 so that its turns are closer together. Conversely, if the indicated fre­quency is too low, stretch L3 so that its turns are further apart. Continue this process until the indicated frequency match­es the station frequency. Don’t worry about adjusting VC4 at this stage. That step is covered in the next section. 10 Local oscillator adjustments RF amplifier adjustment The RF amplifier (Q1) is the next section to be adjusted. The procedure is as follows: Fig.12: this is the full-size artwork for the tuning dial. one end of the band also slightly affect the other end). (5). Tune to a station near 100MHz which gives a reading from 1-3V and readjust T1 for maximum signal. Now adjust L1 for a maxi­mum. (6). Tune to a station which gives a strong signal (above 1V on the multimeter) and adjust VC6 for a maximum reading. Now connect the multimeter between TP4 and TP GND and readjust T4 for a 0V reading. That completes the alignment procedure for the FM Radio Trainer. Check that it can tune stations across the entire FM band from 88-108MHz and that the dial calibrations are correct. Check also that no background noise is evident when you tune to strong local stations (a good antenna helps). If the dial calibrations are incorrect or local stations are noisy, go back and carefully repeat the alignment procedure. Finally, if you wish to operate the FM Radio Trainer for extended periods, you can power it from a 9V DC plugpack instead of a battery. Be sure to remove the battery before connecting the plugpack supply and check the polarity carefully before switching the SC receiver on. May 1995  67