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In this second and final article on the new Superhet Stereo FM Radio, we explain how to assemble and align it for best performance. You can then put it into its superb acrylic case, and your friends won’t believe that you built it! Part 2 By John Clarke The Super-9 Stereo FM Radio Receiver A ssembling the Super-9 Stereo FM Radio is not difficult; everything is mounted on one large PCB. And the alignment is a snap using the small oscillator we’ve designed and some other basic equipment, as we will explain shortly. Construction The Radio is built on one doublesided PCB coded 06109181 which measures 313 x 142.5mm. It is housed in a multi-piece laser-cut acrylic case, available from the SILICON CHIP ONLINE SHOP. This also includes a transparent siliconchip.com.au tuning dial. Station call signs (eg, JJJ for triple J) and frequency markings that are screen printed on the main PCB can be seen through it. Use the PCB overlay diagram, Fig.8, as a guide as you build the Radio. Begin construction with the surfacemounting parts. These are not difficult to solder in using a fine-tipped soldering iron. You need good light and might need a magnifying lens or glasses to see well enough. These parts are Q1-Q4, VC1-VC3, XF1 and IC2, IC5 and IC6. You need to pay particular attenAustralia’s electronics magazine tion to ensure that Q1, Q2, VC3, XF1 and the ICs are fitted with the correct orientation. Mosfets Q1 and Q2 have a larger pad for the source connection, which is marked with an “S” in Fig.8 and on the PCB itself. VC3 has a stripe on the package that indicates the cathode, marked as “k” on the PCB. XF1 has a dot in the corner to indicate pin 1, which lines up with the “1” printed on the PCB. Similarly, the SMD ICs have a dot or divot to indicate pin 1, or you can look for the bevelled edge on that side of the chip package. While the polarity of the other SMDs December 2019  61 also matters, the remaining surfacemount parts have one pin on one side and two on the other, so the correct orientation is obvious as long as you don’t have the parts upside-down (their leads should be touching the surface of the PCB before soldering). Mosfet Q4 is mounted on the opposite side of the PCB from most of the other components; LED1 and LED2 are also mounted on the back (see Fig.9). Everything else is installed on the top side. It’s best to fit Q4 after the other SMDs since it will prevent the board from sitting flat on its back once it is in place. Make sure the NE592 is used for IC2 and LM4865s for IC5 and IC6. These could get mixed up as they are all in the same SOIC-8 style package. For each device, solder one pad first and check its alignment. Re-adjust the component positioning by reheating the solder joint if necessary before soldering the remaining pins. If you accidentally bridge two or more pins, add a little flux paste to the 62 Silicon Chip bridge and then clean it up by applying some solder wick with heat from your soldering iron. Through-hole parts Continue construction by installing the fixed-value resistors. You can use the colour code table from last month, as a guide to figure out which is which. But it’s best to check each set with a multimeter before soldering them in place, as the colour codes are easy to misread. Some colour bands can appear similar to others depending on lighting where red, brown and orange can appear the same. The resistors are not polarised, so they can be inserted either way into the board. Ferrite beads FB1 and FB2 can then be installed by feeding resistor lead off-cuts through the beads and then fitting them in the locations shown in Fig.8. Now is also a good time to fit zener diode ZD1. It must be soldered with its cathode stripe facing towards the top Australia’s electronics magazine of the board, as shown. Follow with axial RF inductor L5, which looks like a fat resistor. It is not polarised. Next, fit through-hole ICs IC1, IC3 and IC4. As with the SMD ICs, check carefully that their pin 1 is orientated correctly as per Fig.8 and that they have been pushed down fully onto the board before soldering the leads. Don’t use sockets as they could prevent the Radio from working correctly. Now mount trimpots VR2-VR5 and VR7. These often are marked with a code. The corresponding codes for these trimpots were shown in the parts list last month. Multiturn trimpots VR3 and VR4 must be orientated with the adjusting screw positioned as shown on the overlay diagram. This is so the adjustment direction will be correct. Fit the 18 PC stakes now. There are two for the antenna and its associated GND point, three to mount the two shields on either side of Q1/VC1, four for pot VR6 (one to ground its body), two above CON3 (TP5V/GND), two siliconchip.com.au Winding the Coils 5 10 This samesize photo of the completed PCB shows all components mounted on the top side. There are three extra (small) PCBs required – the whip antenna support and the two vertical RF shields (all top left corner). 9 L1-L3: 5 turns on 5mm mandrel 0.8mm EnCu wire 5 L4: 0.5 turn on 5mm mandrel 0.8mm EnCu wire 1-2 3-4 T1 Between pins 1-2: 25 turns Between pins 3-4: 5 turns Both 0.125mm EnCu wire near Q3/VC2 in the lower-left corner, and five in the middle of the board, to the left of the battery holder and REG1. Note that while the left-hand shield is held in place with two PC stakes, the right-hand shield only uses one at the top, and is soldered directly to the board ground plane at the bottom. If you haven’t fitted LED1 or LED2 yet, on the underside of the board, now is a good time to do so. Next, install the capacitors. There are three types used in the circuit. The non-polarised MKT polyester types can be recognised by their rectangular prism shape. The ceramic capacitors are also not polarised. Generally, small capacitors are not marked with their actual value and have a code instead. These codes are listed in the parts list last month. The third type of capacitor used in this project is the electrolytics. These are marked with their value in µF and are polarised, so they must be inserted the right way around. The negative lead is marked with a siliconchip.com.au stripe on the capacitor body, while the positive lead is the longer of the two. This goes into the pad marked with a “+” on the PCB. The 2N7000 Mosfets (Q5 and Q6) can go in next. Crank out their leads to match the mounting pad spacing using small pliers. Q7 (the SUP53P06-20) is in a larger package which is mounted flat onto the PCB and secured with an M3 x 6mm screw and nut. Bend the leads to insert into the PCB holes with the flat side down, then secure the tab against the PCB with the screw and nut. You can then solder Q7’s leads and trim them. REG1 is a 7805 three-terminal regulator in a similar package and is mounted in the same way as Q7, except that a small heatsink is sandwiched between it and the PCB. The two shields can be fitted now. They are small PCBs, as shown in Fig.7. These are placed vertically and then soldered to the three PC stakes. Then attach the lower section of the right-hand shield directly to the Australia’s electronics magazine 1-2 L6: Between pins 1-2: 23 turns of 0.125mm EnCu wire 1 5 4 2 3 Base diagram top view SC 20 1 9 ALL DIMENSIONS IN MILLIMETRES EnCu = enamelled copper Fig.5: here’s how to wind the coils and transformer for the Radio. L1-L4 are air cores wound using 0.8mm diameter enamelled copper wire on a 5mm mandrel (a cylindrical former), which is then removed. L6 and T1 are wound using 0.125mm diameter enamelled copper wire on plastic formers, with the ends of the wire soldered to the former pins as shown. L5 is not depicted as it is a pre-fabricated RF choke. December 2019  63 Then trim their leads so that they protrude through the board by about 2mm. L4 is placed against L3. Note that the enamel insulation on the lead ends must be removed before they can be soldered. This can be done by scraping the enamel off with a sharp hobby knife and applying heat from your soldering iron until any remaining enamel melts, after which the leads can be tinned and soldered in the usual manner. Now we need to make a tap connection to L2. Its ends are labelled on the PCB with “1” on the lower end and a “2” at the other end. The middle “3” connection is made using a short length of tinned copper wire soldered onto the coil side as shown. It is placed on the second 330pF A GND10nF 1 F BB207 10nF Silicon Chip 10nF POWER STEREO 10nF 47nF ZD1 470 120k 3.3k 15nF 15nF 1 F REG1 7805 IC6 LM4865 3.3k 1 F L 1 F R 1 Australia’s electronics magazine + BALANCE 1 10k GND 220 F CON3 SPEAKER R 10k 1 F 1 F 100nF VR7 100k IC5 LM4865 3.3k K VC3 BB156 47k 68k VC2 68k 220pF 10k Q3 30C02CH 4.7k 2.2pF E 47k B 10k LINEAR 4.7k 560 10k 10k L3 TP1 Q6 2N7000 L 100nF + 220 F 100k 47k 47 VR6 VOLUME 4.7pF 100 F TP5V 10 F 1 C 100 F IC4 MC1310P 2.2 F 64 220nF 16k 100k 10k LINEAR 470nF TP 19kHz 1k SET 19kHz 3.3k +5V 100k 1 F 3.9k 470pF VR5 5k VR4 500k 220nF TPREF 9V BATTERY HOLDER IC3 CA3089 22nF L6 SQUELCH 1k 22 H 5.1k 0V FM RADIO 100nF TPTUNE L5 TPSIG 10nF 1 20k VR1 TUNING L4 SILICON CHIP Q7 SUP53P06–20 33k 22nF 20k IC1 LMC6482 10k 10k GND VC1 TRIM 10nF 1 10nF 10nF 1 330 330 10k 10k 1 VR3 10k 10 F SHIELD 10M7FA XF1 VR2 10k VC2 TRIM 4.7k 4.7k 39pF SHIELD 10 220pF 47 D 10k 10nF 10nF S FB2 NE592 IC2 10nF 10nF T1 Q2 BF992 330 10nF 10k VC1 BB207 47pF G1 G2 L2 1 68k 68k 2 10nF 330 GND 220pF 1 470k 1k FB1 S 06109184 Antenna Bracket L1 G1 G2 10k 10nF D 47 Q1 3 BF992 10k 47pF 06109183 Fig.6: this tiny PCB is used to attach the antenna to the main board. It allows the antenna to project out from the PCB far enough that it sits outside the plastic case, and the antenna can then be folded along the top of the Radio, or rotated and extended to be vertical during use. SILICON CHIP ANT. Fig.7: this simple shield PCB has copper on both sides along with two exposed pads, to solder to PC stakes on the main board or directly to a pad on the main board. This connects the copper on the shield board with the main PCB ground, preventing signals from coupling between components on either side of the shield. Two identical shield shield boards are used in this design. 15V 1W Fig.5 shows the coil winding details. There are four air-cored coils, L1-L4. L1-L3 are identical and are made by winding 5 turns of 0.8mm diameter enamelled copper wire onto a 5mm drill bit shaft or other cylindrical former. Before winding, stretch the wire straight by clamping one end in a vice and pulling the other end with a pair of pliers. Be sure to wind these coils in the direction shown. If the coils are wound in the wrong direction, they will not fit correctly on the PCB. L4 is made from a half-turn of the same wire. After winding, install the coils on the board with the lower part of each coil 5mm above the top of the PCB. 150 Winding the coils winding from the “1” end. You will need to scrape some of the enamel off the coil so solder will adhere to it. Transformer T1 is made by winding two coils onto a miniature Neosid former. Solder one end of the 0.125mm enamelled copper wire to pin 2. As with L1-4, use heat or a sharp knife to strip away some of the enamel from the wire end, then wind it around the pin, close to the former, and solder it in place. Pass the wire around the indent in the former on the side so the wire won’t contact the metal can when installed. Now, starting from the bottom of the former, wind on 25 turns with each turn adjacent to the previous turn. Do not place windings on the former above the flange near the top. This section is reserved for the cup core to fit. After this, terminate the free end of the winding on pin 1, again making sure that the wire is fed via the indent. The second winding for T1 is placed over the first winding, with five turns terminated to pins 3 and 4. Again, ensure the wires are routed via the former indent. The winding direction is not important. Inductor L6 is wound similarly to T1, except that there is only one wind- 100k board via a solder fillet between the two PCBs. Q5 2N7000 1M LOUDSPEAKER siliconchip.com.au Use a hacksaw to cut the tuning potentiometer (VR1) and the volume control potentiometer (VR6) shafts to 17mm, measured from where the threaded boss starts. Feed VR6’s shaft through the PCB from the component side. The small locating spigot on the side of the pot fits into a corresponding slot on the PCB. Secure the pot with its washer and nut on the other side. Now bend the potentiometer pins so that they touch the PC stakes and solder them in place. You also need to solder the pot body to the fourth PC stake to ground it to the circuit, but the body is passivated to prevent corrosion. This makes it almost impossible to solder, so you need to scrape away S1 9VDC (CENTRE +) CON1 150 PHONES CON2 siliconchip.com.au Existing endstop SILICON CHIP Preparing the potentiometers REQUIRED PIECE 06109185 ing of 23 turns, terminated to pins 1 and 2. Place a cup core over each former, add the metal can and insert into the T1 and L6 positions. Make sure you have the correct one in each place (T1 has the two windings) before soldering the pins. The F16 slugs can now be screwed in the top, but do not use a screwdriver as it will crack the core. Use only a small brass or Nylon trimming tool to screw in the slugs Curved PCB Pot travel stopper SC CUT 20 1 9 Fig.10: make two cuts on the Pot Stopper PCB, where shown here, to separate the thin arc section from the rest of the board. This piece of fibreglass laminate is then placed inside one of the potentiometer bodies, to limit its angle of rotation to exactly 180°. some of the passivation layer with a file or hobby knife first. Potentiometer VR1 needs some modifications before being installed. First, remove the rear metal shell by slightly bending the four flange tabs that clamp it to the pot. We’re doing this so that we can insert a ‘stopper’ piece to reduce the pot rotation to just 180° rather than 300°, to suit the tuning dial. The stopper is a curved piece of a small PCB. You need to cut out the middle section, along the white lines shown on the PCB and in Fig.10. You END STOP MODIFICATION TO ‘ALPHA’ POTENTIOMETER Flange tabs BACKSHELL Fig.11: this shows how the Pot Stopper fibreglass section sits inside the pot body once the shell has been removed. Crimp the existing metal endstop over the pot stopper to hold it in place, then once you have checked the pot travel, reattach the rear shell using the flange tabs. can cut it with sharp side cutters or with a hobby knife, then file the ends smooth. Cut the corner of the original end stop inside the pot rear shell with side cutters and bend the pieces open a little, just enough for the curved PCB piece to be inserted. Locate it centrally around this preexisting end stop, as shown in Fig.11, and then bend the metal pieces back over the PCB piece so it is held in place. Reassemble VR1, making sure that the shaft is rotated so the flat on the Fig.8 (left): this PCB overlay diagram shows where to fit the components onto the board before soldering. Ensure that the polarised components (zener diode, electrolytic capacitors, Mosfets, transistors and ICs) are the right way around. Also, pay careful attention to ensure each component installed is of the correct value and type. Fig.9 (right): and     here are the very few components on the back side of the PCB – LEDs 1 & 2, plus the only SMD on this side of the board, Mosfet Q4. Make sure that the longer lead of each LED goes to the pad marked “A” in each case. Cut open endstop, place curved PCB inside & crimp in place with endstop tabs TUNING C B E Q4 30C02CH VOLUME Australia’s electronics magazine K A LED2 STEREO K A LED1 POWER December 2019  65 shaft is toward the three terminals first. That’s so the plastic rotating piece at the end of the shaft is positioned with its travel stopper opposite the PCB arc piece. Replace the metal shell and test that it now rotates through 180°. Then bend the tabs over again to secure tightly in place. As with VR6, feed it through the PCB and ensure the locating lug is in the slot provided before securing it with the supplied washer and nut. This pot is wired to the PCB using short lengths of 0.7mm tinned copper wire fed through the PCB pad holes and each of the three riveted connection points on the pot. Solder these wires to the PCB and then to the pin ends of the metal tabs on the pot. Do not solder at the riveted points, where the wires pass through, as this could damage the connection to the pot’s carbon resistance element. Now remove some of the pot body passivation near the 0V pin and solder a short length of tinned copper wire from the 0V terminal of VR1 to the pot shell, as shown in the photos. Grounding the pot body provides some shielding to avoid noise pickup through the pot. Now fit the battery holder, on/off switch (S1), DC socket (CON1) and headphone socket (CON2). The battery holder is held in place with short selftapping screws inserted from the underside. The power switch and headphone socket are mounted directly on the board. Also install locking polarised header CON3 now. Its orientation is not critical. Speaker mounting The speaker is fastened to the PCB using four M3 screws and nuts. The board has eight mounting holes, two sets of four on two different circumferences. So use the correct holes for your particular loudspeaker and orientate it with the terminals nearest to the bottom edge of the PCB. Crimp and/or solder a short length of figure-8 wire to the pins for the polarised header plug and insert them into the plastic shell. Connect the other end to the speaker terminals, then plug it into CON3. The two LEDs are located on the dial side of the PCB and sit flat along the PCB, with their lenses pointing towards the dial (see photo). It would be a good idea to fit two different-coloured LEDs so you can later tell which 66 Silicon Chip one is illuminated. Bend the leads to insert into the PCB holes, making sure that in each case, the longer lead is in the anode position, marked “A”. Antenna mounting A small antenna adaptor PCB shown in Fig.6 is used to extend the connection point upwards, so that the antenna can be outside the enclosure. It is attached to the main PCB using an M3 screw and two nuts. One nut is sandwiched between the antenna adaptor PCB and the main PCB, and the other at the underside of the PCB. The electrical connection is made via the antenna input PC stake. The antenna is attached later, once the Radio is in its box. There are several ways to attach the antenna. The antenna mounting hole is smaller than 3mm so it can be tapped for an M3 thread. This allows the antenna to be secured just with an M3 screw. If you don’t have an M3 tap, the hole can be drilled out to 3mm. The antenna then attaches with an M3 screw with washers on each side. An M3 nut holds the antenna reasonably tight in place. You can either use thread lock (such as Loctite 222, 243 or 263) or a locknut to prevent the nut coming loose. A Nylock M3 nut could also be used. Initial testing 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 power it on as it needs to be ‘aligned’ to work properly. Before installing the battery or connecting power, check the assembly carefully to ensure that all parts are in their correct locations and are correctly orientated. The underside of the board should also be checked for missed solder joints and short circuits. Assuming that everything is correct, it should be safe to connect power now. But to be sure there are no immediate problems, you should measure its current draw when you do. Apply 9V with switch S1 off (out) and switch your multimeter to read current. Connect one of its probes to one of the centre pins of S1 and the other to one of its rear-most pins. This will effectively connect power to the circuit via the multimeter. A current reading over 100mA at this stage could mean there is a problem. If so, remove the probes immediAustralia’s electronics magazine ately. Check the board again carefully for incorrectly placed components and shorted pins on ICs or between soldered pads. If the power LED (LED1) does not light, it is either installed with the incorrect polarity or power is not reaching it. There could be a short somewhere across the power rails, which would be indicated by a very high initial current draw. Locate the source of the problem before switching on again. Assuming it’s drawing a modest current and LED1 is lit, proceed to make a series of voltage checks. We listed voltages expected at various parts of the circuit on the circuit diagram. These voltages are approximate and assume a supply voltage of exactly 9V. They were measured on the prototype using a digital multimeter. Check the 5V supply between TP5V and GND. This should be between 4.75V and 5.25V. If any measured voltages differ by more than 20% from our figures, there is probably an incorrectly placed component on the board. Aligning your Radio To make the alignment easy, the circuit should initially be set up according to the following procedure. Note that all adjustments to the ferrite slugs in T1 and T2 must be carried out using a proper trimming tool. Do not use a screwdriver in the ferrite slugs, as this can easily crack them. Measure the length of coil L1 and stretch or compress it until it measures 10mm end to end. Similarly, set the length of L2 to 15mm and L3 to 10mm. Rotate VR2, VR3 and VR4 fully anticlockwise. VR3 and VR4 are multiturn trimpots so you should do this by turning the top screw of each one anticlockwise at least 20 times. You may hear soft clicks when they reach the end of their travel. IF alignment The alignment procedure involves using the IF Alignment Oscillator described 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. There is sufficient signal from the IF oscillator to enable the 10.7MHz signal to pass through the 88-108MHz bandpass filtering of the first stages to reach siliconchip.com.au The front side of the Radio is a shiny black acrylic but the rear, as shown here, is crystalclear – so that you (and everyone else!) can admire your handiwork . . . the mixer IF output. Power for the IF Alignment Oscillator can be derived directly from the Radio’s 5V supply, between TP5V and GND, using a suitable length of hookup wire. The step-by-step alignment procedure for the IF circuitry is as follows: 1) Power up the Radio and alignment oscillator. 2) Connect a multimeter set to a low DC volts range between the “Signal” test point near IC3 and a GND test point. 3) Adjust trimpot VR8 on the IF oscillator for a multimeter reading of 3-4V. 4) Adjust the slug in T1 for a maximum reading. If the reading goes above 4V, adjust VR8 anticlockwise, so the voltage stays in the 3-4V range. Then re-adjust T1 for a maximum reading and repeat until you can’t get it to increase any further. 5) Connect the multimeter between TP REF and TP TUNE and adjust the slug in inductor L6 for a 0V reading. That completes the alignment of the IF stages. But the local oscillator and RF amplifier stages still need to be aligned. Remove the FM IF Oscillator board and attach the telescopic antenna to the antenna extension PCB using a screw and nut. Local oscillator adjustments There are two methods for tuning the local oscillator. It needs to be adjusted so that it tracks the tuned signal frequency, always being 10.7MHz lower. If you have access to a frequency meter or digital oscilloscope that can measure in the 100MHz region and siliconchip.com.au show a frequency reading, that is ideal. A 50MHz oscilloscope would probably be OK, even though the signal level will be down due to its roll-off above 50MHz. Connect a 10:1 probe to TP1 (near coil L3) and connect the ground lead of the probe to the PCB GND. For best results, to prevent any frequency shift due to the probe loading, include a series resistor between TP1 and the probe. A 1kΩ resistor or higher value may be used, but ensure the resulting reduction in signal level does not prevent the signal frequency from being read reliably. During this procedure, keep the probe away from L3, to avoid affecting L3’s tuning. Set the tuning dial to 88MHz, then adjust L3 so that the frequency meter or scope shows 77.3MHz. Squeeze L3’s windings together slightly to lower the frequency, or stretch it to raise the frequency. Then set the tuning dial to 108MHz and adjust VR3 for a reading of 97.3MHz. Now return to the 88MHz tuning dial position and re-adjust L3 for 77.3MHz. Return to the 108MHz position and re-adjust VR3 for 97.3MHz. Repeat until no further adjustments are necessary. If you don’t have access to a frequency meter, then a commercial FM radio can be used instead, as follows: Tune in a strong local station at about 98MHz on the commercial radio and make a note of the exact frequency. Switch the commercial radio off and tune in the same station on your Radio. It will probably not be anywhere near the indicated dial frequency, since the local oscillator has not yet been adjusted. Australia’s electronics magazine 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. This will get the alignment started at the middle of the FM band. Then find a station near 88MHz on the commercial radio, tune it in on your Radio and re-check the adjustment of L3 and the position on the dial. Re-adjust L3 until the indicated frequency matches the station frequency. Find a station up near 108MHz and use the same procedure to adjust VR3 so that the station lines up with the dial calibration. Repeat tuning at the 88MHz and 108MHz ends of the dial until the stations appear at the correct locations on the dial. RF amplifier adjustment The RF amplifier is the next section to be adjusted. The procedure is as follows: 1) Connect a multimeter between TPSIG and GND and tune to a station near 88MHz. Adjust L2 for a maximum reading. Squeeze the coil slightly (so that the turns are closer together) to lower the frequency, or stretch it to raise the frequency, but note that you won’t know which way is ‘right’; you just have to try both and see which helps. Shorten the telescopic antenna should the reading on the multimeter go above 4V. Keep the signal reading within 3-4V while adjusting the antenna to keep this range. 2) Tune to a station around 104108MHz and adjust VR3 until the December 2019  67 received frequency matches the indicated frequency. 3) Adjust VR2 for a maximum reading on the multimeter, again making sure that the reading does not exceed 4V. Re-adjust the antenna length if necessary. 4) Repeat steps 1-3. This is necessary since adjustments at one end of the band also affect the other end. Front end tuning Tune to a station near 100MHz which gives a reading of 3-4V at TPSIG and adjust L1 for a maximum reading by squeezing or opening the coil. Check that the Radio now 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. With a good strong signal being received, the IF slug in T1 can be re-adjusted to peak the reading at TPSIG. Additionally, for the best result, adjust the slug in L6 for 0V between TP REF and TP TUNE. Tuning the stereo demodulator Adjust VR5 for a 19kHz reading at the 19kHz test point. If you don’t have a frequency meter capable of measuring this, you can adjust this while receiving a strong station with the headphones plugged in. Adjust VR5 to the middle of the range where the stereo LED lights up, ie, position it halfway between the two positions where the stereo LED is just off. you’ll want to put it into the purposedesigned acrylic case. Its appearance is not unlike the mantel radios of yesterday, only it is glossy black! The case measures 327 x 155 x 58mm (w x h x d) and the front, sides, top and bottom are made from a very smart high-gloss black acrylic. The back panel is transparent, so that everyone can admire your handiwork. It has holes in the left-end panel for the on/off switch, the DC power plug and the 6.5mm headphone socket. On the front panel, attractive slots are milled for sound output immediately in front of the speaker. At the right end, there’s a matching 105mm hole for the clear acrylic tuning “dial” which reveals the screen-printed PCB underneath, showing the major radio stations. We glued a large knob to the centre of the dial to make it easier to use – this also holds the dial to the shaft. Immediately underneath and to the left of the tuning dial is the single volume control. The case simply slots together and everything is held in place by four 46mm long pillars which go from front to back. We’ve also made provision on the bottom front of the case for a pair of rubber feet which can angle the whole receiver back slightly. Again, this is entirely optional. ~10mm M3 SCREW BACK PANEL 25mm LONG M3 TAPPED SPACER M3 NUTS & WASHERS (SPACE AS REQUIRED TO ADJUST TOTAL LENGTH) Squelch control VR4 sets the squelch control. Squelch is designed to mute the interstation noise while tuning between stations. We found that this adjustment is best left with VR4 in its fully anticlockwise position where there is no muting. You can set the control to a more clockwise position if that is your preference. Putting the case together Because this Radio is self-contained (ie, entirely on one PCB), it is quite happy working without a case. But if you want a really professional finish, or are going to take it with you, 68 Silicon Chip 15mm LONG M3 TAPPED SPACER 46mm ~15mm M3 STUD (15mm M3 SCREW WITH HEAD REMOVED) M3 NUTS & WASHERS AS REQUIRED PCB FRONT PANEL ~15mm M3 SCREW SC 20 1 9 Fig.12: you need four 46mm M3 threaded standoffs, but just try to buy them! We made ours from 15mm and 25mm standoffs, joined with an M3 “stud” made from a headless 15mm screw. Nuts and washers were used to pack it out to 46mm long. Australia’s electronics magazine Remove the nuts from the volume control pot and headphone socket, if fitted. Start with the front panel. Insert four M3 x 15mm screws through the four holes near the edges and put a washer and nut on each to hold them in place. Now slide the receiver PCB down over these screws, orientated so that the speaker sits behind the slots and the dial markings behind the 105mm hole. Slide the left end panel into its slots on the front panel, at the same time engaging the on/off switch shaft and the 6.5mm headphone socket. You will probably have to lift the PCB on this end to allow this. When in position, refit the nut onto the headphone socket. This will hold the end panel in place. You can then slide the bottom, top and right end panels into place, with their tabs fitted into the slots on the front panel and each other. Once you’ve reattached the nuts and washers for the pots and sockets, you can then fit the three knobs (for S1, VR1 & VR6). Threaded standoffs It’s not easy to buy a threaded standoff long enough (45mm+) to hold the rear panel onto the front panel. We made ours with a combination of 15mm and a 25mm M3 threaded standoffs, M3 studs to join them into single 40mm lengths, plus a few M3 nuts and washers to end up with the 46mm length required – see Fig.12. The “stud” which joins the 15mm and 25mm lengths was simply a short (15mm) M3 screw with its head cut off with a hacksaw. You will probably need to clean up the end with a file, then run a nut over the cut-off section to re-form the thread after cutting it. If you use Nylon or Polycarbonate screws, the head can be cut off with side cutters. We used two M3 nuts between the two standoffs as additional spacers. The overall length of the standoff is 46mm. Given that nuts can vary in height, simply vary the number of nuts and/or washers to make your standoffs 46mm long. You will need four of these. The bottom ends screw onto the M3 screws which pass through the case front panel (already fitted with a nut) and then the PCB. The top ends are fastened using four M3 screws which hold the rear panel in place. SC siliconchip.com.au