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2-Chip Stereo Radio Last month, we described the main features and gave the circuit details of our new highperformance AM stereo receiver. This month, we show you how to build it. By STEVE PAYOR The AM stereo receiver is built on a small PC board coded SC 06108891. This board accommodates virtually all the parts and comes in two versions (Rev.1 & Rev.2), depending on the type of headphone socket used. As can be seen from the photos, the PCB assembly is mounted on the lid of a small plastic case. 58 SILICON CHIP Before you start soldering components on the PCB, a little preparatory work needs to be done to ensure that the tuning gang, volume control and headphone socket fit correctly. Also, the empty PCB makes an ideal template for marking out the front panel. The first step is to trim the edges of the PCB with a sharp file, stopp- ing just as you reach the copper. Do not remove the copper border as this forms part of the electrostatic shielding. The small gap in the border track is intentional without it this track would form a short-circuit for some of the magnetic flux around the ferrite rod aerial. Note also that the two corners at the headphone socket end of the board need to be cut away to clear the internal pillars of the plastic case. There are three slotted holes which need to be cut in the board. Two are for the tuning gang lugs and the other is for the anti-rotation lug on the volume control pot. Start each slot by drilling three 1mm holes side-by-side, then open these out into a complete slot with an Arlec Supertool or some similar device. Next, open out the hole for the tuning gang shaft to 6.5mm and the hole for the pot bushing to 11mm. Use a progressively larger series of drills to start the holes, then enlarge them using a tapered reamer until the parts fit perfectly. The tuning gang must be fitted with an 18.5mm long extension shaft. Fig.2 shows the details. It is held in place with a long 1OBA screw and should be a tight fit on the tuning gang shaft. In a pinch, a loose shaft can be pressed into service by coating the inside of the recess with Araldite (not the 5-minute type} or a high-strength anaerobic cement such as green Loctite 601, Permabond A168 etc. Headphone socket The headphone socket requires special attention. Currently, there are three versions available which can be used. The first version (for the Rev.1 board} has a plain, unthreaded bush which doesn't quite reach through the thick wall of the plastic case. To use this socket, the outside of the case needs to be spotfaced to On this style of headphone socket with threaded bush, you have to dig in with a screwdriver and break away the ground contact to prevent it shorting the right channel. After that, use a pair of needle-nosed pliers to crimp the metal bush so that it makes good contact with the ground sleeve of the plug. a depth of a little less than 1mm to enable the plug to reach the socket (more on this later}. The second version (also for the Rev.1 board} has a threaded bush which does reach comfortably through the case wall. Unfortunately, the internal ground contact of this socket rests right on the insulation between the right channel and ground sections of the 3.5mm stereo plug. On some headphones, it shorts the right channel to ground. The best thing to do here is to dig in with a fine screwdriver and bend the ground contact up and out of the top of the socket (see photo}. Next, reach in with a fine pair of needle-nosed pliers and crimp one corner of the metal bush inwards so that it makes firm contact with the ground sleeve of the plug. The horizontal location of the PCB is critical. It must be mounted so that the rectangular shoulder of the headphone socket is in line with the outer edge of the locating ridge on the case lid. OCT0BER1989 59 Virtually all the parts are mounted on the PC board. When installing the parts, keep all leads as short as possible and make sure that you don't mix the coils up. The four test points are used during the alignment procedure. The third style of headphone socket comes from Dick Smith Electronics. It's easily recognised because it has three asymmetrical pins towards the rear of its body and one near the threaded bush for the ground contact (the others have five pins). You don't have to modify the ground contact on this socket but, because its terminal arrangement is different, you do have to use a modified PC board. For this reason, kits for this project from Dick Smith Electronics will use the Rev.2 version of the PC board to suit the DSE headphone socket. Note that the left and right audio channels are reversed with the DSE socket. To correct this, two in- I I0.25± .001 INCH 10 BA SCREW-l¥OUTSl:EDIA ALUMINIUM E j; ""~""H™"1 I ' • ENO OF SHAFT 4mm INSIDE DIAMETER (TIGHT FIT ON TOKO TUNING GANG SHAFT) Fig.2: details for the 18.5mm extension shaft. It should be a tight fit onto the tuning gang shaft and is secured with a long 10BA screw. 60 SILICON CHIP sulated links are used to swap the L & R signals over at the audio amplifier inputs (pins 1 & 9) - see Fig.4. Note also that one of the "R" pins· on the socket is soldered on one side only, to avoid a short to the nearby IC pads. One final point here: the pins on all types of headphone sockets are likely to be quite short - barely long enough to reach through the PCB, in fact. Tin the pins thoroughly before insertion, so that the solder will flow down into the PCB holes. A secure connection is vital here, because the headphone socket will be subjected to quite a bit of mechanical stress during normal use. Five of the holes in the PCB are for the tiny plastic locating pins on the underside of the socket (Rev.1 board sockets only). Drilling the front panel Having satisfied yourself that the tuning gang, volume control and headphone socket all fit snugly in the board, remove all but the headphone socket and place the board against the inside of the case lid to mark the outlines of the front panel holes. There are six holes in all: one for the 1/4-inch diameter tuning gang shaft, one for the 3/8-inch pot bush, one for the 3mm LED, and three for the board mounting screws. Two of the board mounting screws are directly under the ferrite rod, while the third mounting point is located right next to the tuning capacitor for mechanical stability. The horizontal location of the PCB is critical. The rectangular shoulder of the headphone socket must be in line with the outer edge of the locating ridge on the case lid. Vertically, the PCB is centred with respect to the front panel (see photo). Having marked and drilled the holes, the next step is to turn the front panel over and countersink the three holes for the board mounting screws. Do this carefully, so that the screw heads will be exactly flush with the surface. They will be covered over by the front panel label later on. Now fit the three countersunk screws with washers and nuts so that the board will sit 3.5mm from the panel. Tighten the nuts firmly as these screws stay permanently STEREO HEADPHONES 680pF 3300 Fig.3: install the parts on the PCB exactly as shown in this diagram. Don't use IC sockets as the extra stray capacitance could upset the performance of the MC13024. The 3mm LED is mounted on the underside of the PCB (see text). fixed in place on the front panel. One final test of your workmanship: temporarily refit the tuning gang and volume control, then drop the board down over the mounting screws. The capacitor shaft and pot bush should fit snugly in their holes and the two solder pads for the LED should be visible directly below its hole. If you are using the same metal knobs as on the prototype (Jaycar Cat. HK-7024 and HK-7022), the tuning capacitor shaft should extend 12.5mm above the front panel and the volume control shaft 11. 5mm. These shaft lengths should also suit the equivalent metal knobs from Dick Smith Electronics but check first before trimming the pot shaft. PCB assembly You can now start soldering the components to the PCB as shown in Fig.3. Mount the links and low profile components first, otherwise things will get awfully squeezy when the board starts to fill up. PC stakes should be installed at the GND, + 1V REF, AGC and Buffered AGC points along the bottom righthand coner of the board. To save space, we have mounted all the resistors end on. A lot of the resistors have a "hot" end and a "cold" end as far as RF voltage is The PCB assembly is supported on the case lid using countersunk machine screws, full nuts and washers. The screw heads should sit exactly flush with the lid surface so that they can be covered by the label later on. concerned, so it is important to orient the resistors as shown in the parts overlay diagram for best results. For example, if you reverse the 100k0 resistor across 14 (ie, have its other end long), there will be sufficient IF radiation to cause a noticeable whistle as you tune through the weaker stations. Take care with the polarity of the electrolytic capacitors. Note that the two .04 7µF capacitors in the 25Hz bandpass filter should be 2 o/o types with 5mm lead spacing. If you can't get 2 o/o types, 5 o/o greencaps can be used, provided they are selected to be within 2 o/o of value (ie, .046µF to .04 7µF). Now for the tuning capacitor. This must be mounted so that the OCT0BER1989 61 '_/ - R• The ferrite rod antenna is mounted using right-angled aluminium brackets and plastic P-clips. Don't use metal clips, as these will form a shorted turn. The coil can be secured with a cardboard wedge. INSULATED LINKS • Fig.4: this diagram shows the wiring changes for the Rev.2 board with DSE headphone socket. The two insulated links swap over the audio amplifier outputs to compensate for the headphone socket connections. either wide, medium or sharp. These options are selected by bridging solder pads on the PCB as shown in Fig.5. For city dwellers, the wide setting is the one to go for. Mounting the ferrite rod You will need a soldering iron with a fine tip to mount the LED in position. The top of the LED should just poke through the front panel when the board is installed on the mounting screws. wider of the two earth lugs goes towards the bottom of the PCB. Also, if you have keen eyesight, you will see that there is lettering on the back of the tuning gang. Just mount the tuning gang with "C3" towards top left. The Tako coils can be installed next. This job is straightforward since the pinouts are polarised but make sure that you don't mix the coils up. The 3mm LED is mounted on the underside of the board. Use a soldering iron with a fine tip and keep the temperature as low as possible. If you pre-tin the leads, you will only need a quick touch to finish the job. The top of the LED should just poke through above the front panel when the board is in62 SILICON CHIP stalled on the mounting screws but not by more than 0.25mm otherwise is will foul the perspex cursor attached to the tuning knob. Note carefully the orientation of the two !Cs when you install them on the PCB. Don't use IC sockets the extra stray capacitance from the socket could compromise the performance of the MC13024. The volume control pot specified is a PC mounting type. It is installed by bending the lugs for the front section through goo so that they mate with the holes in the PCB. The lugs for the rear section are also bent through goo and are connected to the board via short lengths of tinned copper wire. At this stage, you are ready to select the bandwidth option - As you can see from the photographs, we chose to mount the ferrite rod high above the PCB using two small right-angled brackets and plastic P-clips. Other mounting schemes could also be used, depending upon the available hardware. There are two points to watch here. First, don't use a metal clip around the ferrite rod as it will form a shorted turn. Second, leave one half of the rod unsupported so that the coil can be slid up and down to adjust the inductance. After the rod is mounted, check that it doesn't foul anything on the inside of the plastic case. A 100mmlong rod will fit comfortably. If the rod is any longer you will have to cut it. You can do this by filing right around it with a file, then snapping it like a piece of glass. The aerial coil is a standard prewound unit from Dick Smith Electronics. This normally comes supplied on a short length of flat ferrite rod but can be easily slid off and reshaped to suit the round rod. We wedged a strip of thick cardboard between the coil and the rod to give it a very firm grip. Don't rely on a blob of melted wax to hold the coil in place, because it won't. The coil must not move except when you want it to. The colour code for the coil leads appears to be fairly standard but if there is any doubt, you can identify the large and small windings with an ohmmeter. The "hot" end of the main winding (usually the plain lead) should be the furthest away from the secondary winding and closest to the end of the rod. Trim the leads so they will just reach their respective PCB holes with the coil sitting about 10mm from the end of the rod. Unnecessary lead length must be avoided since any vibration of the leads may cause phase shifts, thus disturbing the stereo image. To prepare the leads, carefully unwind the ends of the three silk covered strands and tin each strand separately with a hotterthan-normal iron. The silk covering and enamel insulation will melt away. Now twist the three strands back together again and solder them as one unit (two of the strands are for mechanical strength only). Does it work? Once the coil has been connected, the radio is electrically complete and you can give it a brief power-up to see if it works. We fitted a 2-pin PCB header plug for the 3V power connection to the board, which made testing and assembly much more convenient. At this stage though, the performance will be far from optimum since you have yet to go through the alignment procedure. Even so, you should be able to tune in local stations. Front panel Accompanying this article are two separate artworks for the front panel. One shows the main Sydney AM station call signs as well as the tuning frequencies in kHz. The other shows no station callsigns, just the frequencies. For Melbourne readers, we will publish an artwork to suit their station callsigns next month. Our apologies to readers from other parts of Australia but the author wanted a proper, oldfashioned tuning scale for his own use, hence the Sydney station callsigns on the upper label. · If yo·u live outside the Sydney or Fig.5: the bandwidth option is chosen by bridging solder pads on the PCB. We recommend that you select the 'Wide' option by bridging two of the pads as shown at far right. Fig.6: here are the dimensions for the perspex cursor. The edges can be smoothed with very fine abrasive paper and polished with "Brasso". Melbourne listening areas, it is well worth the effort to letter in a tuning scale for your own area, as tuning by numbers is a real nuisance when the frequencies are ·all multiples of 9kHz. Letraset is the answer here, and you can use a matt acrylic spray to fix the lettering in place. Tuning dials with station callsigns have all but disappeared these days, since most radios are now made overseas. This puts our little tuner one up on even the most expensive imports. Fixing the label Affixing the Dynamark label on the front panel requires great care to ensure that is accurately centred on the tuning capacitor shaft. The adhesive is so aggressive that you will not have a second chance to reposition it. The 3M literature suggests that you use a weak, mild detergent solution to "float" the label into place, much like the rego label for a car. We chose a different method. After the label has been trimmed, remove it from its backing and cut the backing in half. Now stick the backing back on to the label, leaving a central strip of adhesive exposed. If you have a light table, lay the plastic front panel on it and position the label using the targets and the light through the holes as a guide (the label material is semitransparent). Press down on the centre section to tack it in place, then remove the strips of backing and smooth out the label, always working outward from the centre to avoid bubbles. Small bubbles will disappear by themselves in a day or two, so you don't have to spoil the finish of the label by bursting them with a pin. The best way to open out the holes in the label is with the tip of a very sharp, pointy knife. The PC board can now be fitted to the front panel and secured with three nuts. Knobs and perspex cursor We chose solid aluminium knobs for our prototype. The larger tuning knob is fitted with a clear Perspex cursor which is something you will have to make from scratch. The cursor has a valuable role to play in the alignment procedure, so it is definitely worth the extra effort. Start with a small scrap of 1.6mm thick Perspex and cut and file it to the dimensions indicated in Fig.6. The edges can be smoothed on very fine "wet or dry" abrasive paper, using kerosene as a lubricant. The edges can then be polished by rubbing on a strip of cloth soaked with "Brasso". To scribe a really neat line on the cursor, you will need to grind a piece of old hacksaw blade so that OCT0BER1989 63 can be adjusted to a perpendicular position. You will notice that the tuning capacitor has a total rotation of slightly less than 180°. When tightening the grubscrew in . the knob, rotate the tuning shaft fully anticlockwise and set the cursor exactly in line with the horizontal line on the dial. The clockwise limit of rotation will then be at approximately 1620kHz on the scale. 2Bmm ALUMINIUM KNOB COUNTERSUNK SCREW (UNDER DECORATIVE LABEL) CASE LID PCB Teflon washer Fig.7: this cross-sectional diagram shows the mounting details for the PC hoard, tuning gang, LED and tuning knob. Note the Teflon washer between the Perspex cursor and the front panel. ing ink, allow it to dry for a few hours, then remove the excess ink by rubbing with a piece of clean paper. "Super Glue" (cyanoacrylate) is the only readily available adhesive which will glue Perspex to aluminium. On no account use epoxy. This may hold the Perspex for a few days but it will fall off when the epoxy hardens fully. Degrease the aluminium surface with acetone before glueing but don't let the solvent near the Perspex. With "Super Glue" you only get one shot at it, so fit a short stub of ¼-inch rod into the knob to act as a guide for centring the cursor. The perspex cursor is attached to the bottom of the tuning knob using "Super Glue". The line on the cursor should line up with the groove in the knob. Setting the dial there is just one tooth left on the end. File the tooth down to a width of about 0.25mm. This tool can then be used to scrape a neat, rectangular channel in the Perspex, whereas a scriber will only made a raggedy-edged scratch. Fill the groove with black draw- Now try the tuning knob (plus cursor) on the tuning shaft. Fig.7 shows the assembly details. If the cursor doesn't run parallel to the front panel, you can correct this situation by re-melting the tuning gang solder joints so that the shaft A useful finishing touch is some sort of thin, low-friction washer between the case and the underside of the cursor (see Fig.7). This takes some of the load off the tuning shaft when the radio is bumped and reduces the tendency for it to drop out of stereo mode. The washer is easily punched from Teflon sheet or you can look around for something in Delrin, Nylon, red fibre etc. Just before tightening up the grub screw, lightly squeeze the tuning gang and knob together to preload the washer and tuning gang bearings. Battery clamp The battery clamp consists of four countersunk screws, two tapped spacers and a scrap of Perspex sheet (see Fig.8 & Fig.9). To find out how to bend Perspex, refer to page 45 of the July 1989 issue of SILICON CHIP. Alternatively, you can use 1.6mm aluminium sheet instead of Perspex. Study the location of the batteries carefully from the photographs. They won't fit anywhere else. The top right hand corner of the battery holder comes very close 1.6mm PERSPEX OR ALUMINIUM SHEET .......,,. ..... ~ .! COUNTERSINK~ SNAP CONNECTOR TOP COVER (SEE DIAGRAM) ~ 2xAA BATTERY HOLDER 71mm I Fig.8: the battery clamp can he made from 1.6mm Perspex or aluminium sheet. The corners are cut off the cover plate to clear the dual gang volume control. 64 SILICON CHIP CASE BOTTOM Fig.9: the battery clamp cover is secured to two tapped spacers using countersunk screws. Note that the batteries must he positioned in the bottom left hand corner of the case. POLYSTYRENE CAPACITORS Made here in Australia? Made Special to Type? A Teflon washer must be installed between the front panel and the Perspex cursor. Before tightening the grub screw on the tuning knob, lightly squeeze the tuning gang and knob together to preload the washer and tuning gang bearings. This makes tuning easier and reduces the tendency for the radio to drop out of stereo if it is bumped. +Trim to outside of board outline f SILICON CHIP AM STEREO Fig.10: this full-size artwork is for the Rev.1 version of the PC board. The Rev.2 version will be supplied only with the Dick Smith Electronics kit. to fouling the dual gang volume control, which is why the corners are cut off the cover plate. We also used countersunk screws to keep the height to an a bsolute minimum. Finishing the case The on/off slider switch is located low down on the bottom right hand corner of the case, again to avoid fouling the components on the circuit board. The hole for the headphone socket should be drilled mid way along one edge of the case, at a height determined by measurement from the actual job. This end of the PCB can flex up and down quite a bit, allowing the headphone socket to align itself with the hole which should be a very snug fit. Use a Polystryrene Capacitors - 1 0pF to 1mF Voltage Range 63VDCW to 10,000 VDCW Tolerance - 0.25% to 10% Allied Capacitors Australia manufactures capacitors to the specification of the customers using high quality, imported polystyrene and aluminium foil with a tolerance of 1 micron . Our capacitors are manufactured to the system of Total Quality Control. We can provide Just In Time delivery if required, together with a Certificate of Conformance if requested. Specific values between 1 0pF and . 1 uF are no more expensive than standard value components. You can now design circuits to use a single capacitor rather than a number in parallel or series to achieve a desired value . Personalised labelling is available at no extra cost. Your component code or name can be built into the capacitor. Minimum runs of only 25 allow you to specify a particular value for a prototype run. Delivery lead time for short runs is 3 to 4 working days, and for longer runs is less than 2 weeks. We are also able to produce capacitor styles for applications such as dual mount, end filled and mini style capacitors. INTERESTED? Call us now on: (02) 938 4690 ALLIED CAPACITORS AUSTRALIA PO Box 740 Brookvale, NSW 2100 OCT0BER1989 65 =-..,,;11=1-.T,i -·· 1 : -: 1 7. "J ; 1 ii• :..1 -==~•11 -1,= ':!!!!!f:Z - 'i'=<at> = I.,•, ~ i)\\ ,soo 1-----0-----' l<HZ sharp knife to bevel the upper inside edge of the hole to help guide the socket into it on assembly. If the socket you are using doesn't reach comfortably through the case wall, counterbore the outside surface 0.5mm to 1mm. A 3/8-inch milling cutter will do the job nicely but note that the plastic case must be securely clamped in a drill press, or disaster may result. Final assembly WIDE-BAND AM STEREO® [ Trim label to inside of line - - ••,=r- --=~1-/:.-Jii 1E!f=.<at> -=:a:!E:r-:r:; ::::=. ~ ~ ~ - ~ ~ ., • • ,,. _,:.ilff. 7 I Ii « ,·- = ◊~~ ,~~~ \f;,\\\\ ,soo l<Hz~-------1 o 1-----0-----' l<Hz LOCl<ED ;STEREO: WIDE-BAND AM STEREO ® [ Trim label to inside of line Fig.11: alternative artworks for the front panel. The artwork at top shows the callsigns for the Sydney stations in addition to the kHz dial markings. An artwork showing the Melbourne station callsigns will be published next month. When wiring the battery connector to the switch and the switch to the PCB, keep the power leads as short as possible. The power leads should also be tightly twisted together. This is most important as even a small inductive loop here will couple power supply currents · back into the ferrite rod antenna and may result in oscillation. Finally, if the self-tapping screws supplied with the plastic case are of the thread cutting grooved type, look around for some that will do less damage to the plastic when they are repeatedly removed and replaced. The idea is to use a screw which will cut a proper thread. Even metal theaded machine screws will do. Screw them into the pillars warm, then heat them with your soldering iron until the plastic softens and shrinks around the screw threads. When cold they will come out easily. Next month, we'll conclude with the full alignment details. ~ The batteries must be mounted in the bottom left hand corner of the case (they won't fit anywhere else). Twist the power supply leads together as shown to avoid inductive loops which could radiate supply currents back to the iclntenna. 66 SILICON CHIP