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A synthesised stereo AM tuner, Pt.2 Despite the circuit complexity, our new Synthesised AM Stereo Tuner is easy to build. This month, we show you how to put it together. By JOHN CLARKE The AM Stereo Tuner is housed in a black 44mm high rack-mounting case. There are three PC boards for the tuner circuitry: a main PC board (code SC01101911 , 352 x 120mm), a display PC board (code SC01101912, 341 x 36mm), and a ferrite rod PC board (code SC0l 101913, 101 x 11mm). As can be seen from the photographs, the display PC board is soldered at right angles to the main PC board to give a neat and compact assembly. This technique greatly simplifies construction by eliminating inter-board wiring. The ferrite rod PC board is installed on an adjustable bracket on the rear panel and holds the ferrite rod assembly plus a few other smaller components, including 50 SILICON CHIP varicap tuning diode VCl. All the control switches, the indicator LEDs and the four 7-segment displays are mounted on the display board. With the exception of the 7segment displays, these all protrude through holes in the front panel and are flush-fitting to give a neat appearance. The 7-segment LED displays sit behind a bevelled grey persp ex window, while the front panel itself features gold screen-printed lettering. By the way, we recommend that you buy a complete kit of parts for this project rather than trying to buy the parts separately. That's because some of the parts are rather specialised and are not available from the usual retail outlets. Dick Smith Electronics will be offering a complete kit for around the $200 mark and this will include a pre-punched screenprinted front panel. Building it Before starting construction, make sure that you have the correct tools for the job. Most of the assembly is pretty straightforward but note that you will need a fine-tipped soldering iron when it comes to soldering in the microprocessor control chip (ICl) on the main PC board. Don't try using an iron that's too big for the job you'll damage the copper tracks on the board if you do. Before you start installing components, check the PC boards for shorts between tracks or breaks in the tracks. Also check that the holes have been drilled to the correct size and that all holes have been drilled. For example, the PC board mounting holes, regulator tab mounting holes and cable tie holes (on the ferrite rod PC board) all need to suit the mounting screws and cable ties supplied. Also, the transformer needs holes to accept its plastic mounting bushes as well as its connection pins. Finally, check that the Tako coils fit Left: the AM Stereo Tuner is built into a low-profile rack-mounting case with a pre-punched front panel & gold screen-printed labelling. The four 7segment LED displays sit behind a grey perspex window & provide the frequency readout. into the designated locations (1.2mm holes are required for their earthed case pins). Main PC board assembly Once the PC boards have been inspected , you can begin work on the main board. Fig.1 shows the details. Begin by installing all the PC stakes at the external wiring points (locations C-L, TP1 , TPZ & GND) on the top of the board. This done, install all the wire links , resistors and diodes. Because they are long and so close together, the wire links must be perfectly straight so that there are no shorts. You can straighten tinned copper wire by clamping one end in a vyce and then pulling it taut from the other end using a pair of pliers. Cut the links to length and bend their ends at right angles to form leads before mounting them on the PC board. Note that some of the resistors are 1 % types and these are marked with a star (*) on the wiring diagram (Fig.1). Use your digital multimeter to check the resistor values before installing them on the PC board. When installing the diodes, be sure to use the correct type at each location (check the parts list). Also check that each diode is correctly oriented. The ICs can now all be installed, again taking care to ensure that they are correctly oriented. IC1 is a surface-mount device and so requires special attention. Unlike the other ICs, this device is installed from the underside of the PC board. Check that it fits snugly into its rectangular cutout with its top almost flush with the top surface of the PC board, and that the leads all line up with the PC pads. The dot in one corner indicates pin 1, as shown on the overlay diagram (Fig.1). To solder the IC in place, the fine copper lands which form the solder pads for the IC leads have to be tinned with solder first. Do this with a clean fine-tipped soldering iron by applying the iron to the copper tracks and feeding on a small amount of solder. Note that if the PC board is supplied pre-tinned, you should still carry out this procedure as the tinning layer will be too thin. Once the pre-tinning operation is complete, clean the soldering iron using a damp rag or sponge to remove any excess solder and flux . This done, the IC can be placed in position and the leads soldered in place. The idea is to stroke each lead with the iron so that solder on both the lead and its underlying track melts to form a joint. You may need to push each lead down with a small screwdriver as you solder it. This simple PC board assembly holds most of the parts, including the power transformer & pushbutton switches. When installing the parts, keep the leads as short as possible & take care with the orientation of polarised components. MARCH 1991 51 orientation of the SFZ450C. It must be installed with the cross on the top of its plastic case as shown in Fig.1. You can now complete the main PC board assembly by installing the trimpots, coils and transformer. Once again, the coil types are listed in the parts list. The transformer is secured to the PC board both by soldering its pins and by burring over the two plastic bushes from the transformer body with a hot soldering iron. (We suggest you do this task last because your soldering will need a thorough cleaning afterwards). Display PC board This close-up view shows the mounting details for the microprocessor chip & 3-terminal regulator REG 2. The regulator can be bolted directly to the heatsink (ie, no isolating mica washer is required). The 7-segment LED displays & the pushbutton switches are mounted on Molex pins to stand them off the PC board. Be sure to orient the displays correctly. The three TO-220 regulators - REG 1, REG 2 & REG 4 - are all mounted horizontally and secured with a screw and nut. To keep it cool, the REG 2 device is fitted to a heatsink which mounts between the regulator tab and the PC board. Smear a small amount of heatsink compound on the mating surfaces to ensure good thermal contact (note: a mica insulating washer is not necessary). REG 3 is a smaller TO-92 plastic encapsulated device. It can now be mounted, along with the transistors 52 SILICON CHIP and varicap diodes. Once again, be sure to use the correct transistor or varicap diode at each location (check the parts list) and keep their leads short by pushing them down onto the PC board as far as they will comfortably go. The capacitors can now all be installed, along with the trimmer capacitors (Cl & CZ), the ceramic filters and the crystal (X1). The SFP450D ceramic filter can only be installed one way because of its non-symmetrical pinout, but take care with the Fig.1 also shows the parts mounting details for the display PC board. Begin construction by installing the wire links, resistors and transistors, but don't mount the LEDs at this stage. The four 7-segment LED displays and the 10 switches are all mounted on Molex pins so that they stand proud of the PC board. In the case of the switches, it's best to break the Molex pins off the strip first and push them onto the switch leads. The switches can then all be installed on the PC board (flat side as shown in Fig.1) and the pins soldered. To install the 7-segment LED displays, first install the Molex pins in strips of five . Once they are all in place, snap off the connecting strips and plug in the displays. The decimal point for each display goes towards the centre of the display board (ie, the bottom right hand corner of each display when looking at the display board the right way up). Be careful here - it's very easy to get confused. Finally, the LEDs can all be installed and held in place by splaying their leads. Don't solder the leads yet, though - that step comes later, whe_n we fit the PC board assembly in the chassis. Ferrite rod PC board Fig.2 shows the parts layout on the ferrite rod PC board. Note that the ferrite rod is secured to the PC board using two cable ties. Similarly, the external leads from the PC board are tied in place using a cable tie which passes through a hole adjacent to the 100kQ resistor. The end hole is used for mounting the PC board on a right angle bracket which is secured to the rear of the case. ~ ~ ~ :LED2 ~ ~ IC6 LM324 -{!&•LED3 --1:Iill- : LED4 . •A POWER TRANSFORMER -{JD-o~ --{TID-- :LEDS • LED6 • A 3.3k -- j •• I DD1 . ~1~ ~ . K • -<at>[lo •B w ~w 5~ :,:.. :a = Cl) <I) <I) oc i::, oc "" 0 .c u~ i::, 0 "' Cl) :::: .s"'"' Cl) .d N --- 0. oh ~ SEEK -- +Q 100uF ~ A'' l!)LED13 ~ME K --. 0 0. 0. A [;jLED7 1 K A (!}LEOS 2 K A l;jLED9 3 K (3)• 4 0. 5 A . ~LED10 K A l;jLED11 K A c. ~LEDJ2 K (3)• DOWN 0• UP Fig.I (left): parts layout for the display PC board & the main PC board. Take care with the orientation of the LED displays & note that they & the pushbutton switches are mounted on Molex pins. Use PC stakes at external wiring points C-L, TPl, TP2 & GND on the main board. MARCH 1991 53 (leads towards the centre) and secured by wedging a thin strip of cardboard between the rod and the cardboard coil former. The four leads can then be fed into their respective holes in the PC board and soldered (note: leave the leads at full length so that the coil can be slid along the ferrite rod during the alignment procedure). Finally, the ferrite rod can be strapped in place using the cable ties - see Fig.2. Metalwork The ferrite rod is secured to its PC board using plastic cable ties, while a third cable tie secures the shielded cable runs at the other end of the board. A rightangle bracket is used to fasten the assembly to the back of the chassis. The first step is to mount the two capacitors. These should lie flat on the PC board, so that the lO0kQ resistor and VC1 can be mounted on top of them as shown (ie, bend VCl's leads at right angles so that the flat of its body lies on the 4.7pF capacitor). As supplied, Ll has about 110 turns on the main coil (the plain and dark blue wire ends). Unfortunately, this number of turns gives too much inductance to allow varicap diode VC1 to tune the entire broadcast band, which means that some turns have to be removed. To do this, cut the plain wire where it runs under the main winding and take off 27 turns (the lead running under the winding can be pulled out and discarded). The new end (when trimmed) now becomes the plain wire end. Before going further though, run some wax over the end of the coil to prevent further unwinding, then run the plain wire lead back down the coil (at right angles to the windings) so that it emerges adjacent to the other three leads. This done, the coil should be slipped over the end of the ferrite rod This close-up view on the copper side of the main board shows how the microprocessor is mounted. Use a soldering iron with a very fine point to avoid overheating & damaging the tracks (see text) 54 SILICON CHIP Although the kit will be supplied with a pre-punched front panel, there is still some work to be done on the metalwork. This involves drilling the holes in the rear panel and front subpanel. The first job is to mark out the hole positions for the LEDs, switches and displays on the front sub-panel. This can be done by using the pre-punched front panel as a marking template. Once the holes have been marked out, they can be drilled (and filed) oversize to provide clearance for the front panel components. Don't bother trying to cut square holes to accept the LEDs and switches; it's much easier to drill a large round hole with plenty of clearance. These holes will later be covered up when the front panel is screwed in place. The cutouts for the signal level display and power switch can be made by first drilling a series of holes and then filing to shape. Similarly, the cutout for the 7-segment displays can Be sure to install the mains wiring in a professional manner & use heatshrink tubing to shroud the fuseholder. The paint around the earth solder lug mounting hole should be scraped away to ensure good electrical contact. RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 No . Value 4-Band Code (5%) 5-Band Code (1%) 4 11 1 2 2 7 6 2 4 2 1 6 1.8MQ 1MQ 330kQ 180kQ 150kQ 100kQ 68kQ 47kQ 33kQ 18kQ 15kQ 10kQ 8.2kQ 4.7kQ 3.3kQ 2.7kQ 2.2kQ 1.8kQ 1kQ 680Q 220Q 120Q 100Q 47Q 10Q brown grey green gold brown black green gold not applicable brown grey yellow gold brown green yellow gold brown black yellow gold blue grey orange gold yellow violet orange gold orange orange orange gold brown grey orange gold brown green orange gold brown black orange gold not applicable yellow violet red gold orange orange red gold red violet red gold red red red gold brown grey red gold brown black red gold blue grey brown gold red red brown gold brown red brown gold brown black brown gold yellow violet black gold brown black black gold brown grey black yellow brown brown black black yellow brown orange orange black orange brown brown grey black orange brown brown green black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown orange orange black red brown brown grey black red brown brown green black red brown brown black black red brown grey red black brown brown yellow violet black brown brown orange orange black brown brown red violet black brown brown red red black brown brown brown grey black brown brown brown black black brown brown blue grey black black brown red red black black brown brown red black black brown brown black black black brown yellow violet black gold brown brown black black gold brown 5 7 1 2 1 9 6 2 2 7 4 be made by drilling a series of small holes around the perimeter of the marked area and then punching the centre-piece out and filing to a smooth finish. Once this job has been done, take a look at the rear of the sub-panel. You will find six captured nuts - two at either end and two in the middle. The rear edges of the two in the middle and the two near the tuning switches must now be fil ed flat so that the back of the display PC board can sit flush with the rear lip of th e sub-panel. Don't file the sides of the captured nuts too much though, or you could damage the threads. Fig.3 shows the layout on the rear panel. You will need to drill holes to accept the cord grip grommet, th e fuse, a mounting bracket for the ferrite rod PC board, and a small rubber grommet. In addition, you will have to make a square cutout and drill two holes to mount the RCA socket panel. Once this has been done, the case can be partially assembled by screwing the two side pieces and the rear panel to the baseplate. The fro nt subpanel and fro nt panel can then be fitted. Connecting the PC boards The next step is to connect the main and display PC boards by soldering them together at right angles. To do this , first solder five 5mm-long brass spacers to the mounting points on the copper side of the main PC board. This done, position the display board behind the front panel so that the switches and LED displays all line up correctly, th en slide the main board up to the display board and mark the intersection at either end with a pencil. Now remove the two boards and lightly solder tack them together at either end. Test the assembly by refitting it to the case, make any adjustments necessary, then solder all the CAPACITOR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Value IEC Code EIA Code 0.1 µF .068µF .047µF .01µF .0012µF .001µF 470pF 270pF 120pF 100pF 47pF 22pF 4.7pF 100n 68n 47n 10n 1n2 1n 470p 270p 120p 100p 47p 22p 4p7 104 683 473 103 122 102 471 271 121 101 47 22 4.7 matching pads together. On the prototype, we also soldered two 30mm lengths of 2mm diameter brass tubing across each end of the PC board asM A RCH 1991 55 CORO GRIP GROMMET REAR PANEL ciT~B~ EARTH •- )'4\-c F G H I J K L MAIN BOARD Fig.3: use this chassis wiring diagram to guide you in positioning the components on the rear panel & check that the cord grip grommet grips the mains cord tightly. The power switch (S1) operates at low voltage, so you can use medium-duty hookup wire here. sembly to provid e additional strength (see photos). You can use triangular pieces of scrap PC board or tinplate as an alternativ e to th e brass tubing. Final assembly At this stage , you can reposition the PC board assembly in the case and mark and drill the five mounting hole positions. You also need to drill holes to accept two earth lug mounting screws as shown in Fig.3. Before mounting the earth lugs, scrape away some of th e paint from around the holes to ensure a good electrical contact. The earth lugs can then be secured using a machine screw, star washer and nut. Because the transformer is mounted on the PC board, a pi ece of Elephantide insulation (352 x 120mm) is installed betw een th e bottom of th e main board and the chassis to prevent accid ental shorts. This can be secured by drilling holes through it at the mounting points so that it slips over the 5mm space rs on the bottom of the board. 56 SILICON CHIP Do not mount the PC board assembly in position yet, since the mains wiring still has to be conn ected to the underside of the main board . Before installing the mains wiring however, the ferrite rod PC board should be mounted in position on the rear of the case. First, bolt the rightangl e bracket to the cas e using a machine screw, washer and nut. This done, the PC board can be attached to the bracket using a machin e screw, two washers (one on either side of the PC board), and two nuts. Th e proc edure here is to tighten th e first nut so that the PC board can be still be moved smoothly under friction , then the second nut is tightened onto the first to prevent it from coming loose. The final finishing touch for the ferrite rod PC board assembly cannot be done yet since we need to align the tuner by sliding coil L1 along the ferrit e rod . Following alignm ent , L1 can be held in position with wax and a 140mm length of heatshrink tubing shrunk over the entire ass embly to prevent acc idental damage. The remain der of the rear panel components can now be mounted as shown in Fig.3. Before installing the cord grip grommet, first strip back about 130mm of outer insulation from the mains cord , then feed the cord through th e entry hole. It can now be clamped in position with the cord grip grommet. Check that the cord is h eld tightl y and that it cannot be pulled out of the grommet. The mains wiring can now be run to the fus eholder, earth lug and main PC board. To prevent accidental electric shock, w e strongly recommend that h eatshrink tubing be fitted to the fusehold er to insulate the two terminals. This invol ves slipping a piece of heatshrink tubing over the two leads befo re th ey are soldered to the fusehold er. Th e Neutral and fused Active leads connect to the transformer terminals after first passing through two hol es in th e PC board. Once these connections have been made , you can install the PC board assembly in the case and secure it using machine screws and nuts. Finally, the rest of the wiring can be install ed , along with the 0.1µF capacitor from point C to earth. Be sure to use a cable tie to clamp the shielded cable to the ferrite rod PC board as shown in Fig.2. Note that the power switch operates at low volt- age, so it is only necessary to use medium-duty hookup wire here. That completes the construction. Next month , we will conclude with the alignment details. SC These two photographs show how the main PC board assembly is mounted. It should be positioned so that the back of the display board is flush with the edge of the lip on the sub-panel. This will involve filing four captured nuts, as explained in the text. MARCH 1991 57