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CAR STEREO IN YOUR HOME Here's how to adapt a surplus car radio-cassette player for use in your home. By GREG SWAIN "Those old valve sets sure had a pretty good sound - much better than you get from transistor radios". When was the last time you heard that sort of statement? But don't dismiss it as mere nostalgia. Those old valve sets really did have the edge over the familiar "tranny", at least in terms of sound quality. The reasons are not hard to find. Compared to modern transistor radios, those old valve sets boasted greater power output, better bandwidth (ie, they reproduced a wider range of audio Jrequencies), better AGC, and lower overall noise levels. They also usually had a decent loudspeaker to reproduce the sound. Take a look in the back of one of those old valve radios and you'll in54 SILICON CHIP variably notice a big loudspeaker, cften 15 or 20cm in diameter (6 or 8-inches in the old terminology). What's more, because the chassis was quite large, the cabinet was also quite substantial and could provide a fair amount of baffling for the speaker. The signal-handling capabilities of valve sets was better, too. They could comfortably handle very high input signals without overload or cross-modulation and had much better automatic gain control (AGC) circuits to cope with the wide range of signal strength variations. These days , bandwidths are much narrower, the baffling is woeful, and the miniature speakers used sound dreadful. Add to that the overload and distortion problems inherent in lower-power audio output stages and it's not hard to see why modern table radios don't measure up to the valve sets of yesteryear. The fact is, you can no longer buy a decent table-type radio. All you can get is ghetto-blaster style radio/cassette players which can be quite expensive, or cheaper clock/radios which can sound really crook. But there is an alternative: adapt a car radio to run off the 240V mains for in-home use. Car radio conversion This is such a ripper of an idea, you'll wonder why it hasn't been done before. As we'll show you in this article, converting a car radio to the mains is quick and easy to do. What's more, you can spend as much or as little as you like. What are the advantages of car radio conversion? First, car radios Choosing your radio D7 MEMORY POWER +l 240VAC 4700 e----.. + 13 4V ~_J ~ - - - - - - - - - ~ - 1 - 6 V _ W_ _ _ - - - - - E A R T H TO CASE OF RECEIVER CAR RADIO/CASSETTE POWER SUPPLY 011·1287 Fig. 1 .;~ I GND Fig.1 - the power supply circuit. The output of the transformer is rectified by diodes D1-D4 and fed to a 7812 3-terminal regulator IC. D5 and D6 jack up the GND terminal by 1.4V so that the supply delivers 13.4V to the radio. offer much better station-pulling power and sound quality than virtually any currently available domestic AM/FM radios, regardless of price. Because they are designed for mobile use, car radios have much better sensitivity and selectivity, and they generally have better bandwidth as well. Second, car radios have desirable extra features not usually found on domestic radios. Many have pushbutton station selection, which is a great convenience feature, while upmarket models feature synthesised tuning, digital frequency display and memory storage of favourite stations. They often also have an inbuilt cassette player, bass and treble controls, and a power output of several watts per channel or more. Finally, car radios are very rugged and are well shielded which reduces interference. A car radio can thus form the basis for a very good sound system. All you have to do is add a mains power supply, two loudspeakers and a suitable cabinet. The old adage that "you get what you pay for" holds true here. You can either take the low-cost way out with a no-frills $25 radio, or you can buy something with a cassette player and a few other fancy features. As a matter of interest, we took a quick flick through the Altronics and Dick Smith Electronics catalogs. Both offer a complete AM/FM-stereo radio-cassette player for $79.95, but you can also buy fully synthesised models with electronic tuning and AM stereo starting at around the $350 mark. Tandy Electronics also carries a wide range of models, with features and prices to suit every budget. Of particular interest is a new synthesised model that sells for just $250. They also have a $400 model with a "logic-controlled" cassette deck, Dolby B-C noise reduction and 15 watts per channel output. Of course, you don't have to buy. You may already have a surplus car radio stashed away in the garage or under the workbench. If so, then the price is right and it's the ideal candidate for conversion to mains power. If you don't already have a radio but want to save money, try a local auto wrecker. They often have car radios available for a few dollars but make sure that the model you choose is still in working order. A radio that's been pulled from a wreck may have been damaged in some way. We decided to go the whole hog and convert a Pioneer KE433AM AM/FM radio-cassette player with synthesised tuning, 18-sta tion memory, AM stereo, and 4 watts per channel power output. This is an excellent radio, although unfortunately no longer part of the Pioneer range. The power supply This view shows the assembled power supply PCB, prior to installation in the radio. Note that the four 1N5404 diodes used in the bridge rectifier are larger than the 1N4002 types (don't confuse these). The leads of the 3-terminal regulator are bent so that the device can be bolted to a heatsink which lies flat on the PCB. Take a look now at Fig.1. This shows the circuit of the power supply. As you can see, the circuitry is very simple. A transformer with a 15V secondary feeds a conventional bridge rectifier consisting of diodes D1-D4. The output of the bridge is then filtered by a 4700µF NOVEMBER1987 55 PARTS LIST 1 car radio (see text) 1 timber cabinet, see text 1 Altronics 21 56 1 5V 2A mains transformer 2 car radio speakers, Jaycar Cat.AS3014 or equivalent 1 printed circuit board, code sc011-1287, 109 x 54mm 1 3-pin mains plug and 3-core flex 1 2-way mains terminal block 1 solder lug 1 clamp for mains cord 1 3 .5mm banana plug 1 U-shaped aluminium heatsink, 27 x 25 x 34mm-high, Jaycar Cat.HH-8508 or equivalent Semiconductors 1 7812 12V 3-terminal regulator 4 1 N5404 3A diodes 3 1 N4002 1 A diodes Capacitors 1 4 700µF 25VW PC-mounting electrolytic 1 4 700µF 16VW PC-mounting electrolytic Miscellaneous Insulated hook-up wire, selftapping screws, wood screws, rubber feet for cabinet, grill coth for baffle. electrolytic capacitor to give a smoothed DC voltage of about 20V. Voltage regulation is provided by a 3-terminal 12V regulator (7812). In this case, however, the GND connection of the regulator is jacked up by about 1.4V by series connected diodes D5 and D6. As a result, the supply delivers a 13.4V output to the radio. The inclusion of D5 and D6 may be seen as " gilding the lily" somewhat. After all, the radio will work quite happily at 12V, so why bother? Our reasons are that the diodes are cheap (about 10c each) and the extra 1.4V improves the available power output from the inbuilt audio amplifiers of the car radio and also improves its RF (radio frequency) sensitivity. D5 and D6 also reduce the power dissipation in the 7812 by lowering the voltage between the IN and OUT terminals. A second 4700µF electrolytic capacitor filters the output from the 7812 and provides high short-term current capability when required. Diode D7 is included as a safety measure and protects the output of the 7812 against connection to external voltages (eg, charged capacitors). Strictly speaking, D7 is not required here as the supply will be permanently connected to the radio. We've included it for those readers who want to build the supply as a free-standing unit for use in other applications. It is necessary to fit a small heatsink to the 7812 regulator to obtain the required current rating. This is because the 7812 includes internal thermal overload protection. Our supply delivers about lA on a continuous basis and 1.5A on a shortterm basis, which should be enough for just about any car radio, even high-powered units. PCB assembly All the parts, except for the power transformer, are mounted on a small printed circuit board coded sc0l 1-1287 (110 x 55mm). Fig.2 shows the parts layout diagram. The parts can be mounted in any order but be sure to install the diodes the right way round. Note that 1N5404 diodes (rated at three amps) are specified in the bridge rectifier (D1-D4) because a high surge current rating is required (by virtue of the 4700µF 25VW capacitor). The two 4700µF capacitors must be oriented correctly. Note that the capacitor adjacent to the bridge rectifier diodes must be rated at 25VW. The other capacitor on the regulator output can have a minimum voltage rating of 16VW. Don't use a 16VW capacitor at the output of the bridge rectifier - its voltage rating will be inadequate. Finally, install the 3-terminal regulator by bending its leads at right angles so that they fit the 13.4 V TO RECBVER MAINS CORO POWER TRANSFORMER MAINS CORO CLAMP Fig. 2 ~ E A R T H TO RECEIVER CASE Fig.2 - here is the complete wiring diagram for the power supply. The active (brown) and neutral (blue) leads from the mains cord are connected to a 2-way terminal block, while the earth wire (green/yellow) is connected to a solder lug secured by one of the transformer mounting screws. A second solder lug is used to earth the case of the receiver. 56 SILICON CHIP There's plenty of room in the cabinet for the radio, two loudspeakers, and the power supply components. We used standard oval car speakers (available from Jaycar at $10.95 each, Cat. No. AS-3014). Note the long-wire antenna and the rightangle bracket at the rear of the radio. Take care with the mains wiring. p~~%~R !l SPEAKER EAR14 II QJ I LEFT SPEAKER POWER AMP R RIGHT sm~~-Ri-- - - - - i . . . J I \ I SPEAKER . RADIO 0 ~ - r - - - - O T O POWER POWER \ ANTENNA POWER9"_ _ _ _ +V • POWERO.__ ___,_ l INPUT MEMORY 0---------.0-M~QRY .--------v -!- (a) :oA~IEOROi---r----OTO POWER \ ANTENNA Rg. 3 • 1tii>~T MEMORYo------ME!~RY , - - - - - - - • -V -!- (b) Fig.3 - the output connections for typical car radios. In (a), the speaker outputs are single-ended. Bridging outputs, like those shown in (b), are often found on high-power radios. holes in the PCB. The regulator can then be bolted to the PCB with its heatsink, as shown in the photographs. The cabinet One of the best things about this project is that you can build your own cabinet. We made our cabinet from dressed radiata pine which is relatively cheap and much easier to work than particleboard. Rather than go for tricky mitre joints, we elected to use butt joints as these are much easier to make. Fig.5 shows the dimensions of our cabinet. Cut the timber to the dimensions shown, then bevel the edges using a plane or sanding block. The panels can then be sand- ed to a smooth finish, ready for assembly. The baffle (front panel) board is secured using cleats. These must be inset from the front edge according to the thickness of the baffle and are either glued or screwed in position. Once the cleats have been installed, the cabinet is ready for final assembly. It's up to you how you secure the various panels. We simply glued them in position using "Aquadhere" woodworking glue. Four large G-clamps were used to hold the cabinet together while the glue was drying. After that, the cabinet was given a coat of walnut stain (British Paints Walnut Stain) and three coats of "Feast Watson" satin silk timber finish. Incidentally, if you are using radiata pine for the cabinet, be very careful in handling the timber. Because it is soft, it is easily marked and grubby finger marks can be a problem too. Be careful also not to get excess glue on what will be the visible surfaces of the cabinet. If all traces of glue are not thoroughly removed they will show up as white spots when the stain is applied. It doesn't particularly matter what you use for the rear panel. We used a piece of perforated Masonite we had on hand. The baffle board can be made from scrap NOVEMBER 1987 57 ANTENNA 100cm Of WIRE FADER ANTENNA PLUG .__---;ANTENNA IN LEFT SPEAKER SPEAKERlt--....---t...J\I EARTH RIGHT SPEAKER +Vt-------, CASE CLOCK/MEMORY -v S1 POWER -v POWER SUPPLY N 240VAC A MEMORY +v +vi----__, Fig. 4 Fig.4 - the final wiring hookup for radios with single-ended outputs and front/back fader controls. If your radio uses bridging outputs, connect the speakers as shown in Fig.lb. timber since it will later be covered by speaker grill cloth. It should be at least 12mm thick to provide the necessary degree of rigidity. Before mounting the baffle board, it will be necessary to make cutouts to suit your loudspeakers and radio. The baffle can then be covered with a suitable fabric. We recommend the use of either an acrylic scrim material (looks like hessian) or a stretch fabric. The fabric should be pulled tight over the baffle and stapled in place. The radio and speakers can now be mounted and the baffle fastened to the cleats using screws installed from inside the ea binet. We suggest the use of standard oval car radio speakers which can be either 150 x 100mm (6" x 4") or 175 x 125mm (7" x 5"). These have good sensitivity and will work quite well without any fancy baffling in the form of sealed or bass reflex cabinetry. Don't use high quality speakers. They are usually far less sensitive and require proper sealed or bass reflex cabinets if they are to sound any good. Hooking up the radio Connecting the radio to the loudspeakers, the power supply SC 0 0 011-1287 IT: 0 0 Fig.6 58 here is a full size reproduction of the printed circuit board. SILICON CHIP and an antenna is a fairly easy task. Usually, it will be possible to identify the various leads from the manual, or from a chart pasted to the top or bottom of the radio. If you don't know what the leads are, you'll have to do some detective work (see panel). Fig.4 shows the hookup for radios with inbuilt front/back fader controls which are normally used with four loudspeakers. Connect the front and back outputs together as shown. If your radio is a higher powered unit with bridging outputs, connect the speakers as shown in Fig.3b. On radios without fader controls, the power amplifier outputs are connected direct to the loudspeakers. Note that power switch S1 can be regarded as optional. If your car radio has an inbuilt digital clock or station memory, the switch should be omitted and the radio turned on and off using its own power switch. The power transformer should be mounted in the cabinet with the mains terminals innermost. Before doing this, solder two short lengths of mains-rated cable to the 240VAC terminals, and sleeve the terminals with plastic tubing . The transformer can then be screwed to the side of the cabinet using selftapping screws. Install an earth solder lug under one of the mounting screws. This should be secured with a lockwasher, to ensure a positive connection. The primary leads from the transformer are connected to a two-way mains terminal block. This should be screwed to one of the front-panel cleats. Make sure that the mains cord is securely clamped and terminate the active (brown) and neutral (blue) leads as shown in Fig.2. The earth wire (green/yellow) is soldered to the solder lug secured by one of the transformer mounting screws. The idea to keep in mind when doing the mains wiring is to make sure that it is not possible for anyone to come into contact with exposed active wiring, even if their hands are probing where they shouldn't be. Note that the metal case of the radio must be connected to mains How to identify the connections Let's say that you've acquired a radio with the leads chopped off a few centimetres from the chassis and don't know which is which. Here's how to sort them out using a multimeter. Fig.3 shows the connection details for typical car radios. First, use your multimeter to identify the ground connection - this will usually be a black wire. The main + 1 2V lead will usually be red. To check this, connect your multimeter between this lead and ground and turn the power switch on and off. If the resistance drops each time the switch is turned on, \ .. ~ 375 ALL DIMENSIONS IN MILLIMETRES A = CUT-OUT AND ORILL HOLES TO SUIT RECEIVER USED 8 = CUT-OUT FOR you've found the power connection. When you find the power lead, leave the power switch on and leave the probe connected to it. Now touch the other probe to the remaining leads in turn. The one that gives a zero reading is the hookup for a power antenna. On electronically tuned radios, there will be an extra power lead that supplies the preset station memories and, perhaps, a clock, even when the radio is turned off. To find this lead connect one of the meter probes to ground and touch the other to the remaining ' ~ 150x100mm SPEAKERS FRONT HELO BY SCREWS THROUGH CLEATS FROM INSIDE MATERIAL:- TOP, BOTTOM AND SIDES 20x190mm DAR RADIATA PINE. FRONT 568x142x12mm THICK RAOIATA PINE. Fig.5 - the cabinet dimensions. We mounted the loudspeakers inside the cabinet but you may prefer to build a smaller cabinet and use separate speaker boxes. earth. This means that you must have a wire running from the radio case to the transformer mounting foot, as shown in the wiring diagram. If this wire is omitted, the radio will be completely unuseable on the AM band, due to a very loud rectifier buzz. Note also that the mains wiring and power supply should be mounted as far away as possible from the antenna socket of the radio. This is to minimise mains interference. The next step is to assemble and install a simple antenna . Cut a 1-metre length of stranded hookup wire and solder it to a standard 3.5mm banana plug. The antenna can then be plugged into the radio. unidentified leads. The memory power lead is the one that starts at a low resistance value and rises to a high value. Disregard any connection that continuously gives a low value, or that starts at a low value and rises to infinity. The remaining leads will be the speaker connections. If your radio has a fader control, use your multimeter to find which pairs give a reading of about 40 ohms. Usually, the lead for the other output in a bridging configuration will be coded the same colour, but will have a stripe added. On the prototype, the antenna was stapled to the inside of the cabinet . In most situations, however, it will be better to simply drape the cable out the back but away from the mains cord. Now for the big moment. Disconnect all the in-line fuses to the radio, so that no power can be applied to it. Apply mains power and check that the DC voltage from the power supply is close to 13.4 volts. A voltage within 0.5V of 13.4V can be consider ed OK. If this checks out, switch off, reconnect the in-line fuses and reapply power. Now turn the radio on with its own on/off switch. There · should be a soft thump from the speakers and you should be able to tune stations in the normal way. If your radio is manually tuned, it will be necessary to peak the antenna trimmer for best AM reception. This adjustment will normally be found next to the antenna jack, although it can also be situated on the front of the case behind the dress panel. To make the adjustment, tune in a weak station near 1400kHz, then peak the trimmer for maximum volume. The final step is to fit the rear panel. That's it - your new table radio is now ready for use. Enjoy the sound. Our first version was so successful that one of our staff members decided he had to have one of his own. It NOVEMBER1987 59