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A F lea - P o w er The A M Broad Little Jim AM Transmitter Dubbed “Little Jim”, this low-power “baby” AM radio transmitter has a range of just a few metres, to keep it nice and legal. It’s ideal for sending the output of your MP3 player or personal CD player to your car radio, or for feeding recordings of vintage radio shows to vintage radios, so they sound really authentic. By JIM ROWE W HY WOULD YOU want a broadcast band AM transmitter with a power output so low that it can only be received inside a radius of about four metres? Well, let’s say you’ve just finished building a replica of a classic 1940s’ era AM radio, which you’re entering into a club competition. Wouldn’t it be great if you could tune it into an “authentic” old time radio 32  Silicon Chip program, to recreate the way it might have sounded back then? With this little transmitter you’ll be able to do just that, by rebroadcasting historic radio programs like those available on CD from Screensound Australia (see sidebar). Alternatively, you might want to play the music from your personal MP3 or CD player through your car radio when you’re driving to and from work – but the radio lacks direct audio inputs. With this little transmitter, that’s no problem – although you will need to modify it slightly so that it runs from the 12V car battery. In short, the whole idea of this project is allow any line-level audio signal to modulate an RF carrier in the AM broadcast band, so that it can be siliconchip.com.au Un i t F or c as t Ban d Fig.1: this is the block diagram for “Little Jim”. A tunable RF oscillator sets the carrier frequency and this is them amplitude-modulated by the audio signal. The modulator’s output is then amplified and fed to an antenna. transmitter uses just a handful of parts (including two transistors and the modulator IC) and fits inside a standard UB3 sized plastic jiffy box. It’s also low in cost and easy to build, as all the parts fit on a small PC board. And it’s run from either a plugpack power supply or a 12V battery, so safety isn’t a problem, even for beginners. How it works played through a nearby conventional AM radio. The carrier frequency of the transmitter can be tuned to virtually anywhere in the lower half of the broadcast band – ie, from 550kHz to about 980kHz. This allows you to choose a frequency that’s away from any of the broadcasting stations operating in your area, to ensure interference-free reception. The audio quality from the transmitter’s signal is very close to that of the program material you feed into it, because it uses a special balanced modulator IC. There’s also a modulation level control, so you can easily adjust the transmitter for the best balance between audio volume and minimum distortion. But the best part is that the whole siliconchip.com.au Although it’s extremely simple and designed for very low output power, “Little Jim” uses exactly the same “building blocks” as a full-sized AM radio transmitter. Fig.1 shows the details – it consists of an RF (radio frequency) oscillator, a modulator and an RF output amplifier or “buffer”. The job of the RF oscillator is to generate a continuous sinewave signal of constant amplitude and with a frequency in the range from 5501650kHz – ie, in the AM broadcast band. This provides the transmitter’s “RF carrier”, which is the frequency you tune to with your AM radio to receive the signal. In most full-size AM transmitters, the RF oscillator uses a quartz crystal and is fixed in frequency, so the station concerned is always found at exactly the same place on your radio’s tuning dial. However, in this case, the oscillator is tunable over the range from 550kHz to about 980kHz, so you can set the transmitter’s frequency to a part of the band that’s currently unoccupied in your area, for clear reception. The signal produced by the RF oscillator is fed into the modulator, which is the “heart” of the transmitter. As shown in Fig.1, this also receives the audio signals from your MP3 or CD player. In this case, the stereo signals from the player are fed in via a simple mixing circuit, to convert the signals into mono. The resulting mono signal is then fed to the modulator via a modulation (volume) control, which sets the modulation level. In operation, the modulator uses the audio signal to vary the amplitude of the RF signal (ie, it varies the amplitude of the carrier). When the audio signal swings positive, the amplitude Keeping It Legal The AM transmitter described in this article has very low RF power output (a tiny fraction of a watt) and is designed to have a range of no more than about four metres. Do not attempt to modify the circuit with the aim of increasing its power output or to increase its range by feeding its output into a much larger antenna, because this would greatly increase the risk of interfering with the reception of licensed broadcasting stations. It would also make you liable to prosecution by the broadcasting and spectrum management authorities and probable confiscation of your equipment as well. January 2006  33 Fig.2: the final circuit uses a Colpitts oscillator based on transistor Q1 to generate the carrier frequency which is then modulated by the audio signal fed into pin 1 of IC1 (MC1496). The modulated RF signal is then amplified by common-emitter amplifier stage Q2 and fed to the antenna. Potentiometer VR2 sets the modulation depth. of the carrier is increased and when it swings negative, the carrier’s amplitude is reduced. As a result the RF output signal from the modulator is varied up and down in amplitude, directly in step with the audio signals. In other words, the RF carrier is “amplitude modulated”. The waveforms in Fig.1 show the basic idea. Amplitude modulation or “AM” is just one way of using an RF signal to carry audio or other kinds of information from one place to another. Another approach is to frequency modulate the carrier and this transmission standard is called “FM” (for frequency modulation). The amplitude-modulated RF output from the modulator is very weak, so before it can be fed to our transmitting antenna (which is just a short length of wire) we have to increase its level slightly by passing it through the third building block: the RF buffer amplifier. This stage amplifies the modulated RF signal to 34  Silicon Chip a level that’s just high enough to cause weak radio signals to be radiated from the antenna. Circuit details OK, so that’s the basic operational details of the transmitter. Now let’s take a look at the circuit diagram – see Fig.2 The RF oscillator (which generates the carrier signal) is a simple Colpitts circuit, based on transistor Q1. This uses the primary winding of RF transformer T1 as the inductive arm of its resonant circuit, along with fixed 470pF and 100pF capacitors and a miniature tuning capacitor (VC1). T1 is a miniature local oscillator coil from a low-cost AM receiver coil kit. The output of the oscillator is taken from the secondary winding of T1. This is then fed through a 4.7nF DC blocking capacitor and a series 10kW resistor to one of the two carrier inputs (pin 10) of IC1, an MC1496 balanced modulator specially designed for this kind of use. The second carrier input of IC1 is pin 8 and is tied to ground potential as far as RF signals are concerned using a 10nF capacitor. However, the IC needs both its carrier inputs held at a DC bias level of about +6V and that’s the purpose of the voltage divider network involving the 1.5kW, 560W and 1kW resistors between +12V and ground. The 1kW resistor between pins 8 and 10 ensures that both carrier inputs are biased at the +6V level. It also forms a voltage divider with the 10kW resistor from T1, to reduce the unmodulated carrier level at IC1’s inputs to below 60mV RMS – the maximum level which can be applied to its carrier inputs for undistorted output. IC1’s audio modulating signal inputs are at pins 1 and 4 and these have to be biased lower than the carrier inputs, to about +4V DC. This voltage is provided across the lowest 1kW resistor in the main bias divider and fed to the two audio inputs (pins 1 & 4) via two 1.5kW resistors. In addition, siliconchip.com.au the audio inputs are connected via 10kW resistors to trimpot VR1, which allows fine adjustment of their relative bias – and hence the modulator IC’s operation. The stereo audio input signal is fed into the unit via jack socket CON2 and mixed together via two 10kW resistors to form a mono signal. This signal is then fed to modulation depth control VR2. In addition, two 10kW resistors have been connected between the audio inputs of CON2 and ground. These are used to provide suitable loads for your CD or MP3 player line/headphone outputs. If your particular player needs loads of less than 10kW, these two resistors can be reduced in value. As shown in Fig.2, the modulating signal from VR2 is fed to just one of the modulator’s audio input pins – in this case, to pin 1 via a 4.7mF DC blocking capacitor. The second input (pin 4) is tied to ground via a 100mF capacitor, so the full audio (AC) voltage from VR2 is effectively applied between the two input pins. The 1kW resistor connected between pins 2 & 3 of IC1 is used to set the internal gain of the modulator, while the 10kW resistor from pin 5 to +12V sets the IC’s internal bias and operating current level. or four metres, despite its very low RF power output. Modulated carrier outputs Construction The modulated carrier outputs from IC1 appear at pins 6 & 12, which are both connected to the +12V rail via 3.3kW load resistors. In this circuit, we only use the output from pin 12 and this drives the base of RF amplifier transistor Q2 via a 12kW resistor. Transistor Q2 is connected as a simple common-emitter amplifier stage, with an unbypassed emitter resistor to ensure low gain and stability. Its amplified output is developed across the collector load formed by L1, a broadcast-band antenna coil wound on a very small ferrite rod. As well as forming Q2’s collector load, L1 actually forms part of the transmitter’s antenna, because the ferrite rod inevitably radiates some RF energy. However, its very small size makes it a rather poor radiator, so an external wire antenna (about two metres long) is also connected to Q2’s collector via a 10nF coupling capacitor. This “dual antenna” system gives the transmitter a range of about three Construction is easy, with all the parts mounted on a small PC board measuring 122 x 57.5mm. This board has cutouts in each corner, so it can fit snugly inside a standard UB3 size jiffy box. Note that there are actually two slightly different versions of the PC board, to suit the two different 3.5mm stereo jacks sold by kit suppliers. The board coded 06101061 suits the jack sold by Dick Smith Electronics, while the version coded 06101062 suits the jack sold by both Jaycar Electronics and Altronics. There are no other differences – apart from the provisions for mounting the different 3.5mm jacks (CON2), both board versions are identical. Fig.3 shows the assembly details. Begin the by fitting the PC board terminal pin for the antenna wire connection, located just to the right of the antenna rod, then fit DC input connector CON1 and the audio input jack CON2. That done, you can install the re- siliconchip.com.au Power supply The circuit is powered from a regulated 12V rail and this is derived from the mains via a 12V DC plugpack supply, diode D1 and 3-terminal regulator REG1. A 12V DC plugpack supply is specified, since these typically deliver 1516V when only lightly loaded. The transmitter circuitry draws less than 40mA in operation, which means that REG1 has quite enough “head room” to provide a well-regulated +12V output. Diode D1 provides reverse polarity protection, to prevent the circuit from damage if the supply is connected the wrong way around. Alternatively, for use in situations where no mains power is available, the transmitter can be powered from a 12V battery (eg, a car battery). This involves removing REG1 and replacing it with a wire link between its input and output connection pads. More about this later. Finally, LED1 is used to provide power-on indication. It’s connected across the 12V supply in series with a 1kW current-limiting resistor. (ie, the current through the LED is 10mA). Par t s Lis t 1 PC board, code 06101061 (DSE version) or 06101062 (Altronics and Jaycar versions), 122 x 57.5mm 1 UB3-size jiffy box (130 x 67 x 44mm) 4 M3 x 10mm tapped spacers 9 M3 x 6mm machine screws, round head 1 M3 hex nut 1 mini RF oscillator coil in can (T1 – red slug) 1 Ferrite rod, 55mm long, with BC band coil (L1) 1 Mini tuning capacitor 60160pF, with disc-type knob and mounting screws (VC1) 1 2.5mm concentric DC socket, PC-mount (CON1) 1 3.5mm stereo jack, PC-mount (CON2) 1 mini control knob (to suit VR2) 2 cable ties, 100mm 1 PC terminal pin, 1mm diameter 1 2m length of insulated hookup wire 1 50kW horizontal trimpot (VR1) 1 50kW log pot, 16mm PC-mount (VR2) Semiconductors 1 MC1496 balanced modulator (IC1) 1 7812 +12V regulator (REG1) 2 PN100 NPN transistor (Q1,Q2) 1 3mm green LED (LED1) 1 1N4004 silicon diode (D1) Capacitors 1 220mF 25V RB electrolytic 1 100mF 16V RB electrolytic 1 22mF 16V RB electrolytic 1 10mF 16V RB electrolytic 1 4.7mF 16V tantalum 2 100nF monolithic 1 10nF metallised polyester 1 4.7nF metallised polyester 2 470pF NPO disc ceramic 1 100pF NPO disc ceramic Resistors (0.25W 1%) 2 15kW 3 1.5kW 1 12kW 4 1kW 8 10kW 1 560W 2 3.3kW 1 470W sistors. These are not polarised, so you can fit them either way around although it’s best to have their colour codes all running in the same direcJanuary 2006  35 The PC board is mounted on the lid of the case using four M3 x 10mm tapped spacers and eight M3 x 6mm machine screws. Note how the antenna rod is secured using plastic cable ties. tion to aid checking later on. Table 2 shows the resistor colour codes but you should also check each unit with a digital multimeter before installing it, just to make sure. The non-polarised ceramic, monolithic and metallised polyester capacitors can go in next. Again, these can again go either way around but be sure to fit the correct value in each position. Once they’re in, install the larger polarised capacitors. These include the 4.7mF tantalum unit which goes just below VR2 and the four RB electrolytics. Note that these must all be fitted with the correct polarity, as shown on the layout diagram. The final capacitor to fit is tuning capacitor VC1. This fits on the top of the board, with its spindle stub shaft and three connection tabs passing down through matching holes in the board. The board is then turned over and the capacitor body attached to the board using two of the M2.5 x 4mm screws supplied with it. Don’t lose the third screw, though – you’ll need it later to attach the disc knob to VC1’s spindle. Now solder VC1’s three connection tabs to their board pads. The oscillator coil T1 is next on the list. This is effectively polarised, because there are three connection pins on one side of its base and only two on the other – be sure to orient 36  Silicon Chip it correctly before pushing it all the way down onto the board. There are seven solder connections to make in all – five pin connections plus two for the can lugs. Trimpot VR1 and modulation control pot VR2 can now go in, after which you can fit the semiconductors – diode D1, transistors Q1 and Q2, IC1 and LED1. These parts are all polarised so be sure to install them as shown in Fig.3. LED1 should initially be installed with its body about 20mm above the board (this can be done by sliding a 20mm-wide cardboard spacer between its leads and pushing the LED down onto this spacer). Its leads should then be bent down through 90° at a point about 14mm above the board, so that the LED faces away from the board and will later protrude through a matching hole in the side of the case during final assembly. Mains or battery power If you intend running the transmitter from a mains plugpack, install regulator REG1 in the position indicated. As shown in Fig.3, this is mounted horizontally on the board, with its metal tab secured by an M3 x 6mm machine screw and nut. To do this, first bend its leads down by 90° at a point 6mm from its body, then fit it to the board and secure its tab using the screw and nut. That done, its leads can be soldered to their respec- tive pads. Don’t solder its leads before securing the tab. If you do, the solder joints could fracture due to stress as the screw is tightened. Alternatively, if you intend running the transmitter from a 12V battery, REG1 is left out and a small wire link fitted instead. This link should be fashioned from a short piece of tinned copper wire (or a resistor lead offcut), bent in an inverted-U shape with its centre section just over 5mm long. This is then fitted between the two outer connection lead holes for REG1 and soldered to the pads underneath. Antenna rod & coil The final component to fit to the transmitter board is the antenna rod and coil assembly (L1). This is secured using two small cable ties, each of which loops around under the board through the pairs of 3mm holes provided for this purpose. (Note: do not replace the cable ties with wire or any other metal bands. A metal loop would form a “shorted turn” and this would absorb RF energy and seriously degrade the performance). Unfortunately, making the coil’s connections to the board can be a bit tricky. In most cases, there are four leads and it’s not easy to work out which are the correct two to use – ie, the actual start and finish of the coil. In fact, the only reliable way to identify the start and finish leads is to siliconchip.com.au TUNING Fig.3: install the parts on the PC board as shown here, taking care to keep all component leads as short as possible. Note that board has been designed to accept both 16mm and 24mm pots for VR2 (although a 24mm pot would not allow the board to fit inside the specified UB3 case. check all lead combinations with an ohmmeter and go with the combination that gives the highest reading – typically around 11W. Another little trap is that with many of these coils, the intermediate leads actually consist of two fine gauge insulated wires, twisted tightly and soldered together at their outer ends. This means that if you decide to cut these leads short, they must be bared and soldered together again – otherwise you’ll find that the coil has become an open circuit between start and finish. And of course, the transmitter won’t function very well with L1 open circuit, as this prevents Q2 from drawing current! A word of advice: if you do shorten any of the coil leads, it’s a good idea to check the coil continuity with your multimeter before you solder the start and finish leads to the board. The last step in wiring up the board is to solder the end of a 2-metre length of insulated hookup wire to the “ANT WIRE” terminal pin at the end of the antenna rod. That done, it’s time to fit the tuning “disc knob” to VC1’s shaft and fasten it in place using the remaining M2.5 x 4mm screw. The board assembly is now ready to attach to the box lid (used here as the transmitter’s base). Before doing this, however, you may need to drill and cut the various holes in both the lid and the box itself, if you’re building the project from scratch. The location, size and shape of each of the holes is shown in Fig.5. Alternatively, if you’ve purchased a complete kit, the box will probably be supplied predrilled, with screened graphics for the front panel. The PC board assembly is secured to the lid using four M3 x 10mm tapped Table 1: Capacitor Codes Value 100nF 10nF 4.7nF 470pF 100pF μF Code 0.1µF .01µF .0047µF   NA   NA EIA Code   104   103   472   470   100 IEC Code   100n   10n   4n7    470p    100p spacers and eight M3 x 6mm machine screws (see photo). Once that’s been done, it’s time to check the transmitter’s operation. Checkout & adjustment It’s easy to check and adjust the transmitter’s operation using a frequency counter, an oscilloscope and an audio signal generator. However, these are not essential and you can do the Table 2: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No. 2 1 8 2 3 4 1 1 Value 15kW 12kW 10kW 3.3kW 1.5kW 1kW 560W 470W 4-Band Code (1%) brown green orange brown brown red orange brown brown black orange brown orange orange red brown brown green red brown brown black red brown green blue brown brown yellow violet brown brown 5-Band Code (1%) brown green black red brown brown red black red brown brown black black red brown orange orange black brown brown brown green black brown brown brown black black brown brown green blue black black brown yellow violet black black brown January 2006  37 Fig.4: this is the full-size front-panel artwork. It can be cut out and used directly if required and can be protected using wide strips of adhesive tape. job quite well using just a multimeter (preferably a DMM) and a reasonably sensitive AM radio receiver. The step-by-step adjustment procedure is as follows: (1) Switch the radio on and tune it to a convenient frequency in the lower section of the broadcast band, away from any of the local broadcasting stations (in Sydney, you can tune to about 820kHz). (2) Turn the volume up (you’ll just hear static at this stage) and position the radio near the transmitter, orientated so that its internal ferrite rod antenna is roughly parallel to the transmitter’s ferrite rod. (3) Turn the transmitter’s tuning control (VC1) to one end of its range, set trimpot VR1 well away from its centre position (this is important) and set VR2 (modulation depth) to its midrange position. (4) Turn the adjustment slug in T1 anticlockwise a couple of turns using a small screwdriver or alignment tool. (5) Feed a stereo audio signal from your MP3 or CD player into the trans- mitter by plugging the audio cable into CON2. (6) Apply power to the transmitter and check that the power LED (LED1) lights. If it doesn’t, unplug the power lead and look for your wiring mistake. You’ve probably fitted either D1 or LED1 with reversed polarity. (7) Use your DMM to check the supply voltage at the output pin of REG1, relative to board earth; it should be very close to +12V. Check that the voltage at pin 8 of IC1 is close to +6V (if these voltages check out correctly, your transmitter is very likely to be working correctly). (8) Listen carefully to the radio while you turn the transmitter’s tuning knob very slowly towards the other end of its range. At some point, you should start to hear the music from your MP3 or CD player, after which you should be able to tune the transmitter so that its signal is received at a good strength. Troubleshooting Can’t find the signal? The first thing to do is to try tuning the transmitter Vintage Australian Radio Programs On CD If you’d like to rebroadcast genuine old time Aussie radio programs through your “Little” Jim AM Transmitter, you should know that many of the programs are now available on CD from ScreenSound Australia (the National Screen and Sound Archive). Currently they have some 11 different CDs available, with classic “golden age of radio” programs, including quiz shows, serials like Dad & Dave and 38  Silicon Chip Mrs ’Obbs, comedies like The Bunkhouse Show and McCackie Mansion, and so on. All CDs are currently available for $24.95 each, including GST (but not postage). For more information on what’s available, visit the ScreenSound website at shop.screensound.gov. au. You can even buy the CDs direct via their secure online purchasing system. back the other way but even more slowly and carefully than before. If this still doesn’t bring success, try turning the adjustment slug in oscillator coil T1 anticlockwise another half-turn (or even a full turn if this later proves necessary). This will shift the oscillator’s tuning range up in frequency and should allow you to correctly adjust the transmitter when you tune VC1 over its range again. If you still can’t find the transmitter’s signal, it may be that its output is a little too weak to be picked up by the receiver. In that case, try draping the transmitter’s antenna wire over the receiver, or twist it around the receiver’s telescopic FM antenna if it has one, just to couple in a bit more of the transmitter’s output. Once you’ve found the signal and adjusted the transmitter’s tuning con­ trol for the best reception, try turning up the transmitter’s modulation control (VR2). This should make the reception even louder and clearer but if you turn the control up too far, the music will become distorted. Just back it off again until the distortion disappears. You can also try adjusting trimpot VR1, because a small amount of adjustment one way or the other can also improve transmission clarity. That said, you’ll find that its optimum position is about halfway between the centre and one of the end positions of the rotor (on either side). Don’t set this trimpot (VR1) too close to its midway (centre) position, because this balances out the RF carrier altogether and gives double sideband (DSB) suppressed carrier modulation. And that gives and quite high distortion when you’re using a normal AM receiver. Once all the adjustments have been made, your Little Jim AM Transmitter is working correctly and you’re ready for the final assembly. Final assembly If your UB3 box has vertical PC board mounting ribs inside, you’ll also have to cut some of these away. That’s because the transmitter board assembly is a fairly tight fit inside the box and the ribs foul the ferrite rod and its coil. The ribs to remove are mainly those at the rear side of the box, where they interfere with the ferrite rod. However it’s also a good idea to cut away any siliconchip.com.au Fig.5: this diagram shows the drilling and cutout details for the plastic case. siliconchip.com.au January 2006  39 Why Did We Call It “Little Jim”? Now then, perhaps we should explain the “Little Jim” monicker. Why not “Little Harry”, or “Little Jack”, or “Little Curly” or “Little Mary”? Come to think of it, why “Little” anything? The answer to that question can be found in two May 1938 issues of “Wireless Weekly”, the forerunner to “Radio & Hobbies” magazine which itself later evolved into “Radio, TV & Hobbies” and finally “Electronics Australia”. Those 1938 issues of “Wireless Weekly” described the construction of a 1-valve AM radio receiver which they called – you’ve guessed it – “Little Jim”. The headline to the article was “Little Jim – Brings Test Play To Your Bedside!”. Don’t get excited – they were talking about the cricket! “Little Jim” was pretty simple as AM radios go, using just a single 6A6 twin-triode valve as both a regenerative detector and audio amplifier. It generated sufficient output to drive a pair of headphones and the original was built into an old butter box with an aluminium front panel. A 45V B battery generated the high tension (HT), while the 6.3V AC filament supply was derived from the 240V mains via a transformer. You could build “Little Jim” by scrounging the parts yourself but there was also a kit available. Yes, they had kits back in those days and “Little Jim” was available as a kit of parts (without the cabinet) for the princely sum of four pounds from a company called Foxradio (Fox and MacGillicuddy) of 57 York St, Sydney. Of course, we’re not too sure ribs on the end near the holes for CON1 and CON2, because these can make final assembly more difficult. You should also cut away any ribs on the front of the box, around the holes for LED1 and VR2, as this make the final assembly even easier. The ribs are easy to remove. The ABS material used in these boxes is fairly soft and can be cut away using a sharp hobby knife or small wood chisel. Once the ribs are gone, remove the knob from modulation pot VR2 (if you 40  Silicon Chip The original “Little Jim” was a 1-valve AM receiver built into a modified butter box, with an aluminium front panel. Fig.6: the circuit used a single 6A6 twin-triode valve as both a regenerative detector and audio amplifier. It generated sufficient output to drive a pair of headphones (actually, we have no idea) why the “Wireless Weekly” editors called their receiver “Little Jim” but no matter. That was the name it was given and it proved to be very popular – so popular, in fact, that it was republished in the very first issue of “Radio & Hobbies” magazine, in April 1939. That set was followed by a full battery-powered version dubbed “Little Jim’s Mate” in the May 1939 issue. But it didn’t end there, with lot’s more have fitted it for the checkout) and unscrew the nut from VR2’s ferrule. That done, thread the free end of the transmitter’s antenna wire through the small hole in the rear of the box (from the inside) and pull most of it through the hole. You can now introduce the box to front of the lid/board assembly at a suitable angle, passing VC1’s disc knob through its slot and LED1 and VR2’s shaft through their respective holes. Next, swing the box down over the board assembly, pulling the remain- variations published in subsequent years. In short, there were lots of “Little Jims” and his “mates” published during the valve era. So that’s where we got the name from. When Jim Rowe came in with his new flea-powered AM transmitter, we initially struggled to come up with a good name for it. “Why not Little Jim?”, said the office smart-elec and despite the groans all round, the name stuck. ing antenna wire through its hole as you do so. As it comes down, slide it slightly towards the CON1/CON2 end, so that the ferrule of CON2 enters its clearance hole. That done, you can fit the nut to VR2’s threaded ferrule. Tighten it firmly and then refit the knob. Finally, turn the assembled box over and fit the four supplied self-tapping screws supplied to fasten everything together. Your “Little Jim” AM Transmitter is now finished and ready for SC action. siliconchip.com.au