Tech Directions - December 2008 - (Page 23)

C4 D Q1 NJFET S R3 L4 +V +9 V C10 R2 C8 L1 C7 Q2 NJFET C2 C1 Out L2 C3 C5 R1 C6 G to the new converter board. Once you’ve built the converter, attach a 50'-long wire antenna to it along with a ground. You should start hearing stations on the 25-meter band around 11.5 MHz during daylight hours. Improvements A good challenge for any student building this project would be to research circuits that might make a suitable broadband rf ampliﬁer to place before the mixer transistor Q1. This addition will give you a surprising boost in receiver sensitivity. Another improvement would be to wind a new set of coils for L1 and L2 to allow for tuning other bands. Increasing L1 and L2 turns by 30% will let you tune lower frequency bands. Decreasing the coil windings by 30% will let you sample the higher frequency bands, which are heard best during the daytime hours. If you place solder posts at the end of each coil, you can tack-solder them into place in less than a minute. Resist the temptation to make this a double conversion receiver because many harmonics from the second local oscillator on the old radio will fall across the hf bands. Single conversion works best in this application. Happy listening! C9 L3 GSD Bottom Q1 Shortwave receiver schematic on the AM radio. It will likely have a yellow tuning slug on top. The coil with the red slug will always be the local oscillator coil. Once you have identiﬁed what you think is the proper connection point, try injecting a modulated 455 kHz signal from a signal generator to verify that you have the proper point. Trying other pins on the transformer will show which one gives the greatest signal output from the speaker. Next, disable the local oscillator on the radio. You can generally most easily do this by clipping one or more of the leads on the radio’s converter transistor. This would be the transistor nearest the large ﬂat loopstick antenna coil. Now your old receiver will only accept the i-f frequency of 455 kHz, and it will no longer tune the AM band. None of the part values are critical. The most difﬁcult part to locate will be the variable capacitor. I suggest trying an eBay search for variable capacitors that go up to 100 pF. Be sure to buy types that have a 1/4" shaft for some sort of tuning knob to attach to. From the old AM radio, run a wire from the ﬁrst i-f transformer, a wire from the +9 V power source, and a wire from the negative ground over This project is available for purchase in an expanded format, ready to copy for student use. Visit www. techdirections.com/projectsE.html Parts List C1 & C2 C3 C4 C5 C6 C7 C8, C9, C10 L1 L2 L3 & L4 R1 & R2 R3 Q1 & Q2 0.01 μF 300 pF 100 pF 50 pF 15 pF 100 pF variable 300 pF 25 T #22 enameled wire on a 5/16" form 34 T #22 enameled wire on a 5/16" form 1 mHz 100 kΩ 47 Ω MPF102 10 kΩ Pot Switch for the AM radio The New Down-Converter Next, cut a piece of perfboard 6" × 3" for the new converter stage. This might seem a bit large for a twotransistor circuit, but stick to this size since it will make construction far easier and will allow for circuit expansion at a later date. Refer to the parts list for parts you’ll need to use. www.techdirections.com ELECTRONICS 23 http://www.techdirections.com/projectsE.html http://www.techdirections.com/projectsE.html http://www.techdirections.com

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Tech Directions - December 2008 Contents Advertisers Index Technically Speaking Direct from Washington The Report Technology Today Technology’s Past Mastering Computers Digital Portfolios—Powerful Marketing Tool for Communication Students Choosing the Right CC Welding Unit—Student Success Depends on It Think Green—Increasing Awareness of Garbage and Recycling Think Green—Teach Students Smart Ways to Reduce Home Energy Use Go International! Build a Simplified Shortwave Receiver Mooresville Middle School Snags Web Site of the Month Gifts for Geeks More than Fun

Tech Directions - December 2008

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