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FM radio intercom for motorbikes Last month, we described the circuit and gave the constructional details for our new FM radio intercom. This month, we conclude with the test and alignment details. By JOHN CLARKE & GREG SWAIN Before connecting power to the intercom, you should first go back over your work and check carefully for wiring errors. Are all parts installed the right way around? Are the parts values correct? Are there any missed solder joints or solder bridges? Some careful checking at this stage may save a lot of frustration later on. Once you are satisfied that there are no errors, temporarily hook up the loudspeaker and microphones, 64 SILICON CHIP then connect a suitable power supply. Either batteries (3 x 1.5V) or a variable power supply set to 4.5V can be used to power the unit. Make sure that you don't set the supply above 5V otherwise you could damage the tr~nsmitter chip (IC2). Apply power and switch the intercom to the 'FM' mode. Now, using your multimeter, check that the + 4.5V supply is present on pin 4 of IC1, pin 5 of IC3, and pin 9 of IC4. If these voltages are all correct, switch to the 'intercom' mode and check that the transmit LED lights when you speak into one of the microphones (note: VR1 & VR2 should initially by set to midposition). Now check the voltage on pin 15 of IC2 - it should be about 2.9V (not 2.5V as shown on last month's circuit diagram). Finally, check the voltages across the supply pins of IC3 (pins 5 & 16) and IC4 (pins 9 & 11) in the 'intercom' mode. You should get a reading of about 0.7V when the transmit LED is on and + 4.5V when the transmit LED is off. If you don't get the correct readings switch off and check for wiring errors. If you have problems, there is a brief troubleshooting procedure at the end of the article, so you should also refer to that. L1 The alignment procedure involves making adjustments to the parts indicated here. Be sure to use an insulated alignment tool when adjusting the coils (L1-L3). A metal screwdriver will affect the coil inductance and could crack the ferrite slugs. Alignment If everything is OK, you are now ready for the alignment procedure. This can be done without the use of any test equipment but does require the use of a commercial FM receiver (preferably one with a digital readout). You will also need a plastic alignment tool to adjust the slugs in the coil formers (Ll-13). Do not use a screwdriver here, otherwise you could crack the fer rite slugs. A metal screwdriver will also make adjustments extremely difficult since its presence affects the coil inductance. Make up a suitable alignment tool if you don't already have one. A plastic knitting needle can be easily modified to fit the slot in the ferrite slugs. The slugs should be screwed into 11, 12 and 13 together with a short piece of dental floss (available from supermarkets and chemist shops) or very fine elastic. This will ensure that the slugs stay in position when adjusted. Don't use wax to hold the slugs in position - it won't work and will make a mess. Now for the alignment adjustments. The procedure is set out below on a step-by-step basis to make it easy to follow. • Switch on your reference FM receiver and tune it to a station near the 88MHz end of the dial. Rotate the dial on the intercom so that the dial indicates the same frequency as the tuned station. Switch the intercom to the 'FM' mode and set the volume control to about 25% of full rotation. • Adjust 13 until the same station is heard on the intercom. Now tune the reference FM receiver to a station near the 108MHz end of the band and set the tuning dial on the intercom to the same indicated frequency. Adjust the trimmer capacitor on the top of the tuning gang (VCl) to receive the same station as the reference FM receiver. Note that the trimmer requiring adjustment is the one closest to the front panel. The other trimmer is not used. • Retune the reference FM receiver to the station at the 88MHz. end of the band and set the tuning dial of the intercom to show the station frequency. Readjust 13 until the station is heard. This done, retune the reference FM receiver to the station at the 108MHz end of the band and set the intercom tuning dial to show the frequency of this station. Readjust the tuning gang trimmer so that the station is heard. The above procedure adjusts the tuned circuit in the intercom's receiver so that it covers the commercial FM band. If you like, you can repeat the procedure once more to ensure that everything is correct. • Tune the reference FM receiver to 88.0MHz and check that trimpots VRl & VR2 are at mid-position. Select the 'intercom' mode and activate the transmitter by speaking NOVEMBER 1989 65 The metal shield (at end of pen) must be installed before the alignment procedure. Note how the O.lµF capacitor adjacent to L2 is installed. into one of the microphones (the transmit LED should come on). Adjust slugs 11 and 12 until your voice is heard on the reference receiver. Note that 12 sets the transmit frequency while 11 forms part of a bandpass filter. The two slugs should be adjusted together so that they are at about the same depth in the slug core. Our prototypes tuned to 88.0MHz when the slugs were about 113rd of the way into the core from the top. When the frequency is tuned, adjust L1 for best signal quality. • Adjust VRl and VR2 so that the signal does not sound overloaded or cut out altogether. Rotating the wipers clockwise (as seen from the front panel) will reduce the signal level. Both trimpots should be set to the same position. • That completes the adjustments for the first intercom. Now you need to adjust the second intercom in exactly the same manner. Testing The two intercoms can now be tested for range and clarity. Make The coiled cord is clamped to the front panel using a cord grip grommet. Don't forget the earth connection to the body of the pot. 66 SILICON CHIP sure that the antenna leads are supported with the same orientation. We recommend that the antenna lead be clipped to your collar. Some slight retuning of the transmitters may be required for best results. This is simply a matter of tweaking 12 in each unit to give maximum range (about 50 metres). Don't touch 13 - it's best to adjust the transmitter frequency only. Note also that the components located around tuning coils 11, 12 and 13 will alter the tuning if moved. These parts include the capacitors and the tinplate shield. If you do move any of these components, you may have to go back over the alignment procedure again. As pointed out last month, the O. lµF capacitor adjacent to 12 must be laid flat against the PCB. If you don't to this, it will affect the tuning of the coil, particularly with changes in temperature. Helmet wiring The loudspeakers and microphones are installed in the helmet as shown in Fig.1. Note that the loudspeakers must be small enough to fit behind the helmet lining. Very small headphone loudspeakers can be used if there is little space within the helmet. The electret microphones must be secured in the positions indicated; ie, one directly in front of the rider's mouth and the other mounted to one side. Make sure that the addition of these components will not cause injury if there is an accident. The leads running from the microphones and loudspeakers connect to the 6.5mm stereo and mono line sockets. These leads are run beneath the helmet lining and secured to the base of the helmet. Note that if glue is used to secure the wires and microphones, be sure to use a type which will not affect the helmet. Some glues, such as contact adhesive, give off fumes which may damage the polystyrene helmet liner. Epoxy resin glues should be safe, provided there is adequate ventilation during the curing process. Once the intercom is completed, it can be tested on the motorcycle. The antenna lead should be clipped to the collar of your jacket so that it extends to maximum length. Of course, common sense dictates that you don't attempt to make any adjustments to the intercom while the bike is in motion. Troubleshooting If it doesn't work, don't dive in and replace all the ICs and transistors. That seldom fixes anything and only wastes money. Most faults in kits are due to faulty soldering, wiring errors and incorrect parts. The first step in troubleshooting is to check all these possibilities very carefully. In particular, check the wiring to switch S1. The diagrams show the wiring for the Dick Smith Electronics switch but if you use a switch from another supplier, then the connections may be different. If necessary, use your multimeter to determine the switch terminals. If all the wiring checks out, try to determine which part of the circuit is not working. For example, if the unit won't receive FM stations, then the fault could lie in either IC3 (the receiver chip) or IC4 (the audio amplifier). You can then check IC4 by applying a low-level audio signal from an external source across the volume control (VR3). If the receiver circuitry works but the unit won't transmit, the fault could lie in the microphone preamp circuit (ICla, IClb), the VOX circuit (IClc, ICld & Q1-Q3), or the transmitter (IC2). You can easily check whether or not the microphone preamp and VOX circuit is working by selecting the 'intercom' mode and speaking into the microphone. If the transmit LED lights, the VOX circuit is OK. If it doesn't, there is a fault in the VOX circuit or IC2 is faulty (or the LED is wired back to front). To determine where the trouble lies, short the collector and emitter leads of Ql. If the LED now lights, the fault is in the VOX circuit; if not, the fault lies with IC2. If the VOX circuit isn't working, use your DMM to monitor the voltage on pin 8 of IClc. This pin should normally be close to 0V but should give a reading of over 1V when you speak into the micro- WIRING TO - - - MICROPHONES 6.5mm STEREO LINE SOCKET ANTENNA □-- LOUDSPEAKERS -o MOTORCYCLE INTERCOM -HELMET Fig.1: this diagram shows how the electret microphones and loudspeakers are installed in the helmet. Note that one of the microphones goes directly in front of the rider's mouth while the other is placed to one side. The speakers go behind the helmet lining. phone. If nothing happens, then the microphone preamplifier circuit is at fault. Note: the output of IClc is actually a square wave which is averaged by the DMM to give a reading. If everything is OK here, check the voltages on pins 12 & 13 ofICld. Pin 13 should be at about 0.8V while pin 12 should jump from 0V to over 1V when you speak into the microphone. At the same time, pin 14 of ICld should go from 0V to about 3.2V. Assuming ICld is working, transistors Qt, Q2 & Q3 can be checked by measuring their collector voltages. Normally (ie, VOX not activated & pin 14 of ICld low), Qt & Q2 should be off and Q3 should be on. Qt 's collector should be at + 2.4V, Q2's collector at + 0.7V and Q3's collector close to 0V. If this checks out, connect a jumper lead between the cathode (K) of D2 and the + 4.5V supply rail. Qt & Q2 should both now turn on which means that their collectors should be close to 0V. At the same time, Q3 should turn off and its collector should go to about 4.1 V. Another way of checking the transistors is to measure their baseemitter voltages. When the jumper lead is connected, Qt and Q2 should both have a base-emitter voltage of about 0.7V. When the jumper lead is removed, Q3 should have a base-emitter voltage of 0. 7V (ie, the transistor is turned on). Check the transistors carefully if you fail at get the correct voltage readings at any stage. Their correct operation is crucial for supplying power to the rest of the circuit when in the 'intercom' mode. ~ NOVEMBER 1989 67