TWO TRANSISTOR FM
RADIO MICROPHONE
INSTRUCTION SHEET
Check your kit
Schematic Diagram
PCB Layout
This FM Radio Transmitter uses one transistor to form an audio amplifier
for the microphone and a second to generate and modulate the frequency of the
broadcast signal. The transmitter can be tuned to transmit through the normal
range of FM receivers, viz. 88 to 108 megahertz (MHz) - millions of cycles per
second.
Tuning is best at around 10OMHz which is usually fairly clear of other FM stations. The transmitting frequency is changed by rotating the screw at the centre of the Trimmer-Cap, Final fine tuning is best done by the radio receiver.
Construction
1. Check the PC Board carefully for damage. The tracks should be checked for continuity with either a CdS Circuit Tester or a multimeter. Repair any breaks in tracks.
2. Identify the resistors by their colour bands and bend the legs to fit the respective holes. A multimeter is recommended as a check against correct identification 4 the resistor values. Mount the resistors as the chart shows from the PLAIN SIDE of the PCB, and spread the resistors' legs to hold them in place. They can be soldered if convenient.
3. Identify the ceramic capacitors by their numbers and mount them as the drawing shows. They are not polarised so can go any way round. Please Note that your Kit probably has a 7pF to replace the 6.8pF. They may be soldered. After soldering the excess legs should be trimmed wit nippers.
4. C1 and C2 are electrolytic capacitors and are polarised. There is an arrow on the body pointing down the Negative (K) leg. The layout drawing shows the correct polarity. The value of each capacitor is marked on the body in uF (micro farads). Mount these and solder.
5. The two transistors are interchangeable but must be placed in the PCB with the flat of the body as the drawing shows. Solder as described above, minimising the heat applied to the legs of the transistors.
6. The Mic. Insert has a positive and negative poles, Check the information sheet in the bag to identify these. There is a choice for mounting the Mic. You can use the tinned copper wire soldered direct to the mic, and to the PC Board, or use the Fig.8 wire if you are mounting the mic. insert away from the board. Polarity is important in either case. The layout sheet gives the correct polarity connections.
7. There is a slide switch which connects by hook-up wire to the PCB as shown. Cut a short length from the wire in the Kit and strip the insulation on each end. Push the wire through the MIDDLE and ONE END poles of the switch and solder. The switch is Double Pole Double Throw (DPDT) so the wires are connected to the same side of the switch at centre and one end. Cut the wire, strip the insulation and solder to the two donuts.
8. Push a pin from the Kit into the hole for the Aerial and solder to the PC Board. Strip an end of the aerial wire and solder to the pin.
9. Mount the trimmer capacitor into its holes. You will notice a raised section down the side of the component. This goes to the single hole. You may need to tree the holes a little to get the legs to fit.
10. The Battery Holder is mounted from the side where all solder has been made. Fuse a dob of solder to the donuts in positions that you will be able to get at with a soldering iron from the end of the Board, but make sure the holes are kept clear. Next turn the battery holder legs up and apply some solder to each leg, right up near the plastic case. Push the legs into the two holes and push the holder down towards the board. Keep pressure on the holder while you touch each leg and donut in turn with the soldering iron. The holder should fit close to the board and finally reheat each solder dob to ensure a joint has been made.
11. Tune a radio to around 100 MHz on the FM band and extend its aerial. Turn your radio mic ON and hold the mic near the radio's aerial. Adjust the trimmer until feed-back is heard, and then adjust the radio tuner to bring in a clear signal. Try talking into the mic and adjust either the trimmer or the tuner to improve the signal, Move the aerial on both transmitter and receiver. With a little time spent on tuning the unit you will find that it will transmit 30 to 50 metres with no difficulty.
Troubleshooting
If the unit fails to operate then you will need to check the entire set of components to ensure that you have the correct values and components in the exact places that the layout sheet indicates. Check that you have observed the polarity of the components ; Transistors, Electrolytic Capacitors, Trimmer Capacitor, Microphone Insert. If these checkout then check your soldering. Look over every joint and resolder any that look suspicious. Don't load up extra solder, apply just a touch of solder to get a supply of flux to assist the solder to run. The unit is tried and tested so most will work right away if care is taken in placement and soldering of components. If any components need to be removed from the board after having been soldered, a Solder Sucker Tool gives best result. It is a small vacuum pump with a teflon tip. The soldered joint is heated to molten state, the solder sucker is depressed and its tip held at the solder. The button trips the pump which sucks the solder from the joint, leaving it clear of solder. The component can be withdrawn and replaced, and re-soldered.