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Here is a simple circuit of TV to FM converter. With this simple one transistor BF 494 circuit you will be able to receive FM and VHF stations on your old Black And White TV sets.
The circuit uses only one NPN transistor T1, BF 494 and few other components. A + 6V power supply for this circuit could be taken from a TV set or use a 9V battery separately.
Circuit Description
A transistor BF494 and together with a resistors R1, R2 will biased a transistor and capacitors C1 is connected base of T1 and +9V.C2 is connecte to a base and emitter. And will make a oscillator circuit which oscillates a frequency between 93.5 MHz-113.5 MHz. Transistor T1 will also works as a mixer which will mixed with frequecies between 88.00MHz -108.00 MHz and will give an output SIF signal between 5.5MHz, which is passed to an 5.5MHz IF input of your TV set.
Inductor L* is connected on parallel with VC1 (2-22pF trimmer) and to a + 6 Volts power supply and a collector terminal of Transistor BF 494. An antenna of 75cm long with wire is connected to an emitter of BF 494 through a capacitor C4 (.001uF). This antenna will boost a low signal. Capacitor C1 is connected between a base of BF 494 and to a +Vcc.
Capacitor C2 (10pF) is connected to a collector and an emitter of BF 494 and this capacitor will give a feedback signal to oscillates a circuit. Capacitor C5 is connected between +9V supply and ground will filter a DC power supply.
An output signal 5.5MHz is taken away from a emitter of BF 494 and a capacitor C3 33pF and connected to an input of SIF section of TV through a switch to “ON” switch.
Construction and Calibration
You will need an inductor L* which can be wound of 5 turns on a 6mm aire core of 23 SWG wire. A suitable vero board of size 5cm*7cm can be use to make your FM radio receiver.A trimmer capacitor VC1 is used for tuning different stations. Connect a 9 Volts battery to a circuit and connect an output terminal to SIF section TV set. Now turn ‘ON’ a TV set. You will hear hissing sound from your TV.Now slowly adjust a VC1 or adjust a coil L* by pulling them toward or apart until you’ll hear a nearby FM stations. You may also use an antenna of 75cm long wire to make an antenna if you are receiving a poor signal.*
Parts List
Semiconductor
T1: BF 494
Resistors
R1: 470K
R2: 470
Capacitors
C1: .01uF
C2: 10pF
C3:33pF
C4:.001uF
C5:.01uF
VC1: 2-22pF trimmer capacitor
Coil L*: 5 turns of 23 SWG wire on 5mm air core, wires, 2-pin connectors, 9 V battery, switch to ‘ON’ switch e.t.c.
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