FM radio circuit

Posted on Apr 1, 2012

Two conventional amplifier stages built with SF215 transistors are linked by ceramic 10.7MHz IF filters, and followed by an FM demodulator built with a TBA120T i.c. Straight from the databook. Output goes via a 100k log potmeter (not shown) to the audio stage. (NB SF215 transistors can be replaced by BF240, BF241, BF254, BF255, BF594 or BF595). Since my junkbox does not contain TIP41s, I made a Japanese version, using 2SC945, 2SA564A and 2SD1138s salvaged from damaged Oriental stereos. The amplifier sounds great and is unconditionally stable.

FM radio circuit
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The power supply is also made from junkbox parts. A 2 x 12V, 1.5A wall-wart (computer item) feeds a 6800 µF / 100V smoothing capacitor via two BY298 diodes. A Motorola MC78T12CK i.c. (3A regulator in TO-3 package) provides 12V DC. For stability’s sake I placed a 10µF tantalium electrolytic parallelled by a 10nF ceramic capacitor directly between the output terminal of the MC78T12CK and ground. Audio quality is strongly dependent on the employed loudspeaker. I tried various modern speakers but finally opted for an ancient model salvaged from an empty radio cabinet (Philips AD3700M/06). Valve radio specialist John Hupse ( lists the following specs for this item: dual-cone, diameter 14 cm, power 6 W, impedance 5 Ohms, efficiency 6%, frequency 90-18,000 Hz, strength of the magnetic field 9,500 Gauss (26,200 Maxwell). It dates from 1961 and is still going strong after 46 years.

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