Tabletop FM Radio
Last update: November 23, 2011
One of my favorite mail order companies for the purchase of electronic parts is Pollin Electronic in Germany (www.pollin.de). Nowhere else can you buy so much for so little money, and the service is great. Recently I ordered a bag with five variable capacitors for € 2.95. Such caps are normally quite expensive and rare as hen’s teeth, so I thought this offer could not be resisted.
My bag contained four variables (different values) and a complete FM frontend: an ancient model from the Japanese company Alps, tuned with a beautiful air variable (2 FM and 2 AM sections – the AM sections being unused). A JFET (2SK58) serves as RF amplifier whereas the oscillator and mixer functions are performed by a single-in-line IC (SC-114B-H). There is no datasheet for the latter component available from the web. The first two characters stamped on the IC package indicate that it was produced by Sanyo Corporation (thank’s Jeff!). I considered taking the frontend apart and using the variable capacitor for an AM radio. However, the module is so beautifully made that I decided to leave it intact and use it for its intended purpose.
Another good mail order company is Oppermann Elektronische Bauelemente, also in Germany (www.oppermann-electronic.de). Oppermann sells a kit for an “FM-IF amplifier and demodulator” which costs € 8.95 and is based on devices from the former Eastern bloc (transistors and ceramic filters from the GDR, ceramic capacitors from Russia, an IC from Czechia). The PCB is of high quality (epoxy type with solder mask) and the one-page manual is good, although a schematic is not provided. Alignment is simple (the core of a single coil should be adjusted for minimal distortion).
On a rainy afternoon I have built the kit and linked it to the frontend, using a vertical piece of wire (70 cm) as the antenna. The reception quality was a pleasant surprise. Even weak and distant stations are brought in loud and clear. There is not a single empty channel in the whole FM band. At some locations in our home, FM reception is difficult and most portable radios fail. This radio plays well even there, which indicates a good rejection of spurious signals and a high sensitivity.
A reverse-engineered schematic of the IF amplifier is shown in Fig.1. 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).
Figure 1. Schematic of the IF amplifier for a (monaural) FM radio. A kit for this IF amp can be bought from Oppermann in Germany.
As audio stage I use the “4W Amplifier” presented by SM0VPO on his excellent website – see Fig.2. Harry specifies BC547, BC557 and TIP41 transistors. 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.
Figure 2. Audio amplifier for FM tabletop radio. This amplifier has been developed by Harry Lythall. See his website for more details.
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 (http://www.hupse.eu/radio/speakers/AD3700.htm) 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.
Figure 3. The Philips AD3700M/06 (still going strong after 46 years)
Both the reception and sound quality of this radio are outstanding – it reminds me of the “Bi-Ampli” valve models made by Philips in the early 1960s. In summary, this was a fun project which worked out quite well !
Some suppliers of RF frontends (besides Oppermann and Pollin):
Baco Army Goods (Piet Ouwehand), www.baco-army-goods.nl
Barend Hendriksen, www.xs4all.nl/~barendh (check under “Units, Assemblies”)
Kent Electronics (Rinus Jansen), www.kent-electronics.nl
Van Dijken Elektronica, www.vandijkenelektronica.nl
NB Since tuner modules are surplus items, they are not always available.