Feedback FM Receiver

 

Based on Valvo FD-1A module, TBA120, and MC1310

All other components discrete and mounted on a single PCB

Originally published in Elektuur magazine (nov 1974)

Used in our study

 

Associated equipment:                                                                                                  Last updated: April 14, 2006

 

Sony Discman Compact CD Player D-171

Mini Class A Amplifier (Flavio Dellepiane)

La Petite Audiophile speakers

 

 

 

Figure 1. Feedback FM Receiver seen from the front. From left to right are visible: Stereo LED with accompanying mono/stereo switch, small knob for fine tuning, large knob for coarse tuning, indicator of the tuning frequency (Grundig service item)

 

 

Building FM tuners at home was a popular pastime in the 1970s but little has been published on the subject after 1985. A list of relevant articles is shown at the bottom of this page [1-5]. If you are interested I can send photocopies or scans. The project discussed here was started after I acquired a few Valvo modules from Ebay: the cheap FD-1A and the more expensive FD-11. The latter was employed as the RF section in many high-end tuners (e.g. Restek) during the late 1970s and early 1980s. Since I have no experience at all with the construction of stereo receivers for the FM band, I decided to start with a relatively simple project.

 

The november 1974 issue of the magazine Elektuur contained an article about a homemade FM tuner involving the FD-1A. Although this “Feedback FM Receiver” was rather simple and could be easily reproduced, it provided good quality of reception. The complete tuner consisted of four building blocks which were mounted together on a single PCB: (a) Valvo FD-1A RF module, see Figure 2; (b) IF amplifier with TBA120, see Figure 3;  (c) stereo decoder with MC1310, see Figure 4; and (d) power supply built with discrete components, see Figure 5.

 

Operation of the receiver is based on the phase locked loop principle. The FD-1A contains an RF amplifier, oscillator and mixer (made with BF324, BF451 and BF324 transistors, respectively). Tuning is performed with four varicaps (BB104). There is an additional AFC diode which allows voltage control of the oscillator frequency over a relatively narrow range (see Fig.2). The IF amplifier starts with a 30 dB gain stage using a single transistor (T1 in fig.3). Additional IF gain is provided by the TBA120. A phase detector (which is also part of the TBA 120) compares the 10.7 MHz IF signal with the signal of a 10.7 MHz oscillator built with two RF transistors (T2 and T3 in Fig.3) and using a ceramic filter (SFE10.7MA) in stead of a 10.7 MHz crystal. Via a feedback loop, the phase detector continuously adjusts the frequency of the oscillator in the FD-1A in such a way that there is 90 degrees phase difference between the FD-1A oscillator and the external oscillator (T2/T3). Since the received signal is frequency-modulated, the output of the phase detector is an exact replica of that modulation.

 

I copied the PCB shown in the Elektuur article and mounted the components, but to my dismay the circuit did not work. After careful checking of  part values and wiring, it turned out that a TBA120 i.c. (Telefunken) which I took from the junkbox was damaged, probably by static electricity. Fortunately, the TBA120 is still made by the Czech company Tesla and our local electronics store sells such Telefunken clones. As soon as I replaced the broken i.c., my receiver worked perfectly.

 

 

Figure 2. Circuit diagram of FM tuner module used in this project (Valvo FD-1A)

 

 

Alignment of the receiver involves the trim potentiometers P1 and P2 in the IF amplifier, besides P3 in the stereo decoder. P1 is initially turned completely counterclockwise and P2 completely clockwise. Some stations can now be heard.  If P1 is turned clockwise, the audio volume increases but at a certain point distortion will occur. The right setting of the pot is just before the onset of distortion. Checkpoint 10 (or H) is then about 0.5 Volts positive with respect to checkpoint 9 (also marked J, see Fig.3). P2 is now turned counter-clockwise. The signal-to-noise level improves but at a certain setting of the pot a “crackling” kind of distortion sets in. The right setting is again: just before the onset of distortion. P2 adjusts the bandwith of the IF amplifier. P1 presents a DC bias voltage to the AFC diode in the FD-1A, this should ensure that the diode works in the linear part of its range. Alignment should be performed during reception of a stereo broadcast. The alignment should be repeated a few times since there is some interaction between the two trimpots. Once aligned the circuit is very stable.

 

 

 

Figure 3. The IF section of the receiver employs a TBA120 i.c. and four discrete transistors. The BC177 can be replaced by a BC557.

 

 

Alignment of the stereo decoder is very simple. Tune the receiver to a broadcast in stereo and turn P3 until the stereo LED comes on. If the trimpot is turned further, the LED will go out again. The right setting of P3 is at the center of the area in which the LED is lit.

 

 

 

Figure 4. Circuit diagram of the stereo decoder. The BC147 can be replaced by a BC547.

 

  

 

 

Figure 5. Circuit diagram of the power supply. P4 is the control for fine tuning, P5 the main tuning knob. M1 is the tuning indicator which can be calibrated with P6. The diodes used for B1 are 1N4001 (1N1004 is a printing error). BC107, BC108 and BC177 can be replaced by BC547, BC548 and BC557, respectively.

 

 

Our study is a difficult place to operate a FM tuner since there is no wall outlet of the FM/TV distribution network (cable). The room is also rather heavily shielded by the surrounding houses, resulting in low field strengths of most transmitters - in particular my favorite, Radio 4. For initial tests, I used two pieces of wire as an aerial but this was not a success. Although the tuner received stations at virtually every setting of the dial, stereo broadcasts of the weaker transmitters were plagued with noise and the sound quality was poor. Much better results are obtained with an ancient dipole antenna made from (300 Ohm) twinlead. Optimal results are obtained not with a horizontal but with a vertical orientation of the dipole. The classical music programs of Radio 4 are then received in noise-free stereo. Thus, this was a successful DIY project. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6. Inside of Feedback FM Receiver. The TO-5 transistor at the left (with heatsink) is the BC160 (series regulator) from the discrete power supply. The i.c. at the center is the stereo decoder (MC1310), the other i.c. (TBA120) is part of the IF amplifier. The aluminum box at the right is the Valvo FD-1A.

 

 

References (mostly in Dutch):

 

1. Articles from Elektuur:

P.van Hasselt, FM-ontvanger met CA3089, October 1972.

Anonymus, Phase Locked Loop middenfrekwent met TBA120, November 1972.

Motorola, PLL Stereodecoder, November 1972 [with MC1310P].  

Anonymus, FM Tuner, February 1973.

Anonymus, MF voor FM universeel, June 1973.

Anonymus, MF strip FM, September, October and November 1973.

Anonymus, Universele OTA-PLL, March 1975 [with CA3080]

Anonymus, PLL en Feedback PLL, October 1974.

Anonymus, Feedback FM-ontvanger, November 1974 [uses Valvo FD-1A].

Anonymus, FM kamerantenne, March 1976.

Anonymus, FM76, April 1976.

Anonymus, Variometer FM Tuner, September, October and November 1976

Anonymus, FM-middenfrekwent, June 1979 [with CA3189]

Anonymus, Stereodekoder, June 1979 [with TCA4500A].

 

2. Articles from ETI:

Anonymus, Audiofiele FM Tuner, September 1981 [with Larsholt modules]

 

3. Articles from Radio Bulletin:

Anonymus, Spoelloze stereodecoder, January 1973 [with LM1800].  

 

4. Articles from Radio Electronica:

J.G.Smilde, Moderne FM afstemeenheid met digitale zenderkeuze, January 1978.

 

5. Articles from Wireless World:

L.Nelson-Jones, Stereo FM Tuner Mk II, September 1978.

 

 

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