9 Tube-3 IC STEREO FM Receiver

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I did not build this receiver from a kit or a magazine article. This receiver is the result of many hours of meticulous research and design on my part. Any possible resemblance to other radios on this planet is a pure coincidence!

After succeeding with my 8 tube FM receiver, I decided to upgrade it with a solid-state MPX decoder to make it stereophonic. The decoder I built worked great, but adding it to the receiver was not possible for lack of space. The best solution was to design a larger receiver, and integrate the new decoder in it. The result was the hybrid receiver you see here.

The 9 tube superheterodyne tuner section of the receiver has the following features: a tuned RF (radio frequency) amplifier, an independent VFO (variable frequency oscillator) and mixer, 3 IF (intermediate frequency) amplifiers, an IF limiter, a balanced ratio detector, AFC (automatic frequency control) , AVC (automatic volume control), and a signal and tuning meter amplifier. The tube compliment is as follows: 6BS8 (RF amp.), 12AT7 (VFO & AFC), 6BH6 (mixer), 6BJ6 (1st IF amp.), 6BH6 x 2 (2nd & 3rd IF amps.), 6BH6 (IF limiter), 6AL5 (ratio detector) and 12AT7 (meter amps.).

The AF (audio frequency) section of the receiver is solid-state and is powered from an independent supply. This section uses an NTE743 IC for decoding the stereo MPX signal, and 2 LM386N-4 ICs for amplifying the left and right audio signals. The audio ICs are not run at full power, so hiss which is usually a characteristic of these ICs, is kept at a minimum. Stereo separation from the decoder is excellent, and the sound quality from the 5 inch speakers is rich and clear.

The set runs on 110 volts AC and is sensitive enough to receive stereo transmissions 240 km (150 miles) away from Cyprus on clear days!


Dimensions: 16 x 7 x 7 inches. Controls from left: signal meter zero, tuning meter zero, AFC on/off, sensitivity high/low, speakers/phones, tuning, volume, balance, stereo/mono, and power on/off. Stereo phones jack is left of on/off switch. Stereo LED (light emitting diode) is above tuning dial. Signal & tuning meters are contained in a single unit. Speaker grills were cut out from pre-punched sheets and painted white.


Tuning capacitor with coaxial shafts offers 5.5:1 reduction for smooth tuning. Aluminum dial is attached to outer shaft and rotates slower than tuning knob. Stereo LED indicates stereo transmissions. Stations are accurately tuned when tuning meter needle is in middle position. Signal meter indicates relative signal strength.


The base and holes were drawn in AutoCAD and printed on regular paper. The print-outs were then attached to an aluminum sheet cut to size and the holes punched. The sheet was then bent to form the base.


The front panel followed the same procedure as the base above. Care was taken not to bend the aluminum sheets incorrectly, as this would have rendered them useless.


The base was fastened to the front panel with pop rivets. The radio was now ready for assembly.


The stereo decoder and power supply were assembled and tested first. An MPX signal from my other homemade FM receiver was used for testing. The decoder was monitored with stereo phones while testing the volume and balance controls for the first time.


Although the tuner section was sketched several times before assembly, many changes & improvements were still made during assembly.


The contrast between PC board and point-to-point assemblies is apparent. An "L" shaped shield (lower left) was required for the IF amplifier. Another shield was required between the RF amplifier and mixer tuning circuits. The tuner power supply is below the decoder board.


The tuning capacitor and tubes were shielded for best performance. An RCA jack and switch (center) allow the decoder to monitor other FM tuners with MPX output. The receiver is disconnected and turned off for this purpose.


Supply transformers are on the left. Tuner supply filter capacitor is next to transformer. Tuner on/off switch is below transformers. Telescopic antenna is used for local stations. A dipole antenna can be connected for distant stations. Wide band 5" speakers offer very good frequency response. Aluminum top cover strengthens the unit and doubles as a speaker baffle.


The receiver tuned to a strong station with the tuning needle in center position indicating correct tuning and the red LED indicating stereo reception. Weak stations are usually distorted in stereo mode, so the receiver is switched to mono for clearer reception.


A short history | My radio background | Homemade radios | Tube radios
Transistor radios | World band radios | Kit radios | Reel tape recorders
My other interests | Pictures of Lebanon | Radio links
Home | Showcase | About this site

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