THIS unit is not just another FM con-verter, but a complete eight-tube FM receiver, and it can be built inexpensively. Extremely compact, it is mounted in a cabinet usually sold for four-tube receivers.
A simplified version of the original FM circuit is used, a circuit so rapidly becoming standard that a kit of low-cost components has been put on the market. These parts, used in this design, are a set of three FM coils (antenna, RF, and oscillator), three special FM I.F. transformers peaked at 4,300 kilocycles, a discriminator transformer, also peaked at 4,300 kilocycles, and a three-gang tuning condenser, each section having a capacity of 7-22 mmfd.
The main difference between a standard broadcast superhet and an FM superhet is the use of a limiter tube and a "discriminator" stage. Otherwise this FM receiver consists of the usual R.F. stage (converter stage) and two I.F. stages (second detector and audio stage). The "discriminator" stage makes possible the detection of FM impulses and discriminates against standard or amplitude-modulated impulses.
In an FM receiver all grid and plate leads must be as short as possible, especially in the RF and converter stages. The layout here allows short leads between the tube prongs, condensers, coils, etc. The 10,000-ohm, 1-watt loading resistors across the secondary windings of the coils stabilize and balance the I.F. circuits. An elaborate decoupling system in each plate and screen lead of the first five tubes eliminates feedback between the circuits which would cause oscillation. The rectifier tube and circuit deliver 250 volts at about 60 milliamperes.
Front panel of the FM receiver described in this article
Top view of chassis, showing the compact layout of tubes, transformers, condensers, and speaker, which permits installation in a small, four-tube cabinet.
Bottom of chassis. Note filter choke in center and the special FM coils on extreme right. A phone jack indicated in diagram is not shown.
It is difficult to align an FM receiver, and a dependable dealer will do it for you. Those who are ambitious, however, and can obtain a signal generator can do it at home. Align the discriminator transformer by applying a signal of 4,300 kilocycles to the grid of the 6SJ7 and connecting an O-1 ma. meter with a 100,000-ohm resistor in series with the meter across the 6H6 cathode. Adjust the secondary trimmer of the discriminator until a movement of the meter's needle is noted. As the trimmer is tuned, the meter will go plus or minus, either side of the resonant frequency. Set the trimmer so the meter reads zero voltage. The primary trimmer of the discriminator transformer is adjusted to the maximum reading when it is connected between the center tap of the transformer and ground.
To align the I.F.'s the same O-1 ma. is connected in series with the grounded side of the 50,000-ohm variable resistor (sensitivity control). By-pass the meter with a .02-mfd condenser, and apply a 4,300-kilo-cycle signal to the grid of the preceding 6SK7. Tune the transformer for a maximum reading on the meter. Apply a signal to the next 6SK7 and repeat the procedure. To align the trimmer condensers on the three-gang tuning condenser, tune in a station and adjust each trimmer, by ear, until the station is received at maximum volume.
Adjusting the trimmer condensers. The steel cabinet has a hinged lid.
If the set is located within ten to 15 miles of an FM broadcasting station, a short piece of wire (about ten feet) connected to point "b" on the antenna binding post will suffice, but for best results a special doublet FM antenna should be connected to "a" and "c" on the binding post.
There are two important points about FM reception to keep in mind. First, the discriminator stage will not operate satisfactorily unless a sufficiently strong signal reaches the grid of the 6SJ7. Secondly, the range of FM is limited to about 50 miles.
The five-inch permanent magnet speaker does not do justice to high-fidelity FM reception. It is better to buy one of the high-fidelity speakers now sold for FM reception and use the small speaker just for speech or for monitoring the reception.
Making final adjustments. Since the tubes and power transformer generate heat, two four-inch ventilation holes should be cut in cabinet's back.
View of the RF portion of the FM chassis, showing the three-gang tuning condenser, the 6SK7 RF tube at front, and the 6SA7 mixer tube held in the hand.
The diagram above and on the facing page make if easy to follow the wiring connections. Above, lower right corner, are bottom views of tube..