AI4FR Virtual Shack Tour 

Broadcast band Receivers

 

 Production year 1950

The General Electric model 135 is a super-heterodyne receiver design that covers the AM Broadcast band of between 540 to 1600 Kc.  The radio makes use of the following 5 tubes in its circuit: 35Z5GT/G, 50L5GT, 12BA6, 12SA7, 12SQ7.  The IF(Intermediate Frequency) on this receiver is 455kHz and the circuit incorporates two AF(Audio Frequency) stages.  This radio has a built in PM(Permanent Magnet) speaker. The antenna is a loop which is glued to the inside of the back cover. This radio requires either AC(50 or 60 cycles) or DC at between 105 to 125 volts and draws 25 watts. The model 135 is housed in a brown plastic case.

Operation of the radio is quite simple consisting of two user control knobs. The knob on the bottom left corner is used for on/off and volume control. Rotating this knob to the right turns on the radio and increases the volume.The knob on the bottom right corner is used to change the receive frequeny of the receiver. The operator simply turns this dial to the left or right to receive stations. A reddish orange tuning indicator located behind the display of numbers allows the operator to know where on the broadcast band the radio is tuned. While the radio can receive from 540 kHz to 1600 kHz the numbers marked on the display start at 55 which indicates 550 kHz and go to 160 which indicates 1600 kHz.

The super-heterodyne circuit was invented by Edwin Armstrong in 1918. The circuit of a super-heterodyne(superhet) receiver employs the mixing of signals which provides superior selectivity, frequency stability, and sensitivity when compared with simpler designs such as regenerative, Tuned Radio Frequency(TRF) or Neutrodyne receiver circuits. The superhet, which is still in use today, mixes the received signal with that of an internally produced signal referred to as the intermediate frequency(IF) which allows the receiver to more conveniently process the received radio signal.

The detector, intermediate amplifier, and a-f amplifier of a superhet receiver are actually equivalent to a complete TRF receiver by themselves. The main difference from complete Amplitude Modulation(AM) TRF receiver circuits are that the IF section of a superhet is fixed tuned that is, the resonance is set at one frequency and reception is possible only with signals of the intermediate frequency. The diode detector used in a superhet is somewhat less sensitive than the types used in TRF receivers such as grid leak, square law, linear plate, and similar types.

The signal from the antenna is filtered to reject most of the undesired frequencies and eventually amplified. A local oscillator inside the receiver produces a sine wave(usually 455Kc) that mixes with the received and filtered signal and thus shifting it to a specific intermediate frequency(IF). This IF signal is then filtered and amplified and depending on the circuit it can be further processed in additional ways. Next the demodulator takes this IF signal and demodulates it to recreate a copy of the original information sent in the transmitted radio signal. There are several different superhet circuit designs such as single, double, and triple, conversion.

The word Superheterodyne is a contraction of the words supersonic and heterodyne. In this case the word supersonic(super) indicates those frequencies that are above the range of human hearing. The word heterodyne is derived from the Greek roots where hetero indicates different, and dyne indicates power. Putting all this together superheterodyne simply means frequencies that are above human hearing with a different power.  

The two pictures above were taken after the restoration was complete. 

 

The photograph on the left is of the front showing what the radio looked like before restoration. The photograph on the right is of the inside with the guts removed at the beginning of the restoration process.

In the picture on the right we can also see the PM speaker and on the bottom of the cabinet notice the metal RF shielding plate.

  

  

The photograph on the left is a picture of the top of the chassis before any cleaning or restoration was attempted. While the photograph on the right is a picture of the beginning stages of the restoration process.  

The photograph on the left is proof positive that them Florida roaches can get into any thing. The roach that left the egg behind the dial marker must have really liked the radio station this receiver was turned to.

The photograph on the right is a close up of one of the components found under the chassis. Wow, what happened to that capacitor in the center of the picture? I wonder if all them blisters are a form of capacitor chicken pox? Or maybe this capacitor did not like the radio station as much as the roach did.

  

  

  

The photograph on the left is a view of under the chassis taken during the restoration process. Surprisingly, under the chassis of this receiver was rather clean. So besides some dusting and cleaning of the wires and and components no major work was needed here. The picture on the right is an after restoration photograph showing many of the new parts installed.  

  

  

Resource:

Receiver trouble shooting and repair by Alfred Ghirardi and J. Richard Johnson

  

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