WhitakerAudio AM/FM Stereo Tuner 2012, Assembly & User Manual

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WhitakerAudio
1 Circuit Description
The circuit description is divided into two sections: 1) the AM tuner, power supply, and related
circuits, and 2) the FM tuner and multiplex circuits.
1.1 AM Tuner/Power Supply Circuits
A schematic diagram of the AM receiver section is shown in Figure 1.1.
1
The AM tuner is a five tube superheterodyne receiver employing high-gain tubes and
carefully designed coils in all RF circuitry. Careful layout of components and leads allows this
high-gain to be realized without instability.
Two types of antenna inputs are used for installation flexibility. A low impedance, external
multi-turn loop may be used that provides high sensitivity with good noise immunity and broad
directional characteristics. Very low impedance circuits are not sensitive to electrostatic noise, but
allow efficient transfer of electromagnetic RF signals. Proper impedance matching to the RF
amplifier grid is obtained by use of a large secondary-to-primary winding ratio. An electrostatic
type of antenna input is also provided, which couples a high-impedance straight wire type of
antenna to the RF amplifier grid circuit directly through a coupling capacitor. A much smaller
antenna can be used where necessary, but with poorer noise rejection. Along these lines, a rod
antenna is provided, which may be used in areas of high signal strength.
A 6BA6 tuned radio frequency (TRF) amplifier (V1) is used ahead of the oscillator/mixer
stage (V2) to increase the overall gain and to reduce coupling between the antenna and oscillator
circuits. Signals picked up by the antenna are coupled to antenna coil T1 through either the low-
or the high-impedance windings. This coil is tuned to give maximum gain at the desired signal
frequency and to attenuate all other signals as much as possible. RF energy from the coil is
coupled directly to the control grid of V1 and amplified. An output signal is taken from the plate
of the 6BA6 and delivered to the primary winding of the inter-stage (mixer coil) T2, where it is
inductively coupled to the grid circuit of the V2 6BE6 mixer and tuned once again to further
increase gain and selectivity.
1. The description of the AM tuner section and Figure 1.1 are adapted from: Heathkit: “Assembly and Operation of the Heathkit
Broadcast Tuner Model BC-1A,” the Heath Company, Benton Harbor, MI,