21
AM/FM Stereo Tuner
The output of V57a is also applied to the grid of tube V58A, another amplifier. From tube
V58a, only the L – R signal passes through the bandpass filter consisting of C84, C85, and L55 to
the tuned circuit formed by C91 and L56.
The network consisting of R92, C91, and L56, in conjunction with the bandpass filter, will
also attenuate any SCA (subsidiary communications authorization) signals that may be
transmitted by the radio station. SCA programs are transmitted for special applications and not as
a part of the station’s normal stereo on-air programming.
The output from V58a is also fed through C87 to a keyed oscillator, which accepts only the
pilot tone (19 kHz). This tone keys the oscillator so that it produces a 38 kHz signal. The output of
the oscillator is then coupled through T56 and R94 to the tuned circuit consisting of C91 and L57.
The L – R and 38 kHz signals are mixed together across this tuned circuit.
The resulting composite signal is then demodulated by balanced diode detectors X5l and X52.
Since diodes X5l and X52 are connected in opposite directions in the balanced detector circuit, a
L – R signal will appear across C92 and a L + R signal across C93.
R95, R103, and C94 form a matrix and de-emphasis network for the left channel; R93, R96,
and C95 are the counterparts for the right channel. The matrix network is used to recombine the L
– R and –L + R) signals with the L + R signal as follows:
•
When L + R is added to L – R, the instantaneous sum is 2L. This left channel signal is
coupled from the output of the matrix through C96 to the grid of tube V59
a
.
•
When L + R is added to – L + R), the result is 2R. The output of this side of the matrix
network is applied through C97 to the grid of tube V59
b
.
V59 is a dual cathode-follower stage used to lower the output impedance, thus enabling the
circuit to be connected to an amplifier through long cables without excessive signal loss. R99 and
R102 in the cathodes of V59a and V59b are right and left channel output level controls.
The front panel of the tuner is shown in Figure 1.3. The back panel is shown in Figure 1.4.
Summary of Contents for AM/FM Stereo Tuner 2012
Page 1: ...WhitakerAudio AM FM StereoTuner User and Assembly Manual...
Page 11: ...11 AM FM Stereo Tuner Figure 1 1 Schematic diagram of the AM tuner section...
Page 17: ...17 AM FM Stereo Tuner Figure 1 2 Schematic diagram of the FM tuner section...
Page 34: ...34 WhitakerAudio Figure 3 1 Component layout for the AM tuner PWB...
Page 36: ...36 WhitakerAudio Figure 3 3 Component layout for the FM tuner PWB...
Page 69: ...69 AM FM Stereo Tuner Set the FM tuner board aside it will be used later b...
Page 101: ...101 AM FM Stereo Tuner Figure 5 11 Chassis view of the AM FM Stereo Tuner...
Page 159: ...159 AM FM Stereo Tuner...
Page 160: ...WhitakerAudio AM FM StereoTuner...
