McIntosh MR 73, Руководство пользователя

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FM SECTION: This is divided into two separate
modular sections:
A. The 100 MHz Radio Frequency (RF) Section.
This section houses the complete FM-RF front-
end and part of the AM-RF circuits. A special,
four section, variable tuning capacitor provides
a high degree of RF selectivity and excellent
spurious response rejection. The problem of
image rejection has been greatly reduced in
the RF section of the MR 73.
The two stage parallel-fed-cascode junction
field effect transistor (JFET) RF amplifier gives
better sensitivity and higher gain than conven-
tional one-stage amplifiers. The use of JFET's
helps to further reduce the problem of front end
overload in strong signal areas.
A mixer using JFET has been designed for
high sensitivity and freedom from overload.
Low temperature coefficient components have
been designed into the FM local oscillator to
prevent frequency drift. The frequency stability
inherent in the local oscillator makes automatic
frequency control (AFC) unnecessary. The rate
of drift of the local oscillator is less than ten
parts per million per degree centigrade.
Both the FM and AM-RF front ends have
been designed in the same completely encased
metal modules. This design gives protection
against radiation or interference. The RF cir-
cuits of the MR 73 exceed the FCC require-
ments for suppression of oscillator radiation.
The MR 73 has antenna connections for
either 300 ohm twin lead transmission line or
75 ohm coaxial cable. The normal input imped-
ance of the first RF amplifier is 75 ohms. Im-
pedance match to 300 ohms is provided by a
Mclntosh designed balun transformer which
has negligible losses. Connections for a 300
ohm line are made with new push type termi-
nals. No tools are required. A type F male con-
nector is furnished for 75 ohm coaxial cable.
For greater signal transfer and lower distor-
tion, a special matching transformer has been
designed to interface the FM-RF to the FM-IF
amplifier. This matching transformer consider-
ably enhances the linear phase characteristics
of the IF amplifier.
B. The FM-IF and Detector Section. The FM-IF and
Detector design required extended engineering
time and testing and uses the latest concepts
in integrated circuits and crystal filters. The
FM-IF consists of two integrated circuits and
two phase linear crystal filters. They combine
to give a total gain of over 120 dB (the signal
is amplified to over 1,000,000 times its original
level). The response curve has a nearly flat top
with linear phase characteristics. The skirts of
the response curve are very steep. The maxi-
mum width is 240 kHz at - 3 . 0 dB and 400 kHz
at - 6 0 dB. The response curve is symmetrical
each side of the center frequency. The crystal
filters are permanently sealed and do not re-
quire adjusting. The IF cannot drift nor vibrate
out of adjustment.
Each of the two integrated circuits used in
the FM-IF of the MR 73 contain 16 transistors,
3 zener diodes, 5 diodes and 23 resistors, all
on a single monolithic silicon chip.
The exceptionally high gain of the two inte-
grated circuits assumes "hard limiting" at very
low levels of input signals.
A "phase" or "Foster Seeley" discriminator
has been designed to complement the inte-
grated circuit IF section. The IF section has
exceptionally high gain with hard limiting char-
acteristics. It develops a capture ratio that is
very low. The detected output signal of the
discriminator is extremely low in distortion con-
tent. De-emphasis of the discriminator output
restores the frequency amplitude character-
istics to the same level they were before trans-
mission.
C. FM Stereo Multiplex Section. Mclntosh Labora-
tory has developed a special detecting circuit
in the multiplex section. A particular advantage
of this circuit is the elimination of the critical
adjustments necessary with commonly used
matrixing circuits. The circuit detects the L-R
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