McIntosh MR 74 Owner's Manual page 14

TUNING MECHANISM AND DIAL DRIVE
In the MR 74, unique design and careful manufac-
ture of the mechanical dial drive assembly gives
smooth flywheel tuning.
By controlling the relationship of mass and me-
chanical resistance, and by dividing the workloads
in the dial drive system, it becomes nearly impossi-
ble to detect any backlash. Yet, the entire dial drive
is a model of mechanical stability.
For added ease and increased tuning accuracy, a
section of the dial pointer is illuminated.
FM
The Radio Frequency (RF} section houses the
complete FM-RF front-end and part of the AM-RF
circuit.
A seven-section variable capacitor is the heart of
the RF section. Four sections of the variable capaci-
tor are in the FM front end and the remaining three
are in the AM section. By interleaving the sections
(FM-AM-FM-etc.) spurious responses and oscillator
pulling are significantly reduced. The four FM sec-
tions of the variable tuning capacitor provide a high
degree of RF selectivity and excellent spurious re-
jection. Use of the latest "state of the art" field
effect transistors with a well-designed variable tun-
ing capacitor provides an excellent RF front end.
All of the RF circuits, including the selectivity cir-
cuit and the AM sections of the variable capacitor
are encased in a metal module. Each FM-RF section
is isolated in a separate compartment by metal
shielding. Careful design and manufacturing increase
the protection against radiation and interference.
The MR 74 exceeds the FCC requirements for sup-
pression of local oscillator radiation.
A dual insulated gate metal oxide silicon field ef-
fect transistor (MOSFET) is used as first and second
RF amplifier. Each gate of the transistor is internally
protected by back-to-back diodes against incoming
transients. Use of MOSFET's greatly reduces the
cross-modulation products over a wide dynamic
range. A wide dynamic range permits the input cir-
cuits to accept extremely strong signals without
overload. Since both RF amplifiers have insulated
gate configurations, external neutralization is not
required. This design results in a very stable RF
amplifier circuit.
Low temperature coefficient components for the
FM local oscillator prevent frequency drift. The fre-
quency 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.
The mixer design uses a junction field effect tran-
sistor (JFET) for high sensitivity and freedom from
overload. The mixer delivers the composite FM sig-
nal at the 10.7 MHz intermediate frequency. The path
of the IF signal is controlled by the front panel
SELECTIVITY selector switch.
At the NORMAL position, the SELECTIVITY switch
directs the signal through an IF preamplifier stage
that uses a JFET and a double-tuned IF transformer.
The signal then goes to the FM-IF and discriminator
module for further amplification. Setting the SELEC-
TIVITY selector switch to NARROW routes the signal
through two double-tuned transformers, a ceramic
filter network, and a single-gate MOSFET. The sides
of the IF curve are compressed by this circuit nar-
rowing the IF bandpass. In this mode of operation
weak stations adjacent to strong stations can be
tuned.
Antenna connections for either 300 ohm twin lead
transmission line or 75 ohm coaxial cable are pro-
vided on the back panel. The normal input imped-
ance of the RF amplifier is 75 ohms. Impedance
match to 300 ohms is provided by a negligible loss
balun transformer designed by Mclntosh. Connec-
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