McIntosh MR 7083, Руководство пользователя, Страница: 15 / 20

TUNING SYSTEM
The MR 7083 has three modes of tuning, MANUAL,
SCAN, and PRESET.
In the MANUAL mode, two photo electric sensors
sense the direction and degree that the tuning knob
is rotated and move the tuned frequency
accordingly. The smoothness and ease of tuning
provided by this system is a result of only light
coming in contact with small metal veins attached to
the tuning shaft. Therefore, friction is almost
non-existent and flywheel action is superior.
When a SCAN pushbutton is pressed, a ramped
tuning voltage is generated, increasing in voltage for
SCAN up, decaying for SCAN down. The ramp
continues until the zero crossing of the detector's S
curve is sensed at the input of the controller. It then
stops and maintains that voltage with any necessary
correction added.
Once a station has been tuned by either the
MANUAL or SCAN mode , you may ENTER it into
any of the eight memory locations. Simply press
ENTER, then within 5 seconds, press the desired
STATION button, 1 through 8. The station is then
stored in the tuning controller's memory.
A momentary press of a preset STATION
pushbutton will instantaneously recall the station
from memory. A lithium battery maintains the
memory for more than 10 years. Each time a
pushbutton is pressed, a mute pulse suppresses any
noise that might occur during the tuning process.
During the scan mode, an output from the controller
lowers the tuner sensitivity. This prevents weak noisy
stations from being captured. If one wishes to listen
to weak stations, they may tune them manually.
FM TUNER
A type F connector is provided on the rear panel
for connection of a 75 ohm antenna or cable system.
Following the antenna matching circuit is an RF
tuner of exceptional performance. It uses a
DMOSFET RF amplifier, Double Balanced Mixer and
4 RF circuits that are tuned by matched varactor
diodes. The results are high spurious rejection and
great sensitivity. This circuitry and high tuning
voltage eliminate RF intermodulation distortion
caused by diode nonlinearities.
The FM IF section uses 2 transistors, 3 linear phase
piezoelectric filters, and 2 integrated circuits. They
combine to provide over 120dB of gain and a
selectivity greater than 70dB. Limiting, muting, signal
strength drive, and FM detection are all functions of
the last IF integrated circuit.
A phase locked loop (PLL) stereo decoder
integrated circuit is the heart of the multiplex
section. It has a high signal-to-noise ratio, low
distortion, high channel separation, and high SCA
rejection. The PLL MPX 1C eliminates inductors to
minimize drift, provides integral lamp driving
capability for stereo indication, and has transient free
mono/stereo switching.
Following the PLL MPX decoder, an LC tuned dual
notch filter (19 and 38kHz) is used to prevent tape
recorder bias interference. The LC filter is driven
from the MPX output amplifier and is terminated by
an operational amplifier. This provides the necessary
filter input and output impedances for proper phase
response.
AM TUNER
The first element in the input section is a new Low
Impedance Loopstick Antenna. It receives all the AM
stations and rejects noise and other interference. A
J FET-Transistor cascode amplifier follows. This
amplifier provides excellent sensitivity and spurious
response rejection. It also has a delayed AGC system
that enhances the overload performance margin.
Two varactor tuned circuits and a double balanced
mixer are next. This type of mixer has excellent
image and IF rejection.
The mixer's 450kHz output feeds a matching
transformer for the wide band linear phase
piezoelectric 4 element lattice filter, then the IF
amplifier, another transformer, and finally the AM
detectors, filter and muting circuits.
Push terminals are provided for use with an
external AM antenna. Due to the unusual design of
the AM RF input section, almost any type of antenna
can be used.
AUDIO PROCESSOR
The MR 7083 contains an audio processor that
looks at L + R and L - R information and
ENHANCES the sound by modifying the frequency
and phase response.
If the tuner is in the MONO mode, and tuned to
an AM or a weak FM station, it provides a stereo
effect. Since this is done at audio frequencies, it
does not degrade the signal-to-noise ratio. When
receiving a strong FM station in stereo, it ENHANCES
apparent separation giving a wider stereo image.
TECHNICAL DESCRIPTION 13