The McIntosh
MR 73 is a precision engineered, highly sensitive solid state AM/FM tuner.
The research
staff of mcIntosh Laboratory, using the latest technology in solid state
physics, has developed many circuit advances:
Phase linear
crystal shaped IF amplifiers
Integrated
Circuits that are the equivalent of 16 transistors, 3 zener diodes, 5 diodes
and 23 resistors
Special
multiplex (stereo) detecting circuits that eliminate critical adjustments
Ultrasonic
muting for complete silence between stations while tuning.
Automatic
stereo-mono switching that is "clickless" and completely electronic.
Visual
multipath indicator to help avoid multipath distortion showing as a visual
indicator of correct antenna direction for minimum multipath reception
New
D'Aarsonval movement meters of increased sensitivity which do not need zero
setting adjustments.
A new
computer designed filter to reduce noise from stations broadcasting subcarrier
auxiliary music services. No other tuner uses such a filter.
Exclusive
McIntosh PANLOC is used for installation.The PANLOC system gives you absolute
ease of installation, operation and maintenance.
PANLOC is
the first professional installation technique to be used on stereo
instruments. In the PANLOC system a
metal shelf is mounted first. The tuner slides into position on this shelf.
Depressing the front panel PANLOC buttons, locks the tuner firmly into position
against the mouting panel.
Onec you
have enjoyed the outstanding performance of the Mr 73, you will understand why
McIntosh products have earned their reputation as "the best". Your
McIntosh MR 73 tuner will give you years of the finest possible reception, and
will be a valued part of your home music system.
FM SECTION
This is
divided into two separate modular sections:
1 - The 100
mHz Radio Frequency (RF) Section
2 - The
FM-IF and Detector Section
The first
section houses the complete FM-RF front-end 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 conventional 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 a JFET has been designed for high sensitivity and freedom from overload.
Low temperature coefficient components have been designed into the FM local
oscillator to revent 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 cantigrade.
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
circuits of the MR 73 exceed the FCC requirements for suppression of oscillator
radiation.
The MR
73 has antenna connections for either
300 Ω twin lead transmission line or
75 Ω coaxial cable. The normal input
impedance of the first RF amplifier is 75 ohms. Impedance match to 300 Ω is provided by a McIntosh designed balun
transformer which has negligible losses. Connections for a 300 Ω line are made with new push type terminals. No
tools are required. A type F male linear phase characteristics of the IF
amplifier.
The FM-IF
and Detector section design required
extended engineering time and testing and uses the latest concepts in
integrated circuits and crystal filters. The FM-IF consist 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 maximum
width is 240 kHz at -3 dB and 400 kHz at -60 dB. The response curve is
symmetrical each side of the center frequency. The crystal filters are
permanently sealed and do not require 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 zenar diodes, 5 diodes and 23 resistors, all on a single monolithic silicon
chip.
The
exceptionally high gain of the two integrated circuits assumes "hard
limiting" at very low levels of input signals.
A
"phase" or "Foster Seeley" discriminator has been designed
to complement the integrated circuit IF section. The IF section has exceptionally high gain and hard limiting
characteristics, yet the capture ratio is quite low. The detected output signal
of the discriminator is extremely low in distortion content.
De-emphasis
of the discriminator output restores the frequency amplitude characteristics o
the same level they were before transmission.
C.FM Stereo
Multiplex Section
McIntosh
laboratory has developed a special detecting circuit in the multiplex section.
A particular advantage of this circuit in the
elimination of the critical adjustments necessary with commonly used matrixing
circuits. The circuit detects the L-R sidebands, then automatically matrixes
the recovered information with the L+R main carrier signal. This yields the
left and right program output with maximum separation.
The 19 kHz
pilot signal is filtered from the composite stereo input signal, amplified by a
special limiting amplifier, doubled to the 38 kHz carrier sognal. The new combination of signals is fed to the
special detector circuit mentioned above. Balanced full wave detectors are used
to cancel the 38 kHz conponents in the output.
The SCA
(Subsidiary Communication Authorization) signal must be removed from the
composite output. This is accomplished by the use of a new "Image
Parameter" band elimination filter that has been computer designed. The SCA filter
rejects SCA signals without impairing stereo performance.
FM muting in
the McIntosh MR 73 operates by detecting ultra-sonic noise which is present
when tuning between stations or when receiving a weak station. The muting
circuit can be activated or defeated by the use of the muting switch on the
front panel. The level of muting desired can be adjusted by the muting level
control on the top panel. Varying the muting control adjusts the threshold at
which the muting takes effect.
When the 19
kHz carrier of a stereo signal is received, the automatic FM stsreo switching
circuit activates the multiplex decoding circuit. This lights the stereo
indicator. The circuit switching is all done electronically with no clicks. The
automatic stereo switching can be defeated by turning the mode selector switch
to FM Mono (In this position the stereo indicator will still light to indicate
the presence of a stereo signal). On monophonic
transmissions the stereo switching is inactive at all times, assuring a
greater signal to noise ratio. The stereo switching circuit has been designed
so that noise will not activate it.
AM SECTION
The
"State-of-the-art." in electronic technology has been incorporated by
McIntosh into the MR 73 with the use of metal oxide silicon field effect
transistors (MOSFET's) in the AM section.
An AM-RF amplifier circuit includes a three section variable tuning
capacitor in the metal enclosed shielded module, which also houses the FM-RF
front end. The Am-RF amplifier uses a
dual-insulated gate MOSFET to obtain more overload protection against strong
local stations than can be obtained with conventional bipolar transistors.
The AM-RF
mixer is also a dual-insulated gate MOSFET. The use of MOSFET's in the RF
amplifier and mixer provides the MR 73 with very little cross modulation
(spurious response) and the image rejection is very good.
Three double
tuned IF transformers are used to obtain a high degree of selectivity yet still
allow good Amfidelity. A 10 kHz whistle filter has been incorporated at the
output of the AM detector. Its purpose is to supress hetrodyning that occurs
between adjacent AM carriers.
To increase
the "pulling power" of the AM section of the MR 73, connect an
external antenna at the rear apron of the unit by using a push connector. An
internal transformer matches the external antenna to the input impedance of the
AM front end. An optional highly sensitive loop stick antenna is also provided
for local or strong stations. A switch on the back panel selects either
loopstick or external antenna.
An AM
sensitivity switch has been provided to decrease both the gain of the RF
amplifier and the noise between stations.
Audio
Preamplifier Section
An audio
amplifier has been incorporated in the McIntosh MR 73. It in creases the audio
level from the AM or FM multiplex outputs to a level sufficient to drive a
preamplifier or other accessory equipment. The audio amplifier consists of 2
separate amplifiers, each amplifier having three transistors. The design uses
considerable negative feedback. Negative feedback helps to achieve low
distortion, wide frequency response, and unexcelled stability. Each audio
amplifier has two low impedance outputs. One is a fixed output set at 2,5
volts. The second output is variable by the volume the control on the front
panel.
By
cross-coupling a small portion of one channel into the other in the audio
amplifier, the residual alternate channel information can be cancelled, thereby
further increasing the stero channel separation.
Also
included in the audio section is a Stereo Filter switch. With the Stereo Filter
turned on and waek stations are being received, stereo noise is reduced, but
still allows ample stereo listening performance.
Power Supply
Special
design attention has been given to the power supply section of the McIntosh MR
73. Two separate rectifier circuits are used in the MR 73.
The 16 volt
regulator is the power heart of teh Mr 73. All signal stages throughout the
unit are powered from this regulator. The 16 volt regulator is very elaborate
in design, using a specially selected transistor and associated circuit. The
regulator uses electronic filtering to insure the lowest possible background
hum level, maximum stability and extremely good regulation.
A half wave
rectifier and filter, supply the DC high voltage needed for the anode of the
multipath indicator. The second power supply is a full wave rectifier which
supplies DC to the multiplex indicator and to the voltage regulator.
Specifications
FM Section
Frequency:
88 to 108 MHz
Useable
Sensitivity: 2,5 microvolts at 100%
modulation (±75 kHz deviation) for less than 3% total noise and harmonic
distortion in accordance with IHF standards
Audio
Frequency Response: Flat from 20 Hz to
20 kHz with standard de-emphasis and 19 kHz pilot filter.
Harmonic
Distortion:
Mono : Less
than 0,3% at 100% modulation ±75 kHz deviation.
Stereo : les
than 0,7%
Capture
Ratio: Better than 1,5% dB at 100%
modulation
Muting: AF injected ultrasonic muting at least 60 dB
noise reduction between stations
Oscillator
Drift: less than 25 kHz
Image
Rejection: Better than 80 dB at 90
MHz; better than 70 dB at 100 MHz
Hum: better
than 70 dB below 100% modulation
Output: Approx. 2,5 volts, low impedance
FM Antenna
Inputs: Built-in loopstick or external
antenna switch selected
IF: two Ics and two phase linear crystan filters
Radiation:
Substantially below FCC requirements
Stereo
Separtion: Better than 35 dB at 1 kHz
Stereo
Filter: Greater than 48 dB suppression of 38 kHz multiplex carrier
Stereo
Indicator: Front panel multiplex stereo
light activated by 19 kHz carrier-only
SCA
Filter: 50 dB down at 67 kHz to 74 kHz ,
275 dB per octave slope
Automatic
Mono/Stereo Switch: McIntosh developed;
all electronic automatic mono/stereo switching circuit
Semiconductor
Complement: 22 diode, 24 Transistors, 2
Integrated Circuits, 1 Indicator Tube
AM Section
Frequency: 55 to 160 kHz
Sensitivity:
Better than 12 microvolts at 1000 kHz (using the external antenna input)
Frequency
Response: Down 6 dB at 5 kHz
Harmonic
Distortion: less than 1% at 39 %
modulation
Selectivity: -3 dB at 10 kHz
Image
Rejection: 60 dB or greater at 1000 kHz
Signal-to-noise
Ratio Better than 55 dB
General
Power
Consumption: 20 watts, 105 to 125 volts,
50 to 60 Hz
Dimensions
Front Panel
: 16 inches wide by 5-7/16inches high
Chassis : 15
inches wide by 13 inches deep, including PANLOC shelf and back panel connectors
Knob
Clearance : 1-½ inches in front of
mouting panel
Weight: 24 pounds net, 36 pounds in shipping carton
Finish: Front panel. Anodized gold and black with
special gold/teal panel nomenclature illumination.
Chassis: Chrome and black
Mouting: McIntosh
developed professional PANLOCK