THE MR 75 AM/FM TUNER
THE MR 75 AM/FM TUNER
Reading Time: 32 Minutes
Price $2.00
VARIOUS REGULATORY AGENCIES REQUIRE THAT WE BRING THE FOLLOWING
INFORMATION TO YOUR ATTENTION. PLEASE READ IT CAREFULLY.
WARNING: TO PREVENT FIRE OR SHOCK
HAZARD, DO NOT EXPOSE THIS UNIT TO
RAIN OR MOISTURE.
The Mclntosh you have purchased is a Model MR 75.
It has a serial number located on the rear panel of the
chassis. Record that serial number here:
Serial Number
The model, serial number and purchase date are
important to you for any future service. Record the
purchase date here:
Purchase date
Upon application, Mclntosh Laboratory provides a
Three-Year Service Contract. Your Mclntosh authorized Service Agency can expedite repairs when you
provide the Service Contract with the instrument for
repair. To assist, record your Service Contract number
here:
Service Contract Number
Your MR 75 AM/FM Tuner
can give you many years of
satisfactory performance. If you
have any questions, please contact:
Contents
CUSTOMER SERVICE
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: 607-723-3512
SERVICE CONTRACT INFORMATION
INSTALLATION
HOW TO CONNECT AND
BACK PANEL INFORMATION
FRONT PANEL INFORMATION
AND USE OF CONTROLS
PERFORMANCE LIMITS
PERFORMANCE CHARTS
TECHNICAL DESCRIPTION
BLOCK DIAGRAM
Take Advantage of 3 years
of Contract ServiceFill in the Application NOW.
McINTOSH THREE YEAR SERVICE CONTRACT
An application for A THREE YEAR SERVICE
The terms of the contract are:
1. Mclntosh will provide all parts, materials
and labor needed to return the measured
performance of the instrument to the
original performance limits. The SERVICE CONTRACT does not cover any
shipping costs to and from the authorized service agency or the factory.
2. Any Mclntosh authorized service agency
will repair Mclntosh instruments at normal service rates. To receive service
under the terms of the SERVICE CONTRACT, the SERVICE CONTRACT CERTIFICATE must be presented when the
instrument is taken to the service agency.
3. Always have service done by a
Mclntosh authorized service agency. If
the instrument is modified or damaged
as a result of unauthorized repair, the
SERVICE CONTRACT will be cancelled.
Damage by improper use or mishandling is not covered by the SERVICE
CONTRACT.
4. The SERVICE CONTRACT is issued to
you as the original purchaser. To protect you from misrepresentation, this
Copyright 1981 © by Mclntosh Laboratory Inc.
1
CONTRACT is included with this manual.
contract cannot be transferred to a second owner.
5. To receive the SERVICE CONTRACT,
your purchase must be made from a
Mclntosh franchised dealer.
6. Your completely filled in application for
the SERVICE CONTRACT must be postmarked within 30 days of the date of
purchase of the instrument.
7. To receive the SERVICE CONTRACT, all
information on the application must be
filled in. The SERVICE CONTRACT will
be issued when the completely filled in
application is received by Mclntosh
Laboratory Incorporated in Binghamton,
New York.
8. Units in operation outside the United
States and Canada are not covered by
the Mclntosh Factory Service Contract,
irrespective of the place of purchase.
Nor are units acquired outside the
U.S.A. and Canada, the purchasers of
which should consult with their dealer
to ascertain what, if any, service contract or warranty may be available locally.
1
2, 3
4-6
6, 7
8
9, 10
11, 12
13
THE MR 75 AM/FM TUNER
Installation
1. Open the carton and remove the PANLOC brackets, hardware package, and mounting template. Remove the MR 75 from its plastic bag and place it upside down on the shipping pallet, then unscrew the
four plastic feet from the bottom of the chassis.
The PANLOC system of installing equipment conveniently and securely is a direct result of Mclntosh
research. By depressing the two PANLOC buttons on
the front panel, the instrument either can be locked
firmly in place or it can be unlocked so that the
chassis can slide forward, giving you easy access to
the top and rear panels.
2. Mark the cabinet panel
Place the mounting template in the position on the
cabinet panel where the instrument is to be installed,
and tape it in place. The broken lines that represent
the outline of the rectangular cutout also represent
the outside dimensions of the chassis. Make sure
these lines clear shelves, partitions, or any equipment. With the template in place, first mark the six A
and B holes and the four small holes that locate the
corners of the cutout. Then, join the four corner markings with pencil lines using the edge of the template
as a straightedge.
The trouble-free life of an electronic instrument is
greatly extended by providing sufficient ventilation to
prevent the build-up of high internal temperatures
that cause deterioration. Allow enough clearance so
that cool air can enter at the bottom of the cabinet
and be vented from the top. With adequate ventilation
the instrument can be mounted in any position. The
recommended minimum space for installation is 15
inches (38.1 cm) deep, 17 inches (43.2 cm) wide, and 6
inches (15.2 cm) high.
3. Drill Holes
Use a drill with a 3/16 inch bit held perpendicular to the
panel and drill the six A and B holes. Then, using a drill
bit slightly wider than the tip of your saw blade, drill
one hole at each of two diagonally opposite corners.
The holes should barely touch the inside edge of the
To install the instrument in a Mclntosh cabinet,
follow the instructions that are enclosed with the
cabinet. For any other type of installation, follow
these instructions:
penciled outline. Before taking the next step, make
sure that the six A and B holes have been drilled.
2
4. Saw the Panel Cutout
Saw carefully on the inside of the penciled lines. First
make the two long cuts and then the two short cuts.
After the rectangular opening has been cut out, use a
file to square the corners and smooth any irregularities in the cut edges.
each side, attach the PANLOC brackets to the cabinet panel; the short flange is mounted against the
front (face) of the cabinet panel. The screws pass
through the PANLOC bracket flange, the cabinet panel, and then through the mounting strips previously
mounted.
5. Install the Mounting Strips
In the hardware package you will find two mounting
strips and two sets of machine screws. For panels
that are less than ½ inch thick, use the ¾ inch
screws; for panels that are more than ½ inch thick,
use the 1 ¼ inch screws.
7. Install the Instrument
Guide the AC power cord through the panel opening
to the back of the cabinet; then, slide the instrument
into the opening carefully so that the rails on the bottom of each side of the chassis engage the tracks on
the mounting brackets. Continue to slide the instrument into the cabinet until it is stopped by the adjust
position latches. Press the latches inward, this permits the instrument to slide into the cabinet until its
front panel is flush with the cabinet panel. Depress
the PANLOC buttons at the lower left and right corners of the instrument panel to lock the unit firmly in
the cabinet. Depressing the PANLOC buttons again
will unlock the instrument so that it can slide forward
to the adjust position; if you press inward on the adjust position latches then you can remove the instrument from the cabinet.
Starting at the right-hand side of the panel, insert a
screw of proper length into the center hole in the
panel, marked B on the template. On the back of the
panel, align a mounting strip with the holes in the
panel and tighten the screw until the screwhead is
pulled slightly into the wood.
Repeat this procedure to attach the mounting strip
to the left side of the panel.
6. Attach the PANLOC Brackets
Using two screws of proper length in the A holes on
3
How to Connect and
Back Panel Information
Refer to the drawing on page 5.
AUDIO OUTPUTS
dipole to the ANTENNA 300
Use the FIXED OUTPUT jacks on the rear panel to
feed program to a stereo control preamplifier or other
equipment which has its own volume control. The
front panel VOLUME control does not affect the loudness of the tuner at the FIXED OUTPUT jacks. The output level is a nominal 1 volt for 100% FM modulation.
FM push connector.
The flexibility of the twin flat wire assembly permits
it to be placed under a rug, tacked behind the stereo
or placed in any other convenient location. In some
cases, it may be necessary to "position" the antenna
for best signal reception. This should be done before
it is permanently located.
Use the VARIABLE OUTPUT jacks to connect to
equipment such as a power amplifier or a tape recorder
where control of the volume at the tuner is desired. The
output at the VARIABLE OUTPUT jacks is a nominal 2.5
volts for 100% FM modulation. There is no difference in
the signal quality at either pair of output jacks.
Avoid locating the antenna next to other wires or
metal objects. Any indoor antenna may be ineffective
in houses having metal siding or metal foil insulation.
FM PRESELECTOR SWITCH
FM receivers can be overloaded by very large antenna input signals when the receiver is located very
near to a FM broadcasting station or when a high gain
directional antenna is used in a metropolitan area. An
example of overload is where a FM station can be tuned in at two or more places on the dial. A preselector
tuned circuit is built into the MR 75 to prevent
overload.
Both pairs of OUTPUT jacks may be used simultaneously. The output impedance at both outputs is
very low so that long audio cables can be used without a loss of high frequencies due to cable capacity.
CONNECTING AN FM ANTENNA
One of three antenna systems can be used: (1) an
outdoor FM antenna, or (2) an VHF-TV antenna, or (3)
the indoor dipole supplied.
The preselector may be switched into or out of the
FM RF circuit by a slide switch located on the rear
panel.
An outdoor antenna is recommended for optimum
performance in all areas. In fringe (outlying) areas,
best results will be obtained with a highly directional
FM antenna used in conjunction with a rotator. If the
antenna uses a 300 ohm lead, connect it to the
ANTENNA 300 FM push connectors.
We recommend the FM PRESELECTOR switch be
used in the "out" position for all FM listening conditions except where there is evidence of overload by
strong stations. In that case, the "in" position will
protect against overload. For most stations there will
be no listening difference between the two switch
positions.
The coaxial cable of an unbalanced 75 ohm antenna
connects to the rear panel ANTENNA 75ft FM type F
coaxial connector.
An outside antenna system should not be located
in the vicinity of overhead power lines or other electric light or power circuits, or where it can fall into
such power lines or circuits. When installing an outside antenna system, extreme care should be taken to
keep from touching such power lines or circuits as
contact with them might be fatal.
AM ANTENNA
For local and most moderately distant AM reception, the built-in ferrite loopstick antenna may be
used. The AM loopstick antenna is on a swivel base
and must be adjusted away from the chassis for best
reception. The illustration at the top of page six
shows examples of the many positions the AM antenna may be placed in. Try several positions until you
locate the best performing location for your listening
area.
A VHF-TV antenna can be effective when it is
designed for both FM and TV reception. Connect the
two leads from the VHF-TV antenna to the ANTENNA
300ft FM push connectors.
Distant reception can be improved with the use of a
copper antenna wire 50 to 150 feet in length. Suspend
the wire in a straight line as high as possible. Attach
the wire at each end with suitable glass or ceramic in-
The flexible folded dipole antenna (300 ohm) supplied with the MR 75 is for use in urban or high
strength signal areas. Connect the two leads from the
4
FM ANTENNA
ALTERNATE
AM ANTENNA
TO
EXTERNAL
GROUND
OPTIONAL
75 ANTENNA
LEAD-IN
TO AC
POWER
SOURCE
MAXIMUM PERFORMANCE
INDICATOR
OR
OSCILLOSCOPE
PREAMPLIFIER
POWER AMPLIFIER
LEFT SPEAKER
RIGHT SPEAKER
5
CONNECTING A MAXIMUM
PERFORMANCE INDICATOR
The scope vertical and horizontal jacks on the rear
panel are used to connect a Mclntosh maximum performance indicator or an oscilloscope. Follow directions outlined in the maximum performance indicator
owner's manual. The maximum performance indicator gives a display of signal strength versus frequency and allows adjusting a directional antenna for
the best multipath free FM reception condition.
UNSWITCHED AC POWER OUTLET
This outlet may be used to power auxiliary equipment such as an antenna rotator or a maximum performance indicator. Power is available whenever the
MR 75 is connected to a power source.
sulators. Connect a lead-in wire at any convenient
point on the antenna. It is recommended that a
lighting arrester be used with an outdoor AM antenna.
The arrester should be well grounded to a ground rod
or cold water pipe.
FUSE
A 0.5 AMP fuse protects the MR 75 circuits. The
fuse does not protect additional equipment connected to the back panel AC outlet.
Front Panel Information and
Use of Controls
The MR 75 Tuner has a flexible control system. Proper use of these controls will yield a high level of performance.
MUTE:
The mute switch has two positions: ON-background noise and
hiss heard between FM stations
is suppressed. OFF -inter-station
muting is defeated.
INPUT SELECTOR
The two position input selector
functions as follows:
AM: Connects the AM tuner circuits for AM reception.
FM: Connects the FM tuner circuits
for FM stereo or mono reception.
VOLUME CONTROL AND
AC POWER SWITCH
The VOLUME control has been
precision tracked throughout the
listening range (0 to - 6 5 dB) for
accurate stereo balance. It sets
the output level of the VARIABLE
OUTPUT jacks and the front panel
HEADPHONE jack. The FIXED OUTPUT jacks are not affected by the position of the
VOLUME control. The AC power switch is part of the
VOLUME control. Turning the VOLUME control fully
counterclockwise turns the AC power off.
MODE:
The mode switch has two positions: STEREO - In this position
the tuner automatically switches
from FM mono to FM stereo depending on the type of broadcast
being received. MONO - In this
position of the mode switch, all
broadcasts are received monophonically.
6
FRONT PANEL INDICATORS
The MR 75 has three dial scales: 1. AM - marked 550
to 1600 kHz; 2. FM - marked 88 to 108 MHz; 3. Logging
scale 0 to 100.
As the tuning knob is turned, the pointer moves
across the dial. There are two LEDs on the pointer.
When you have selected FM, one LED glows behind
the FM scale, on AM the other glows behind the AM
scale.
The Logging Scale is useful for retuning to your
favorite station with great accuracy.
easy but can cause the MR 75 to not return to the FM
station when power is turned off and on again. This
condition can be avoided if, when tuning in a station,
you stop turning the Tuning Knob as soon as the
center bar indicator lights. Then the MR 75 will return
to the same station when power is turned on again.
TUNING KNOB
The Tuning Knob selects AM or FM stations. An AM
station is tuned in correctly when the vertical column
signal strength indicator shows a maximum. An FM
station is correctly tuned when the center bar indicator at the bottom of the signal strength column illuminates and the signal strength display is at a maximum. There are indicators marked + and - on each
side of the FM center bar. These light as you approach FM stations and indicate if you are above ( + )
or below ( - ) the station. When the center tuning bar
lights, the FM AFL (automatic frequency lock) circuit
is active and the tuner is locked to the FM station.
This locked condition will give the best FM reception
with lowest background noise, lowest distortion and
best stereo separation.
The remaining LED marked "mute" indicates when
the mute switch is in the "on" position.
The bar at the top indicates reception of a FM
stereo multiplex (MPX) station. The 10 little dots arranged in a vertical column indicate relative station
signal strength for AM or FM. The three bars at the
bottom of the display indicate if you are tuned above
( + ), below (-), or right on-station when receiving FM.
When tuning a FM station, the proper adjustment is
when only the center on-station vertical bar indicator
lights.
Because of the locking action of the AFL circuit,
the Tuning Knob can be tuned mechanically slightly
away from the frequency of the FM station without
detuning the MR 75. This makes tuning the MR 75 very
HEADPHONE JACK
A HEADPHONE jack is provided to drive low impedance dynamic headphones from an internal power
amplifier.
INPUT INDICATORS
At the bottom right of the tuning dial area are three
LED lamps; two indicate the status of the SELECTOR
switch. The illuminated LED indicates whether AM or
FM is selected.
SOLID STATE TUNING INDICATOR
The MR 75 utilizes an all solid state tuning indicator. Fourteen LEDs make up the display, which
consists of four bars and 10 small dots.
7
Performance Limits
Adjacent Channel Selectivity
30 dB minimum IHF
Performance limits are the maximum deviation from
perfection permitted for a Mclntosh instrument. We
promise you that when you purchase a new MR 75
from a Mclntosh Franchised dealer it will be capable
of performance at or exceeding these limits or you
can return the unit and get your money back.
Mclntosh is the only manufacturer that makes this
statement.
Image Rejection
65 dB minimum from 550 kHz to 1600 kH
GENERAL INFORMATION
Audio Output
Variable: 2.5V into 5k
Fixed: 1V into 5k
FM SECTION
Tuning
88 to 108 MHz
Audio Hum
75 dB down from 100 % modulation
Antenna Inputs
One 300ft balanced and one 75
Power Requirement
120 Volts 50/60 Hz, 20 Watts
unbalanced.
Semi
22
4
23
15
19
2
Intermediate Frequency
10.7 MHz
Usable Sensitivity
2µM (11.2dBf) IHF
Quieting Sensitivity
5µV (19.1dBf) IHF - 5 0 dB (Mono)
50µV (39.5dBf) IHF - 5 0 dB (Stereo)
Conductor Complement
Bipolar Transistors
Field Effect Transistors
Silicon Diodes
Integrated Circuits
LED's
Bridge Rectifiers
MECHANICAL INFORMATION
SIZE:
Front panel measures 16 inches wide (40.6 cm) by 5
7/16 inches high (13.8 cm). Chassis measures 14 3/4
inches wide (37.5 cm) by 4 13/16 inches high (12.2 cm)
by 13 inches deep (33 cm), including connectors.
Knob clearance required is 1 1/4 inches (3.2 cm) in
front of mounting panel.
Signal To Noise Ratio
70 dB IHF minimum both Mono and Stereo
Harmonic Distortion
0.18% (Mono) IHF maximum
0.38% (Stereo) IHF maximum
Alternate Channel Selectivity
75 dB IHF minimum
FINISH:
Front panel is anodized gold and black with special
gold/tea! nomenclature illumination. Chassis is black.
Image Rejection
100 dB IHF minimum
Stereo Separation
45 dB minimum at 1 kHz
MOUNTING:
Exclusive Mclntosh developed professional PANLOC.
Audio Frequency Response
20 Hz to 15 kHz + 0 , - 1 dB
WEIGHT:
23 pounds (10.43 kg) net, 35 pounds (15.88 kg) in
shipping carton.
Capture Ratio
1.8 dB
Spurious Rejection
100 dB IHF minimum
SCA Rejection
60 dB minimum
AM SECTION
Sensitivity
75µV IHF with external antenna
Signal To Noise Ratio
45 dB minimum IHF or 55 dB at 100% modulation
Frequency Response
+ 0 - 6 dB from 20 Hz to 3500 Hz
Harmonic Distortion
0.8% maximum at 30% modulation
8
Performance Charts
FM SIGNAL PERFORMANCE
0
RELATIVE OUTPUT IN dB
-10
-20
-30
-40
-50
-60
-70
RESPONSE AND SEPARATION IN dB
HARMONIC DISTORTION IN PERCENT
0.1
1
10
100
SIGNAL INPUT IN MICROVOLTS
1k
10k
F.M. HARMONIC DISTORTION
100% MODULATION
0.5
0.4
0.3
0.2
0.1
0
20
100
1k
FREQUENCY IN HERTZ
10k
20k
10k
20k
F.M. FREQUENCY RESPONSE AND
STEREO SEPARATION
+ 10
0
-10
-20
-30
-40
-50
20
100
1k
FREQUENCY IN HERTZ
9
A.M. FREQUENCY RESPONSE
80% Modulation
RELATIVE OUTPUT IN dB
0
-10
-20
-30
-40
20
100
1k
10k
20k
RF INPUT USING DUMMY ANTENNA IN MICROVOLTS
FREQUENCY IN HERTZ
AM MINIMUM USABLE
SENSITIVITY (IHF)
120
100
80
60
40
20
0
500
600
700
800
900
1000
1100 1200
1300 1400
1500
FREQUENCY IN kHz
RELATIVE OUTPUT IN dB
AM SELECTIVITY
AT 1000 kHz
-20
-40
-60
-80
-100
970
980
990
1000
1010
FREQUENCY IN kHz
10
1020
1030
1600
1700
Technical Description
special systems, an automatic variable separation
control circuit to reduce background noise when
receiving weak stereo stations, and tri-level digital
waveform generation which eliminates interference
from SCA signals and from the sidebands of adjacent
channel FM signals.
FM SECTION
The radio frequency (RF) assembly houses the com
plete FM-RF front end and part of the AM-RF circuitry.
An eight section variable capacitor is the heart of the
RF assembly. Five sections, 1, 2, 4, 6 and 8 are used
for FM and three sections, 3, 5 and 7 are for AM. Sections 1 and 2 of the capacitor are for the FM antenna
RF input circuits. Section 2 is always in use, however,
if additional RF selectivity is desired to protect
against strong signal overload, additional preselector
filtering is accomplished by adding section 1. This is
accomplished by operation of the preselector switch
on the rear panel. This switch directs a DC voltage
that controls PIN semiconductor diodes to perform
the RF switching functions electronically.
The variable separation control is operated from
the IF amplifier's signal strength detector. A smooth
transition is provided from mono to stereo or from
stereo to mono at weak signal levels to provide the
optimum signal to noise ratio and best stereo separation for the prevailing signal conditions. The circuit
operates only during stereo reception. It switches
automatically to monophonic if the 19 kHz pilot tone
is absent.
A MOS-FET RF amplifier follows section 2 to provide exceptional sensitivity while maintaining superior cross-modulation rejection over a wide dynamic
range. Capacitor sections 4 and 6 follow the RF amplifier to increase selectivity and to provide correct impedance matching for the following fully balanced
MOS-FET mixer.
In the PLL the internal oscillator operates at 228
kHz, locked onto the 19 kHz pilot tone. The 228 kHz
feeds a 3 stage Johnson counter via a binary divider
to generate a series of square waves. Suitably connected NAND gates and exclusive OR gates produce
the tri-level drive waveform for the various
demodulators in the circuit. The usual square
waveforms have been replaced in the PLL and
decoder sections by tri-level waveforms. These trilevel forms contain no harmonics which are multiples
of 2 or 3. This eliminates frequency translation and
detection of interference from the side-bands of adjacent stations since the third harmonic of the subcarrier (114 kHz) is excluded. It also eliminates interference from SCA broadcasts since the third harmonic of the pilot tone (57 kHz) is excluded. Unwanted spurious audible components and phase jitter
in the PLL are inherently eliminated by this technique.
The local oscillator is tuned by capacitor section 8.
A MOS-FET buffer amplifier is employed between the
oscillator and the mixer. The high input impedance of
this amplifier insures that oscillator loading is
minimal, therefore, oscillator stability is greatly
enhanced. The oscillator is fine tuned by a varactor
diode operated by a correction voltage generated by
the Automatic Frequency Lock (AFL) circuit. AFL is a
Mclntosh patented circuit that turns on a lock voltage
when manual tuning reaches the center of the FM station carrier. This AFL tuning aid insures the tuning is
accurate for minimum distortion and best performance.
The FM-IF system that follows the mixer consists of
five integrated circuit amplifiers and four piezoelectric
filters. They combine to give a total amplification of
140 dB. The IF response characteristic is very selective, being 170 kHz wide at -3 dB and only 500 kHz
wide at - 5 0 dB. The response is symmetrical about
the center frequency of 10.7 MHz. The IF filters are permanently sealed and cannot drift nor vibrate out of adjustment. The exceptionally high gain of the IF system
assures proper limiting at all input signal levels. A
Foster-Seely discriminator completes the IF system.
The detected output of this discriminator has a high
signal to noise ratio and extremely low distortion. This
output feeds to the FM stereo multiplex section.
Additional advantages of the phase locked loop
stereo demodulation are the elimination of inductors
to minimize drift, integral lamp driving capability to
indicate the presence of the 19 kHz pilot carrier, excellent channel separation over the entire audio frequency range, extremely low distortion, low output
impedance, and transient-free mono/stereo switching.
After multiplex detection, 19 kHz pilot and 38 kHz
carrier suppression circuits are used to prevent tape
recorder interference.
The FM muting circuit is unusual. It operates both
by detecting ultrasonic noise and by sensing correct
tuning of the detector circuit. To 'un-mute' it is
necessary for the signal to have an adequate signalto-noise ratio and to be tuned to the center of the FM
carrier. The MUTING circuit can be activated or defeated by the front pane! muting switch. The muting
of the audio signal is done with FET analog switches.
FM STEREO MULTIPLEX
The heart of the multiplex section is a new third
generation phase locked loop (PLL) stereo decoder integrated circuit (IC). This PLL IC incorporates two
11
AM SECTION
The AM-RF amplifier circuit includes a three section variable tuning capacitor in a metal enclosure.
This three section variable capacitor is used for
greater spurious rejection. The Mclntosh AM-RF amplifier circuit is unique. It has constant sensitivity, constant selectivity and high image rejection across the
complete AM band. Ordinary AM-RF circuits cannot
do all of this simultaneously. This design achieves
equal sensitivity even down to the low end of the
band. Spurious, image and intermediate frequency rejection are all superior. The same circuit delivers
equal selectivity across the entire band. Thus, there is
no loss of audio frequency response at the low end of
the band. Another advantage of the Mclntosh circuit
is freedom from cross modulation and overloading by
strong local stations.
system was designed to prevent bursting or thumps
when the AM is tuned through a strong signal. Distortion at low audio frequencies is minimized by using a
two-section AVC filter instead of the conventional
single section.
OUTPUT/HEADPHONE AMPLIFIER
The output audio amplifier is a push-pull complementary class AB amplifier using a signal inverting differential stage at its input. This amplifier
drives the variable output and the headphone jack.
TURN ON DELAY
A turn-on delay circuit is located ahead of the output amplifier. This circuit uses a light emitting
diode/light dependent resistor network that functions
to transmit no signal until two seconds after the
power switch is turned on and to remove the output
signal almost instantly upon power turn-off. Thus the
turn-on and turn-off is transient and noise free.
A loopstick antenna is provided which can be
rotated for maximum sensitivity over 180 degrees.
Each MR 75 loopstick is individually tuned. This
custom matching of the loopstick to the AM-RF front
end maximizes the performance of the antenna. A
rear panel antenna connector is provided for connecting an external antenna if desired.
POWER SUPPLY
To minimize hum, radiation, and thus improve
signal to noise ratio the MR 75 power transformer is
triple shielded. Shielding includes a copper strap, a
silicon steel strap, and finally a steel outer shell. The
transformer secondary output voltage is fed to a full
wave bridge rectifier and 2,200 microfarad filter
capacitors to provide the +24 volts for powering the
plus regulator and - 24 volts for the minus regulator.
The ± 18 volts needed for low level amplifier stages is
supplied by integrated circuit voltage regulators.
To maintain the excellent image rejection and spurious cross modulation rejection of the AM-RF amplifier, an autodyne circuit is used for the AM mixer. The
AM-IF uses two double tuned IF transformers designed to obtain a high degree of selectivity yet allowing
good audio fidelity. A 10 kHz active filter eliminates
the whistle and irritating "monkey chatter" caused by
an adjacent station. The automatic volume control
12
Block
Diagram
13
McINTOSH MR 75 AM/FM TUNER
McINTOSH LABORATORY INC.
2 CHAMBERS ST., BINGHAMTON, N.Y. 13903
607-723-3512
The continuous improvement of its products is the policy of
Mclntosh Laboratory Incorporated who reserve the right to
improve design without notice.
Printed in U.S.A.
039281
BE112003
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement