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Page 1
The Mclntosh MR-55 is a superior quality AM-FM Tuner for, use in home music systems and profes-
sional applications where distortion free reception is needed. Typical professional uses are "off the air" re-
cording, broadcast monitor, rebroadcasting, etc.
A thorough perusal of this instruction manual will give greater enjoyment and better listening. The MR-55
is the finest tuner available and complete familiarity with its functions and operation will result in the out-
standing performance you expect from a Mclntosh product.
SPECIFICATIONS:
AM
Sensitivity:
1.5 microvolts
Selectivity:
Narrow Position: 4 K.C. Bandwidth, ± 10 K.C. from
center down 53 D.B.
Medium Position: 13 K.C. Bandwidth, ± 10 K.C. from
center down 20 D.B.
Broad Position: 20 K.C. Bandwidth
(Measurements include R.F. and LF. circuits. Charac-
teristics substantially unchanged over entire R.F. tun-
ing range.)
A.V.C.:
Exceptionally strong; less than 4 D.B. audio output
change with input changes from 10 microvolts to
100,000 microvolts.
Distortion:
Less than 1% at 100% modulation
Hum:
50 D.B. below full signal
Bandwidth:
LF. at 600 K.C.-20 K.C.; at 1600 K.C.-20 K.C.
R.F. at 600 K.C.-21 K.C.; at 1600 K.C.-23 K.C.
Audio Bandwidth:
Broad Position ± 3 D.B.-20 to 10 K.C.
Medium Position ± 3 D.B.-20 to 6.5 K.C.
Narrow Position ± 3 D.B.-20 to 2.0 K.C.
Sensitivity Selector:
Three Positions
Front End:
21 K.C. Constant bandwidth, band pass input
Whistle Filter:
70 D.B. rejection 10 K.C.
FM
Sensitivity:
3 Microvolts at 100% modulation (±* 75 K.C.
Dev.) for less than 3% total noise and distortion
(Based on I.H.F.M. (proposed) standards for
measuring F.M. Tuners)
Capture Ratio:
1 to 0.8
I.F. Amplifiers:
Four
I.F. Bandwidth:
200 K.C.;Flat on Top
I.F. Transformers:
Mechanically captive
Limiters:
Two; zero time constant grated-beam followed by
a 0.5 microsecond pentode limiter
Limiter and Detector Bandwidth:
2 Megacycles
Frequency Response:
± 3 D.B.; 20-20,000 cycles
Hum:
75 D.B. below full signal
A.F.C.:
Separate detector; strong, distortion free, com-
pletely variable
Drift:
± 30 K.C. without A.F.C.: negligible with A.F.C.
Antenna Input Impedance:
300 ohm balanced; 72 ohm unbalanced
Radiation:
Substantially below F.C.C. requirements
TUBE
V1-6BZ7 F.M. R.F. Amplifier
V2-6BZ7 F.M. Reactor and F.M. Os
V3-6AB4 F.M. Mixer
V4-6BJ6 1st LF. Amplifier
V5-6AU6 2nd I.F. Amplifier
V6-6Bf6 3rd I.F. Amplifier
V8-6AU6 4th LF. Amplifier
V7-6AL5 A.M. Detector and A.V.C. Detector
V9-6AL5 P.M. A.V.C.
V10-6BN6 1st Limiter
COMPLEMENT:
V11- 6AU6 2nd Limiter
V12-6AL5 F.M. Detector
V13-6BV8 AFC Detector
V14-6BA6 A.M. R.F. Amplifier
V15-6BE6 A.M. Converter
V16-6AL5 A.V.C. Clamper
V17-12AU7 Squelch Amplifier and F.M.
Metering
V18-12AU7 Audio Amplifier and Cathode
Follower
SIZE:
4¾ Inches High x 14¾ Inches Wide x 12 Inches Deep
WEIGHT:
Tuner Only: 17 Pounds, 3 Ounces; In Shipping Carton: 28 Pounds
-2-
Page 2
INSTALLATION INSTRUCTIONS FOR THE
MclNTOSH MR-55 AM-FM TUNER
The MR-55 may be mounted in its own cabinet or shelf mounted in a custom cabinet. The tuner has been
designed to mount on a shelf rather than hang from the cast tuner panel. Rubber shock mounting feet have
been provided to offer the optimum in mechanical isolation from extraneous vibrations.
Enclosed in the instruction manual envelope are two copies of the "TUNER PANEL CUTOUT TEM-
PLATE" and two copies of the "SHELF MOUNTING TEMPLATE" for mounting the MR-55. Prepare the
cabinet for the mounting of the MR-55 by first cutting the wooden cabinet panel to receive the cast tuner
panel.
1. On the back of the wooden cabinet panel make a vertical center line on the exact center of the area
in which the tuner will mount. Extend this center line to the shelf.
2. Fold the "TUNER PANEL CUTOUT TEMPLATE" on the "FOLD LINE." The "FOLD LINE" is
the line of the top of the shelf on which the tuner will mount.
3. Place the "TUNER PANEL CUTOUT TEMPLATE" in the proper location on the inside of the cabi-
net with the fold line against the shelf. The template center line should match the center line scribed on
the panel in step 1.
4. In the center of the "Cutout Area" on the "Tuner Cutout Template" arc two holes marked "LOCA-
TION HOLES." Carefully and accurately drill these two holes with a 5/32 inch drill.
5. Transfer the "TUNER PANEL CUTOUT TEMPLATE" to the front of the wooden cabinet panel.
Match the "LOCATION HOLES" for the proper positioning of the template. Proceed with marking the lo-
cation of the six holes and the outline of the cutout on the front of the wooden cabinet panel.
IMPORTANT: DRILL THE FOUR 3/16 INCH AND THE TWO 5/32 INCH HOLES BEFORE
CUTTING OUT THE TUNER PANEL HOLE.
The cast tuner panel has a 1/4 inch overlap to cover the edges of the cutout in cabinet panel.
Next prepare the shelf for mounting the tuner.
1. Measure the thickness of the wooden cabinet panel.
2. "FOLD LINES" have been supplied on the "SHELF CUTOUT TEMPLATE" for standard pane!
thickness up to one inch. Fold on correct fold line for your wooden panel thickness.
4. Place the folded edge of the template against the back of the front panel. The template center line
must match the center line extended to the shelf in step #1 of the front panel cutout instructions.
5. Carefully mark the locations of the four 1/4 inch holes and the 8 by 10 1/4 inches, ventillation hole.
6. Drill and cut out the shelf.
Next prepare the MR-55 for mounting in the cabinet by removing the pressed wood shipping pallet from
the bottom of the tuner.
Remove the knobs by gently pulling them toward you. Remove the felt washers.
The cast tuner panel has been attached to the tuner by two angle brackets that are for shipping pur-
poses only. These are removed by unscrewing the four screws (two on each side) that hold the angle brackets
to the tuner chassis. After removing these four screws the tuner panel can be slipped off over the knob shafts.
After removing the tuner panel lay it face down on a protected surface for the following operations.
Remove the four 6/32 screws (one on each corner) that hold the two shipping brackets to the tuner panel
-3-
Page 5
FM.
TO
300ohm
FM
ANT.
A substitute for the folded dipole can be an existing VHF T. V. antenna. The MR-55 is supplied with
an internal switching arrangement that eliminates the need for two set couplers. The VHF T. V. antenna
lead-in is connected to the antenna terminals marked "FM. ANT'." Connect the terminals marked "TO T. V.
SET" to the antenna terminals on the T. V. receiver with 300 ohm antenna lead. When the MR-55 selector
switch is in cither of the "FM" positions the antenna is disconnected automatically from the T. V. receiver.
In any other position of the selector switch the antenna is connected to the T. V. set and disconnected from
the tuner.
TO 300 ohm VHF TV.
ANT.
TO
T.V.
SET
-6-
Page 6
75 OHM UNBALANCED
ANTENNA INPUT
75 OHM ANTENNA: The MR-55 has provision for the connection of a 75 ohm unbalanced antenna.
The 75 ohm terminal screws are next to the chassis on the back of the brown bakelite balun card located
approximately 2 inches behind the tuning knob flywheel. The center conductor of the 75 ohm coaxial lead
is attached to the screw terminal located 3/8 inch above the chassis. The outer conductor or ground side is
connected to the screw terminal located close to the chassis. Solderless spade lugs are supplied for your con-
venience.
-7-
Page 13
FM
-
Generator
CW-
Oscillator
98MC- 75
KC
50cps 90MC-
CW-Oscillator
Preferably
Battery
Operated.
With
Generator
Set the
Level
at
98MC
Increase
Over,
all
Gain
With
External Amplifier
If
Necessary. Then
With
CW-Oscillator
at
90MC.
Measure Hum. Then
Switch
to
Phono
and
Measure
Hum
With
Phono
Input
Shorted
With
1Mohm.
Measure Distortion.
Listen
88-98-108MC.
75 KC.
Deviation
400cps.
Decrease FM-Generator Output
for 3%
Distortion.
Reading
of
Generator
Then
Is
Measure
of
Overall
Sen-
sitivity.
Increase
Generator
Output
Until
1
Div.
or
Sensitivity
Meter
Is
Reached. Generator Output Reading
Now is
Measure
of
Gain
in RF and IF
Stages.
Tune/Listen
FM
-
Generator
98MC
75KC.
Deviation
400cps.
AFC-Conrrol
Fully
Clockwise
Distortion Analyser
Audio
Output
R81 and
Upper
Slug
of T12 for
Center
of
FM-Meter
Vary
Tuning
Knob
Until
FM-Meter
Remains
Stationary.
For
Tune
and
Listen
Positions
With
R81, Bring FM-Meter
to
Center,
With
Selector
Switch
in
Tune
Position.
Decrease
Output FM-Generator
Until
3%
Distortion
and
Noise
Is
Found. Then
in
Listen
Position, Adjust
T12
for
Minimum
Distortion
and
Noise.
Overall
Tune
FM
-
Generator
Center Freq's.
88-
108MC.
75 KC.
Deviation
50cps.
Thru Balun
to
Antenna
Input,
or
75ohm
Input
With
Termi-
nating
Resistors
Oscilloscope, Synchronized
to
Input Sweep
Test Point
T3 >
Upper
and
Lower
Slugs
T5 > for
Best
Response
Curve
Vary
Freq.
From
88 to
108MC.
and
Follow
With
Tuner,
for
Gesf
Response
Curve
Over FM-Band.
Give
Special
Attention
to
96.3MC
for
Interference
of 9 x
10.7MC.
IF-Strip
Tune
CW
- Oscillator
10.7MC.
Point
(R)
(Pin 6-V3)
Oscillator
Coupled
to (H)
Thru
.0! MF
Condenser
and
Terminating
-Resistance.
V.T.V.M.
Test
Point
T2
T3
Upper
and
lower
Slugs
T5 for
Maximum
T7
T9
Defeat AVC-Action
at
Point
J
(R36-R38)
Load
Transformer-Winding
No!
Under Adjustment
-
with
10Kohm
Resistor
Front-End
Tune
SWEEP
GENERATOR
CW-
Oscillator
Center Freq's.
88-
108MC.
90 and
105MC.
Thru Balun
to
Antenna
input
or
75ohm
input
With
Termi-
nating
Resistors
Any
Suitable
Injeclion-
Point
To Be
Used
as 90-
105MC,
Marker Points
Oscilloscope, Synchronised
to
Input
Sweep
FM-Tuning
Meter
Point (G)
( Pin
7-V3
)
L9 - C18 - L10 -
C19:
For
Best
Response,
Keeping 90-1 05MC.
Markers
in
Corres-
pondence
With
the 90 -
I05MC.
Dial
Points
L11 -
C23:
For
Calibrating
the
Tuner
FM-
Local-Oscillator
to 90 -
105MC.
Dial
Points
Adjustments
of
LP-C18-L10-C19
and
L11-C23
Are
Inter
Related,
Therefore Both
Series
of
Adjustments
Should
Be
Checked
and
Rechecked.
Disable
the
Oscillator When Adjusting
L9-C18-L10-C19
Point
(E)
(Pin
5-T12)
T12:
Lower
Slug
for
Man.
R81 : For
FM-Tuning-
Meter
Center
TI2:
For
FM-Tuning-Meier Center
Ground
Point
F (
High
Side R82)
10.7MC
Point
(C)
(Pin
7-V12]
Point
(D)
(R70-C88)
T11 :
Lower
Slug
for
Max.
Upper
Slug
for
Zero
10.4MC.
T10:
Upper
Slug
for
Max.
Lower Slug
for
Max.
Load Primary
With
1.5Kohm
Resistor
Load Secondary
with
1.5Kohm
Resistor
Limiters,
Detector
and
Audio
Tune
CW-
Oscillator
10.7MC.
Point
(A)
(Pin
5- V9)
Oscillator Coupled
to (A)
Thru
.01 MF
Condenser
and
Terminating
Resistor,
V.T.V.M.
Point
(B)
(Pin 1-V11)
R63:
For
Max.
100Kohm
Resistor
in
Series
With
D.C. Probe
of
V.T.V.M.
CIRCUIT
UNDER
TEST
POSITION
OF
SEL SW.
SIGNAL
GENERATOR
FREQUENCY
AT
SIGNAL-SOURCE
REMARKS
MEASURED
WITH
AT
ADJUST
SIGNAL-OUTPUT
REMARKS
F.M.