McIntosh MC24O Owner's Manual

Page 1
STEREO POWER
AMPLIFIER
MC24O
CONTENTS
GENERAL DESCRIPTION 1 TECHNICAL DESCRIPTION 1
PANEL FACILITIES 4
INSTALLATION 5
Input—Stereo 5 Input—Twin Amp 5 Input—Mono 6
Output—Stereo or Twin Amp 6
Output—Mono 6 Line Output—Stereo or Twin Amp 6 Line Output—Mono 7
AC Power 7
OPERATION 7 ADJUSTMENTS 7
GUARANTEE 8
OWNER'S MANUAL
MC24O
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Page 3
MC240
STEREO
POWER
AMPLIFIER
GENERAL DESCRIPTION
Over one million watts of amplifier output
power capacity have been manufactured by Mclntosh since 1947. In this 1,000,000watts of audio power there are less than 10 watts of distortion capacity and less than 1/100 watt of noise capacity!
The dramatic difference in the quality of music reproduction when you listen through Mclntosh instruments is directly due to low distortion performance. Careful, devoted research is a way of life at Mclntosh. The
world's finest amplifier is the creation of per­severing, resourceful Mclntosh engineers. Mclntosh Laboratory is the only manufac­turer in the entire industry to guarantee the
lowest distortion at all audio frequencies, at
full power. The U.S. Patent Office has rec­ognized the advanced technology of the
Mclntosh circuit by granting 6 patents.
1
Long life, flexibility, highest quality con­struction are characteristic designs in every Mclntosh instrument. Wide electrical and
thermal margins of safety for all components and tubes, advanced engineering, and cool operating design add to the long life built
into every Mclntosh product. Reliability pro-
longs your investment without expensive maintenance costs.
The MC240 has on one chassis two 40
watt power amplifiers. In addition to use as a
stereo amplifier, the flexibility of the MC240 permits it to be used as a monophonic
amplifier that delivers 80 watts, or as two
separate 40 watt amplifiers with each chan­nel amplifying completely separate pro-
grams, or as two amplifiers for use with an
electronic crossover network. Such flexi­bility permits maximum use for greatest return from your investment.
TECHNICAL DESCRIPTION
The patented Mclntosh Unity Coupled circuit and output transformer have estab­lished Mclntosh amplifiers as the unchal­lenged leaders in the audio field.
Before 1947, low distortion at high power
and high efficiency was impossible. A com-
pletely new engineering approach resulted in an amplifier that for the first time permitted high power with distortion below 1%. That new engineering produced the Mclntosh Unity coupled circuit and the Mclntosh bifilar wound output transformer. With the intro­duction of the Mclntosh amplifier new stand­ards for distortion-free performance were established.
turn of primary number one is next to the
same turn of primary number two. There is almost complete magnetic coupling between the two wires. The magnetic coupling is rein­forced by the capacitance between the two wires.
INPUT
OUTPUT
Fig. 1 —Representation of bifilar winding.
The Mclntosh output transformer is
unique. It has two primary windings which are
wound bifilarly. In the bifilar technique both
primary wires are wound side by side. Each
Fig. 2 —Representation of Mclntosh output circuit.
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U.S. Patent No's: 2,477,074; 2,545,788; 2,646,467; 2,654,058; 2,860,192; 2,929,028.
Page 4
In the Mclntosh Unity Coupled circuit one of the bifilar primary windings is connected through the power supply to the plate and cathode of one of the output tubes. The other bifilar primary winding is similarly connected
to the other tube.
All low distortion high power amplifiers
use push-pull output circuits known as Class
AB1, AB2 or B. Two tubes are arranged in a
balanced circuit. This permits each tube to
operate alternately somewhat over half the time. Compared to full time operation of the tubes, the push-pull method reduces heating and permits more power from a given type of tube. Despite this advantage of the conven­tional push-pull circuit one problem in par­ticular remained to be solved. When current in each tube is cut off to begin the idle period distortion is produced at the instant of cut off and again at the instant when current flows. This form of distortion is known as Notch Distortion and was well illustrated by Mr. Pen Tung Sah in the "Proceedings of the
I.R.E." Volume 24, pp 1522-1541 in 1936.
Imperfect coupling between the primary windings found in all conventional output transformers produces the condition which
permits notch distortion. Trying to improve coupling in a conventional transformer de­creases the power response at both low and high frequencies, heating the output tubes and lowering the available power output.
The Mclntosh Unity Coupled output cir­cuit and bifilar transformer is the first com­mercial breakthrough that eliminates notch distortion by coupling both output tubes almost to perfection. In the Mclntosh trans­former the extremely close coupling of the bifilar windings removes the condition which permits notch distortion. Furthermore the the two output tubes are arranged as partial cathode followers. Half of the output circuit is in the cathode and half in the plate of each tube. The output tubes now are operating in a local feedback loop which, reduces their
distortion, reduces their internal generator
resistance, and reduces their balance
requirements. The Mclntosh circuit in reality perfects push-pull high efficiency output circuits.
Leakage inductance (lack of coupling) between the primary and secondary wind­ings of the output transformer limits the high frequency response of an amplifier. The primary and secondary windings of the Mclntosh output transformer are interleaved
five times to improve coupling. The inter­leaving is accomplished by winding groups of
primary layers, then secondary layers alter­nately until the total transformer is wound. Interleaving helps to extend the Mclntosh power band width to over 100,000 cycles.
Since 1947 this new technology has been built into every Mclntosh power amplifier.
Good voltage regulation in the power supply permits overloads without overshoot or blocking, good transient response, and complete stability. To improve regulation a silicon rectifier power supply is used in the MC240. In addition to better voltage regula­tion, the silicon rectifier allows even higher operating efficiency, cooler operation, and
longer amplifier life.
To greatly extend tube and component life a thermistor in the MC240 limits current surges produced when the equipment is turned on. The thermistor is a special type of resistor. Its resistance depends on its tem­perature. When the amplifier is off the therm­istor has a high resistance value (about 79 ohms). Just after the amplifier is turned on the current which flows through the therm­istor heats it and causes its resistance to decrease to a low value (less than .1 ohms). Current is thus limited when the MC240 is first turned on but is not limited as the unit warms.
ELECTRICAL SPECIFICATIONS
Power Output:
Stereo or twin amplifier—40 watts con­tinuous per channel Monopnonic—80 watts continuous
Harmonic Distortion:
Less than 0.5% at rated output or less,
20 cycles through 20,000 cycles. Typical performance is 0.3% or less at 20 cycles and 20,000 cycles at full power. Typical performance in the mid frequencies is
0.1% or less at full power.
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Intermodulation Distortion:
Less than 0.5% for any combination of frequencies from 20 cycles through 20,000 cycles if instantaneous peak power is below twice the rated power.
Phase Shift:
Less than +6° 20 cycles through 20,000
cycles
Frequency Range:
At rated output: +0, -0.1 db 16 cycles through 40,000 cycles +0, -0.5 db 16 cycles through 60,000 cycles At one-half rated output: +0, -1 db 10 cycles through 100,000
cycles
Noise and Hum:
90 db or more below rated output
Output Impedance:
Stereo or Twin Amp:
(Isolated from ground) 4 ohms, 8 ohms, 16 ohms
(Center tap internally grounded) 600 ohms (One side internally grounded) 125 ohms
Mono:
(Isolated from ground) 2 ohms, 4 ohms, 8 ohms, 16 ohms, 32 ohms
(Center tap internally grounded) 300 ohms (One side internally grounded) 62 ohms
Output Voltage:
Stereo or Twin Amp:
(Isolated from ground) 25 volts
(Center tap internally grounded) 140
volts
(One side internally grounded) 70.7
volts
Mono:
(Isolated from ground) 25 volts
(Center tap internally grounded) 140 volts (One side internally grounded) 70.7 volts
Internal Impedance:
Less than 10% of rated load impedance
Input:
Input impedance—250,000 ohms
Input sensitivity—0.5 volts to 30 volts through gain control with input switch in MONO or TWIN AMP position. With the switch in the STEREO position, sensi­tivity is 2.0 volts through a balance control.
Power Requirements:
105 volts to 130 volts at 50/60 cycles:
145 watts at zero signal output: 270 watts at rated output. The power trans­former is tapped for 117 volts and 125 volts.
Tube Complement:
Voltage Amplifier—One 12AX7 (drives
both amps)
Phase Inverter—Two 12AU7
Drive Amplifier—Two 12BH7 Cathode Follower Drive—Two 12AX7 Output—Four 6L6GC or 7027A
MECHANICAL SPECIFICATIONS
Dimensions:
Chassis dimensions: 17¼" long x 10¾" wide x 8" high. (See INSTALLATION on
Page 5 for minimum mounting space.)
Weight:
Chassis only—56 pounds. In shipping carton—65 pounds.
Finish:
Chromium and Black
Accessories Supplied:
Owners Manual
Mounting Template Mounting Flanges
Octal output plug with cap
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PANEL FACILITIES
Fig. 3—The end panel of the MC240 showing all input and output facilities.
INPUT
The input of the MC240 has a three posi­tion lever switch to permit the amplifier to be used in any one of three ways:
(1} As a monophonic 80 watt amplifier (2) As twin 40 watt amplifiers used with an
electronic crossover network, or two completely separate amplifiers, or in stereo applications requiring 0.5 volt sensitivity in the power amplifiers
(3) As a stereo amplifier used with a
Mclntosh preamplifier or other high output stereo program sources.
The input marked STEREO has a sensi­tivity of 2.0 volts controlled by a BALANCE control. The input impedance is 250,000 ohms.
The input marked TWIN AMP has a sensi­tivity of 0.5 volt. Each TWIN AMP input is controlled by a GAIN control. The GAIN con-
trol allows signal sources with output from
0.5 volt up to 30 volts to be connected with­out overloading the input to the amplifier. The input impedance is 250,000 ohms.
The input marked MONO has a sensitivity
of 0.5 volt. The MONO position of the input
switch parallels the inputs to both ampli-
fiers. When the outputs are properly con-
nected the MC240 becomes an 80 watt monophonic amplifier. The MONO input has
a GAIN control that permits connecting signal sources up to 30 volts without overloading the amplifier's input. The input impedance
is 250,000 ohms.
OUTPUT
The two barrier terminal strips marked
OUTPUT provide stereo connections for the
normal speaker impedances of 4 ohms, 8 ohms, and 16 ohms. For monophonic oper­ation connections for 2 ohms, 4 ohms, 8 ohms, 16 ohms, and 32 ohms are provided.
The terminal strips may also be connected
for a constant voltage output of 25 volts in either stereo or mono.
The secondary of the Mclntosh output transformer may be operated either grounded or not grounded. As supplied, the secondary of the MC240 is not grounded. If grounding is desirable a wire may be connected be­tween chassis and common of either or both output terminal strips.
The barrier strip output connections are
fed from the secondary of the output trans­former. LINE OUTPUT (See also pages 6 and 7)
The octal socket marked LINE OUTPUT
has connections for 70.7 volts, 125 ohms,
140 volts, or 600 ohms for stereo operation. For monophonic operation the octal socket provides connections for 70.7 volts, 62
ohms, 140 volts, or 300 ohms.
The 600 ohm winding and the 300 ohm winding are balanced to ground. Pin 1 of the octal socket is the center tap for these wind-
ings and is grounded.
The connections at the octal socket are
fed from taps on the cathode winding in the
primary of the output transformer.
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FUSE
The MC240 uses a 3.2 ampere slo-blo type fuse. The auxiliary AC socket is not fused.
AC OUTLET
The auxiliary AC outlet can be used to supply power to other equipment in the
system. The outlet will provide a maximum of 300 watts of power. The AC outlet is not fused.
LINE VOLTAGE
The MC240 operates on any line voltages between 105 volts and 130 volts, 50 cycles to 60 cycles. The switch marked LINE VOLTAGE changes the tap on the primary of the power transformer. The switch can be adjusted at any time.
INSTALLATION
Adequate ventilation extends the trouble­free life of electronic instruments. It is generally found that each 10° centigrade
(18° F) rise in temperature reduces the life of electrical insulation by one half. Adequate ventilation is an inexpensive and effective means of preventing insulation breakdown that results from unnecessarily high operat­ing temperatures. The direct benefit of ade­quate ventilation is longer, trouble-free life.
The suggested minimum space for mount-
ing the MC240 is 20" long x 14" wide x 10"
high. Always allow for air flow either by ventilation holes or space next to the bottom of the amplifier and a means for the warm air to escape at the top,
The MC240 can be mounted in any posi­tion except upside down. If the amplifier is to be installed on a vertical surface it is recom­mended that the transformers be on the down side. The advantage of this position is that the flow of heat from the tubes rises vertically and does not tend to heat the transformers.
If the MC240 is to be permanently mounted use the two mounting flanges supplied with the amplifier. The flanges are shipped sepa­rately and must be attached to the amplifier bottom at each end.
Turn the MC240 over with the transform­ers down and place it on a piece of cloth or cardboard to prevent scratches. The ampli­fier is shipped with 4 plastic feet on the bot­tom cover. Remove the 4 plastic feet. Re­move the three self tapping screws at one
end of the amplifier that hold the bottom cover to the chromium chassis. Place the square edge of the mounting flange with the three round holes positioned over the three
holes in the bottom cover. The three holes in the mounting flange will line up over the three holes in the chassis. Replace the three
self tapping screws through the mounting flange and bottom cover. The mounting flange is now firmly attached to the ampli­fier. The mounting flange on the other end of the amplifier is installed in the same way.
Fig. 4—Bottom of MC240 with mounting flanges attached.
Place the MC240 mounting template on the area where the amplifier is to be mounted. Center punch the mounting shelf through the 4 slotted holes in the mounting flanges
shown on the template. Remove the template and install four #10 round head screws in the center punch marks. Do not tighten these screws. Place the amplifier over the screw
heads and slide the amplifier to either side
in the slotted holes of the mounting flanges. The # 10 wood screws can then be tightened securely.
INPUT-STEREO
The shielded cable from the left output of the Mclntosh preamplifier is plugged into the jack marked STEREO INPUT-L. The shielded cable from the right output of the Mclntosh preamplifier is plugged into the jack marked STEREO INPUT-R.
CONNECTING THE MC240
INPUT-TWIN AMP
The shielded cable from the left program
source is plugged into the jack marked
TWIN INPUT-L. The shielded cable from the
right program source is plugged into the
jack marked TWIN INPUT-R.
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INPUT-MONO
The shielded cable from the program
source is plugged into the jack marked
MONO INPUT.
OUTPUT-STEREO OR TWIN AMPLIFIERS
For stereo or twin channel operation it is not necessary to use the same impedance load on each output. Simply connect each
output for the impedance desired.
Speakers are connected at the barrier
strips marked OUTPUT on the left end of
the panel.
In compliance with the National Electrical
Code, Class II wiring can be used between
the speaker and the amplifier at the 4 ohm,
8 ohm, or 16 ohm connection. Class II wiring is lamp cord, bell wire, or other wire with
this type of insulation. For the normally short
distances of under 100 feet between the amplifier and speaker, #18 wire or larger can be used. For distances over 100 feet between the amplifier and speaker use larger wire.
The loudspeaker impedance is usually
identified on the loudspeaker itself. Connect
one of the leads from the left loudspeaker to
the screw marked COM on the left barrier strip. Connect the other lead from the left loudspeaker to the screw marked with the number corresponding to the speaker im­pedance on the LEFT barrier strip.
If the speaker impedance is:
4 ohms 8 ohms
16 ohms
Connect one left speaker lead to screw LEFT-COM
and the other to:
LEFT-4 LEFT-8
LEFT-16
Connect one right speaker
lead to the screw marked
RIGHT-COM and the other to:
RIGHT-4 RIGHT-8
RIGHT-16
OUTPUT-MONOPHONIC
When the MC240 is to operate as an 80 watt monophonic amplifier, the outputs of the two channels combine to produce a
-single 80 watt output. This chart lists the proper connections and interconnections for monophonic operation.
Warning: Do not parallel the amplifier out-
puts when using the amplifier as a stereo amplifier or twin channel amplifier. Damage to the output tubes may result if parallel operation is attempted when the output volt­ages are not the same phase and amplitude.
If the
speaker
impedance
is:
2 ohms
4 ohms
8 ohms
16 ohms
32 ohms
Connect one speaker
lead to the screw
marked LEFT-COM
and the other to:
LEFT- 4
LEFT- 8
LEFT-16
RIGHT-8
RIGHT-16
Connect a
wire between:
LEFT-COM and
RIGHT-COM
LEFT-COM and RIGHT-COM
LEFT-COM and RIGHT-COM
RIGHT-COM and LEFT-8
RIGHT-COM and LEFT-16
Connect another
wire between:
LEFT-4 and
RIGHT-4
LEFT-8 and RIGHT-8
LEFT-16 and RIGHT-16
— —
When connected as outlined the MC240
operates as an 80 watt monophonic amplifier.
LINE OUTPUT-STEREO or TWIN AMPLIFIERS
The octal socket marked LINE OUTPUT
has connections for 70.7 volts, 125 ohms and
140 volts or 600 ohms. The 25 volt line is fed from the barrier strips.
For 25 volt line operation connect one of the left leads to the screw marked COM on the LEFT barrier strip. The other left lead is connected to the screw marked 16 on the
LEFT barrier strip. Connect the right leads in the same manner on the RIGHT barrier
strip.
The 125 ohm and 70.7.volt connections are the same. To connect for 125 ohm or 70.7 volt operation the octal plug supplied in the hardware bag is used. Solder one of the left leads to pin #2 and the other to pin #1. Solder one of the right leads to pin #4 and the other to pin #1. Pin #1 is the ground side.
The 600 ohm connections for the left side are made between pin #2 and pin #3. The 600 ohm connections for the right side are made between pin #4 and pin #5. The 600 ohm winding is balanced to ground. Pin #1 on the octal socket is the center tap of the 600 winding. Pin #1 is grounded.
If the speaker impedance
is between:
3.2 to 6.5 ohms
6.5 to 13 ohms 13 to 26 ohms
Connect the
speaker leads
between COM and:
4 ohms 8 ohms
16 ohms
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The only adverse effect on the operation
of a Mclntosh amplifier when it is improperly
matched is a reduction in the amount of
distortion-free power available to the loud-
speaker. Close impedance matching is de­sirable for maximum distortion-free power.
Use this table to determine proper speaker
connections:
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For all applications using 600 ohms use Class I wiring if the installation must meet the requirements of the National Electrical Code.
LINE OUTPUT-MONOPHONIC
The octal socket marked LINE OUTPUT has connections for operating the MC240 monophonically at 70.7 volts, 62 ohms or 300 ohms. For operating at 25 volts connec­tions are made at the barrier strips.
To feed a 25 volt line, connect one side to
the screw marked COM on the LEFT barrier
strip. Connect the other side to the screw
marked 16 on the LEFT barrier strip. Then connect with a wire the screw marked COM on the LEFT barrier strip to the screw marked COM on the RIGHT barrier strip. Connect, with another wire, the screw marked 16 on the LEFT barrier strip to the screw marked 16 on the RIGHT barrier strip.
The 62 ohm and the 70.7 volt connec­tions are the same. To connect for 62 ohms or 70.7 volts solder one of the leads to pin
#1 in the octal socket. The other lead is soldered to pin #2. A wire is connected be-
tween pin #2 and pin #4.
To connect for 300 ohms one lead is soldered to pin #2. The other lead is sol­dered to pin #3. Connect pin #2 to pin #4 with a wire. With another wire connect pin
#3 to pin #5. Pin #1 is the center tap of the
300 ohm winding. The 300 ohm winding is
balanced to ground.
AC POWER
The MC240 operates on 105 volt to 130
volt, 50 to 60 cycle power. The amplifier will
be turned on and off if its power cord is
plugged in one of the auxiliary AC outlets on
the program source.
OPERATING THE MC240
Before turning the MC240 on check all
connections and plugs to see that they are
firmly and correctly connected. Check to
make sure that the tubes are firmly seated
in the proper sockets. After the following adjustments have been completed the MC-
240 will operate without any further attention.
ADJUSTMENTS
LINE VOLTAGE
The LINE VOLTAGE switch should be set to the position that more closely matches the average line voltage in your area. The local electric power company can usually tell you
the average line voltage in your area. If the
line voltage is between 105 volts and 120
volts put the LINE VOLTAGE switch in the 117 volt position. If the line voltage is between
120 volts and 130 volts put the switch in the
125 volt position. If the line voltage is un­known, it is safer to operate with the switch in the 125 volt position.
INPUT SWITCH
STEREO
With the input switch in the left position
marked STEREO the input of the MC240 is
connected to the two jacks marked STEREO
INPUT located below the BALANCE control.
The amplifier has an input sensitivity in this
position of 2.0 volts. This position has been provided to be used with Mclntosh preampli-
fiers or any other high output preamplifiers.
The BALANCE control assists in balancing
the overall stereo system for equal loudness.
Equal loudness is affected by room acous­tics, furniture placement, speakers and many other things. With a monophonic signal adjust this control until both loudspeakers
are equal in sound output.
TWIN AMP
With the input switch in the center posi-
tion marked TWIN AMP, the input of the
MC240 is connected to the two jacks marked
TWIN INPUT located below the two center control knobs marked GAIN. The input sensi­tivity in this position is 0.5 volt. This position has been provided for use with a low output stereo preamplifier, an electronic crossover network, or as two completely separated monophonic 40 watt amplifiers.
The two center control knobs marked
GAIN should be adjusted to match the input sensitivity of the MC240 to the output voltage of the program source equipment.
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To balance a stereo system using the TWIN AMP input position use a monophonic signal from the program source. A mono-
phonic signal supplies the same voltage to both channels. Turn the volume control on
the source equipment to the 12 o'clock posi-
tion (half rotation). Turn up the GAIN con­trol on the LEFT input until the loudness in
the room is not quite as loud as you like to
listen. Then turn the GAIN control on the
RIGHT input until it is equally as loud as the left channel. The system is now balanced for loudness and provides the most convenient amount of loudness change when using the
volume control on the source equipment.
MONO
With the switch in the right position marked MONO the inputs of the MC240 are internally connected in parallel to the jack
marked MONO INPUT. This jack is located
below the right control knob marked GAIN. The input sensitivity in this position is 0.5 volt. The GAIN control should be adjusted to
match the input voltage sensitivity of the
MC240 to the output voltage of the program source.
For maximum flexibility set the GAIN con­trol as follows: turn the volume control on the program source equipment (i.e. tape
recorder, tuner, preamplifier, etc.) to the 12 o'clock (half rotation) position. Turn the
GAIN control on the MC240 until the loud-
ness of the sound from the speaker is just a little louder than you normally like to listen.
The volume control on the program source
equipment now has the most convenient amount of loudness change in either direc­tion from the 12 o'clock position.
IMPORTANT
The excellent performance that is inher-
ent in all Mclntosh amplifiers does not
depend on the critical adjustment of bias or
balance controls in the output circuit. The
patented Mclntosh circuit delivers its adver­tised specifications without any need for these controls and is not dependent on care­fully balanced tubes for its performance. With Mclntosh you can install the amplifier and forget it.
The MC240 you have purchased will give you years of pleasant and satisfactory per­formance. If you have any questions con­cerning the operation or maintenance of this amplifier please contact:
Mclntosh Laboratory Incorporated guar­antees this equipment to perform as adver­tised. We also guarantee the mechanical and electrical workmanship and components of this equipment to be free of defects for a
period of 90 days from date of purchase.
This guarantee does not extend to compo-
nents damaged by improper use nor does it extend to transportation to and from the factory.
3-YEAR FACTORY SERVICE CONTRACT
An application for a FREE 3-YEAR FAC-
TORY SERVICE CONTRACT is included in the
pocket in the back cover of this manual. The
FREE 3-YEAR FACTORY SERVICE CONTRACT
will be issued by Mclntosh Laboratory upon
receipt of the completely filled out applica-
tion form. The term of this contract is defined
in the 3-year factory service contract. If the application is not mailed to Mclntosh Labo­ratory only the services offered under the standard 90-day guarantee will apply on this equipment. TAKE ADVANTAGE OF 3 YEARS
OF FREE FACTORY SERVICE BY FILLING
IN THE APPLICATION NOW.
In Canada: Manufactured under license by:
McCurdy Radio Industries, Ltd. 22 Front Street West
Toronto, Canada
Design subject to change without notice.
GUARANTEE
Customer Service Mclntosh Laboratory, Inc. 2 Chambers Street
Binghamton, New York
Our telephone number is 723-5491.
The direct dial area code is 607.
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2 CHAMBER S STREET , BINGHAMTON , N. Y.
Phone-Are a Code 607 723 549 1
LABORATORY INC.
Made in U.S.A.
PRINTED IN U.S.A.
1M 038-00 2
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