The McIntosh
MR67 is a precision engineered, highly sensitive FM Multiplex Tuner. The finest
possible reception of FM monophonic and FM multiplex stereophonic broadcasts is
assured by another McIntosh First a built -in Multipath Indicator. Multipath
reception of FM transmissions causes noisy and distorted sounding programs.
Multipath
distortion is caused by receiving both a direct signal and one or more
reflected signals. Multipath signals may be reflected from a buildin, water
tower or even a nearby hill or mountain. The multipath reflected signals rerach
the antenna at slightly later times than the direct signals due to the longer
distances they direct signals at the antenna can cause moderate or severe
distortion at the tuner outputs. Multipath can be eliminated or greatly reduced
by proper use of a directional antenna. The MR67 multipath indicator shows when
the antenna is rotated to the correct position for picking up only the desired
direct signal.
The MR67
also uses the new, exclusive McIntosh PANLOC method of installation.
Technical
Description
The MR67
tuner has a 6DS4 high-gain Nuvistor for the first RF amplifier stage. The
Nuvistor (triode) operates into a triode tube to form a cascode amplifier.
Careful design of the operating characteristics allows the cascode amplifier to
handle a wide dynamic range of signals with excellent signal-to-noise ratio.
Proper design of circuit constants, together with extreme care in
manufacturing, reduces spurious signals to a minimum. Sensitivity is increased
to the highest possible degree, while still retaining high selectivity. A
low-noise triode mixer follows the RF stages.
The high
frequency oscillator mechanical construction and layout are engineered for
minimum responses to temperature variations. The combination of both mechanical
and electronic design in the MR67 oscillator circuit is so precise that a
compensating automatic frequency control is not needed.
The
mixer output is amplified through four intermediate frequency stages with
flat-topped highly selective response curves. The IF transformers are designed
for wide band response with maximum adjacent channel rejection. Construction of
the IF transformers assures electrical stability and resistance to mechanical
shock and vibration.
Two
limiter stages are used followed by a wide-band discriminator. The second
limiter is part of the muting circuit which automatically suppresses
interstation noise while tuning. Weak or distant stations that don't override
the background noise and interference are also suppressed by the muting
circuit. A front panel switch allows muting to be cut out for listening to
these weak stations. A back panel muting adjust control is factory-set to the
proper threshold level.
The MR67
multiplex decoder uses a special Mclntosh developed detector. One of the many
advantages of this circuit is the elimination of critical adjustments necessary
with commonly used multiplex matrixing methods. The L-R (left minus right)
stereo sidebands are detected and then automatically combined with the L+R main
carrier signal. The left and right channel signals are then produced with
excellent separation.
A
temperature-stabilized 19 KC amplifier locks-in a highly stable 38 KC
synchronous ' oscillator. This method provides exceptional noise immunity.
Balanced detectors cancel the 38 KC signal components in the output for lowest
distortion and reduced susceptibility to spurious signals. Tape recording
interference is also minimized by this method.
The MR67
has a multiplex stereo indicator that lights whenever the dial pointer crosses
a station broadcasting multiplex stereo. A unique circuit using a transistor as
a switch operates the MPX stereo indicator lamp. The transistor is controlled
by a special noise rejecting differential detecting bride-circuit. - This
circuit automatically discriminates between noise and the 19 KC multiplex pilot
signal. The indicator will light only on the 19 KC signal present in a
multiplex stereo broadcast. It will not light on noise or interference.
The dual
electron ray tube at the left on the tuning dial serves three separate
indicator functions. The upper half of the tube is a multipath signal
indicator, a unique and exclusive Mclntosh development. Multipath signal
reception causes moderate or severe distortion and poor stereo channel
separation. By knowing the presence of multipath reception, a directional FM
antenna may be properly rotated to reduce or eliminate the multipath signal and
the poor sound quality it causes. The MR67 multipath indicator shows whether
multipath signals are present by rapid fluctuations of the electron ray beams.
When the antenna is correctly positioned, the indicator beams remain steady,
showing that multipath distortion has been eliminated.
A switch
behind the front panel allows this section of the electron ray tube to also
function as a signal strength indicator.
The
lower half of the electron ray tube indicates correct station tuning.
Two
identical audio amplifier circuits are used in the MR67. Negative feedback
reduces distortion and lowers the tuner output source impedance for best
operation with any type of control preamplifier.
A new
type of mechanical tuning assembly gives the MR67 an extremely smooth, quiet,
flywheel tuning action. The tuning capacitor is driven directly, which in turn
drives the dial pointer. Backlash is virtually eliminated with this design. A
Teflon-lined pointer carriage and nylon pulleys are used to practically
eliminate friction and wear. The overall result is unusually smooth, quiet and
precise tuning action.
Specifications
Useable
Sensitivity: Better than 2.5 microvolts at 100% modulation
(+75KC deviation) for less than 3% total noise and distortion in accordance
with IHFM standards
Audio
Frequency Response: Within ½ dB 20 to 20,000 cycles,
including 75 microseconds de-emphasis
IF
Bandwidth: 200 KC flat top response,
down 3 dB at 200 KC
Limiters: Two
stages
Radiation: Substantially below FCC requirements
Distortion: Less
than 0.5% at l00% modulation, ±75KC
Capture
Ratio: 1.7 dB at 100% modulation
Muting: IF
injected; at least 60 dB quieting between stations
Drift: Less than 25 KC
Image
Rejection: Better than 60 dB
Hum: Better
than 70 dB below 100% modulation
Output: 2.5
volts, low-source impedance
Antenna
Inputs: 300 Ω balanced; 75 Ω unbalanced
RF
Amplifier: Cascode with 6DS4 Nuvistor in first stage
IF
Amplifiers: Four stages ; AGC used to insure that limiting
occurs only in the limiter stages
Tuning
Indicators: Dual electron ray tube;
lower section for precise tuning indication; upper section for signal strength
presentation (can be switched to indicate multipath distortion)
FM Multipath
Distortion Indicator: An exclusive new
McIntosh development; the upper section of the dual electron-ray tube can be
switched to indicate multipath distortion or signal strength
Multiplex
Channel Separation: Better than 30 dB at 1000 cycles
Multiplex
Filter: Greater than 40 dB suppression
below 100% modulation of 19 KC pilot and 38 KC carrier
Multiplex
Indicator: MPX stereo light activated by
19 KC carrier
Multiplex
Type: peak-detecting,, self-matrixing
circuit
Tube and
Semiconductor Complement:
1 x 6DS4
Nuvistor 1st RF amplifier;
1 x 12AT7
2an RF amplifier and mixer;
1 x 6AB4
Oscillator;
1 x 6AU6 1st
IF;
1 x 6AU6 2nd
IF;
1 x 6AU6 3rd
IF, 1st limiter;
1 x 6AU6 4th
IF, 2nd limiter;
1 x 6AV6
Muting amplifier, AGC clamp;
1 x EMM801
Tuning indicator, signal strength
indicator, Multipath distortion indicator
1 x 6U8 MPX
amplifier and 19KC separator- Indicator control
1 x 12AU7
MPX 38KC oscillator
2 x 6BL8
left and right audio amplifiers
1 x MA113
Transistor (multiplex indicator lamp switch)
2 x 1N542
Diodes, wide band discriminator
4 x 1n542
Diodes; balanced MPX detector;
2 x 1N542
Diodes, balanced detector for MPX indicator;
1 x 1N541
Diode, muting and tuning indicator detector;
2 x Selenium
rectifiers, high-voltage supply;
1 x 1850
Pilot lamp, MPX indicator
Power
Consumption: 50 watts, 105 to 125 volts,
50/60 cycles
Dimensions:
Front
Panel: 16" wide by 5-7/16"
high,
Chassis: 15" wide by 5-1/8" high by 13
" deep (including Panloc shelf, connectors), clearance in front of
mounting panel including knobs, 1 ½"
Weight: 24,5 pounds (tuner only)
Finish: Anodized gold and black front panel, chrome
chassis
Installation: Convenient professional PANLOC
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