McIntosh MR67

 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