McIntosh MR 73

 The McIntosh MR 73 is a precision engineered, highly sensitive solid state AM/FM tuner.

The research staff of mcIntosh Laboratory, using the latest technology in solid state physics, has developed many circuit advances:

Phase linear crystal shaped IF amplifiers

Integrated Circuits that are the equivalent of 16 transistors, 3 zener diodes, 5 diodes and 23 resistors

Special multiplex (stereo) detecting circuits that eliminate critical adjustments

Ultrasonic muting for complete silence between stations while tuning.

Automatic stereo-mono switching that is "clickless" and completely electronic.

Visual multipath indicator to help avoid multipath distortion showing as a visual indicator of correct antenna direction for minimum multipath reception

New D'Aarsonval movement meters of increased sensitivity which do not need zero setting adjustments.

A new computer designed filter to reduce noise from stations broadcasting subcarrier auxiliary music services. No other tuner uses such a filter.

Exclusive McIntosh PANLOC is used for installation.The PANLOC system gives you absolute ease of installation, operation and maintenance.

PANLOC is the first professional installation technique to be used on stereo instruments.  In the PANLOC system a metal shelf is mounted first. The tuner slides into position on this shelf. Depressing the front panel PANLOC buttons, locks the tuner firmly into position against the mouting panel.

Onec you have enjoyed the outstanding performance of the Mr 73, you will understand why McIntosh products have earned their reputation as "the best". Your McIntosh MR 73 tuner will give you years of the finest possible reception, and will be a valued part of your home music system.

 FM SECTION

This is divided into two separate modular sections:

1 - The 100 mHz Radio Frequency (RF) Section

2 - The FM-IF and Detector Section

The first section houses the complete FM-RF front-end part of the AM_RF circuits. A special four section, variable tuning capacitor provides a high degree of RF selectivity and excellent spurious response rejection. The problem of image rejection has been greatly reduced in the Rf section of the MR 73.

The two stage parallel-fed-cascode  junction field effect transistor (JFET) RF amplifier gives better sensitivity and higher gain  than conventional one-stage amplifiers. The use of JFET's helps to further reduce the problem of front end overload in strong signal areas.

A mixer using a JFET has been designed for high sensitivity and freedom from overload. Low temperature coefficient components have been designed into the FM local oscillator to revent frequency drift. The frequency stability inherent in the local oscillator makes automatic  frequency control (AFC) unnecessary. The rate of drift of the local oscillator is less than ten parts per million per degree cantigrade.

Both the FM and AM-RF front ends have been designed in the same completely encased metal modules. This design gives protection against radiation or interference. The RF circuits of the MR 73 exceed the FCC requirements for suppression of oscillator radiation.

The MR 73 has antenna  connections for either 300 Ω twin lead transmission line or 75 Ω coaxial cable. The normal input impedance of the first RF amplifier is 75 ohms. Impedance match to 300 Ω is provided by a McIntosh designed balun transformer which has negligible losses. Connections for a 300 Ω line are made with new push type terminals. No tools are required. A type F male linear phase characteristics of the IF amplifier.

 The FM-IF and Detector  section design required extended engineering time and testing and uses the latest concepts in integrated circuits and crystal filters. The FM-IF consist of two integrated circuits and two phase linear crystal filters. They combine to give a total gain of over 120 dB (the signal is amplified to over 1,000,000 times its original level). The response curve has a nearly flat top with linear phase characteristics. The skirts of the response curve are very steep. The maximum width is 240 kHz at -3 dB and 400 kHz at -60 dB. The response curve is symmetrical each side of the center frequency. The crystal filters are permanently sealed and do not require adjusting. The IF cannot drift nor vibrate out of adjustment.

Each of the two integrated circuits used in the FM-IF of the MR 73 contain 16 transistors, 3 zenar diodes, 5 diodes and 23 resistors, all on a single monolithic silicon chip.

The exceptionally high gain of the two integrated circuits assumes "hard limiting" at very low levels of input signals.

A "phase" or "Foster Seeley" discriminator has been designed to complement the integrated circuit IF section. The IF section has  exceptionally high gain and hard limiting characteristics, yet the capture ratio is quite low. The detected output signal of the discriminator is extremely low in distortion content.

De-emphasis of the discriminator output restores the frequency amplitude characteristics o the same level they were before transmission.

 C.FM Stereo Multiplex Section

McIntosh laboratory has developed a special detecting circuit in the multiplex section.

A  particular advantage of this circuit in the elimination of the critical adjustments necessary with commonly used matrixing circuits. The circuit detects the L-R sidebands, then automatically matrixes the recovered information with the L+R main carrier signal. This yields the left and right program output with maximum separation.

The 19 kHz pilot signal is filtered from the composite stereo input signal, amplified by a special limiting amplifier, doubled to the 38 kHz carrier sognal. The  new combination of signals is fed to the special detector circuit mentioned above. Balanced full wave detectors are used to cancel the 38 kHz conponents in the output.

The SCA (Subsidiary Communication Authorization) signal must be removed from the composite output. This is accomplished by the use of a new "Image Parameter" band elimination filter that has  been computer designed. The SCA filter rejects SCA signals without impairing stereo performance.

FM muting in the McIntosh MR 73 operates by detecting ultra-sonic noise which is present when tuning between stations or when receiving a weak station. The muting circuit can be activated or defeated by the use of the muting switch on the front panel. The level of muting desired can be adjusted by the muting level control on the top panel. Varying the muting control adjusts the threshold at which the muting takes effect.

When the 19 kHz carrier of a stereo signal is received, the automatic FM stsreo switching circuit activates the multiplex decoding circuit. This lights the stereo indicator. The circuit switching is all done electronically with no clicks. The automatic stereo switching can be defeated by turning the mode selector switch to FM Mono (In this position the stereo indicator will still light to indicate the presence of a stereo signal). On monophonic  transmissions the stereo switching is inactive at all times, assuring a greater signal to noise ratio. The stereo switching circuit has been designed so that noise will not activate it.

 AM SECTION

The "State-of-the-art." in electronic technology has been incorporated by McIntosh into the MR 73 with the use of metal oxide silicon field effect transistors (MOSFET's) in the AM section.  An AM-RF amplifier circuit includes a three section variable tuning capacitor in the metal enclosed shielded module, which also houses the FM-RF front end.  The Am-RF amplifier uses a dual-insulated gate MOSFET to obtain more overload protection against strong local stations than can be obtained with conventional bipolar transistors.

The AM-RF mixer is also a dual-insulated gate MOSFET. The use of MOSFET's in the RF amplifier and mixer provides the MR 73 with very little cross modulation (spurious response) and the image rejection is very good.

Three double tuned IF transformers are used to obtain a high degree of selectivity yet still allow good Amfidelity. A 10 kHz whistle filter has been incorporated at the output of the AM detector.  Its  purpose is to supress hetrodyning that occurs between adjacent AM carriers.

To increase the "pulling power" of the AM section of the MR 73, connect an external antenna at the rear apron of the unit by using a push connector. An internal transformer matches the external antenna to the input impedance of the AM front end. An optional highly sensitive loop stick antenna is also provided for local or strong stations. A switch on the back panel selects either loopstick or external antenna.

An AM sensitivity switch has been provided to decrease both the gain of the RF amplifier and the noise between stations.

 Audio Preamplifier Section

An audio amplifier has been incorporated in the McIntosh MR 73. It in creases the audio level from the AM or FM multiplex outputs to a level sufficient to drive a preamplifier or other accessory equipment. The audio amplifier consists of 2 separate amplifiers, each amplifier having three transistors. The design uses considerable negative feedback. Negative feedback helps to achieve low distortion, wide frequency response, and unexcelled stability. Each audio amplifier has two low impedance outputs. One is a fixed output set at 2,5 volts. The second output is variable by the volume the control on the front panel.

By cross-coupling a small portion of one channel into the other in the audio amplifier, the residual alternate channel information can be cancelled, thereby further increasing the stero channel separation.

Also included in the audio section is a Stereo Filter switch. With the Stereo Filter turned on and waek stations are being received, stereo noise is reduced, but still allows ample stereo listening performance.

Power Supply

Special design attention has been given to the power supply section of the McIntosh MR 73. Two separate rectifier circuits are used in the MR 73.

The 16 volt regulator is the power heart of teh Mr 73. All signal stages throughout the unit are powered from this regulator. The 16 volt regulator is very elaborate in design, using a specially selected transistor and associated circuit. The regulator uses electronic filtering to insure the lowest possible background hum level, maximum stability and extremely good regulation.

A half wave rectifier and filter, supply the DC high voltage needed for the anode of the multipath indicator. The second power supply is a full wave rectifier which supplies DC to the multiplex indicator and to the voltage regulator.

Specifications

FM Section

Frequency: 88 to 108 MHz

Useable Sensitivity:  2,5 microvolts at 100% modulation (±75 kHz deviation) for less than 3% total noise and harmonic distortion in accordance with IHF standards

Audio Frequency Response:  Flat from 20 Hz to 20 kHz with standard de-emphasis and 19 kHz pilot filter.

Harmonic Distortion:

Mono : Less than 0,3% at 100% modulation ±75 kHz deviation.

Stereo : les than 0,7%

Capture Ratio:  Better than 1,5% dB at 100% modulation

Muting:  AF injected ultrasonic muting at least 60 dB noise reduction between stations

Oscillator Drift:  less than 25 kHz

Image Rejection:  Better than 80 dB at 90 MHz;  better than 70 dB at 100 MHz

Hum: better than 70 dB below 100% modulation

Output:  Approx. 2,5 volts, low impedance

FM Antenna Inputs:  Built-in loopstick or external antenna switch selected

IF:  two Ics and two phase linear crystan filters

Radiation: Substantially below FCC requirements

Stereo Separtion:  Better than 35 dB at 1 kHz

Stereo Filter: Greater than 48 dB suppression of 38 kHz multiplex carrier

Stereo Indicator:  Front panel multiplex stereo light activated by 19 kHz carrier-only

SCA Filter:  50 dB down at 67 kHz to 74 kHz , 275 dB per octave slope

Automatic Mono/Stereo Switch:  McIntosh developed; all electronic automatic mono/stereo switching circuit

Semiconductor Complement:  22 diode, 24 Transistors, 2 Integrated Circuits, 1 Indicator Tube

AM Section

Frequency:  55 to 160 kHz

Sensitivity: Better than 12 microvolts at 1000 kHz (using the external antenna input)

Frequency Response:  Down 6 dB at 5 kHz

Harmonic Distortion:  less than 1% at 39 % modulation

Selectivity:  -3 dB at 10 kHz

Image Rejection:   60 dB or greater at 1000 kHz

Signal-to-noise Ratio Better than 55 dB

General

Power Consumption:  20 watts, 105 to 125 volts, 50 to 60 Hz

Dimensions

Front Panel : 16 inches wide by 5-7/16inches high

Chassis : 15 inches wide by 13 inches deep, including PANLOC shelf and back panel connectors

Knob Clearance : 1-½ inches in front of mouting panel

Weight:  24 pounds net, 36 pounds in shipping carton

Finish:  Front panel. Anodized gold and black with special gold/teal panel nomenclature illumination.

Chassis:  Chrome and black

Mouting:  McIntosh  developed professional PANLOCK