Specifications

The MR 78 tuner uses linear phase filters at the
NORMAL and the NARROW positions of the SELEC-
TIVITY switch settings.
LINEAR PHASE BRIDGE DISCRIMINATOR
The excellence of the IF filter would be useless if
it had to work into an ordinary FM detector. Thus a
new detector with suitably low distortion had to be
developed. A search of the available literature
revealed a little-known bridge circuit with a
theoretical distortion of zero! However, designing a
practical working circuit for a commercially feasible
stereo tuner took some doing. A U.S. patent is held
by Mclntosh on this circuit.
Distortion performance of the new Mclntosh bridge
detector is close to" the theoretical zero. In addition
to its excellent distortion performance, the bridge
detector also exhibits a capture ratio close to 0 dB.
FRONT END
Instead of using a conventional, easily overloaded
transistor or FET as a straight RF amplifier, the MR
78 uses a rugged Junction Field Effect Transistor as
an impedance converter to drive a 5-watt power tran-
sistor. This combination, a cascode circuit, makes
the RF amplifier virtually impossible to overload or
cross-modulate. As an example, if you are tuned to a
3 microvolt signal at 96.3 MHz, the MR 78 will reject
signals elsewhere on the dial which are at least,
4,000,000 times stronger. Thus, a 12 volt signal
received at 104.3 MHz would not interfere with the
signal at 96.3 MHz.
The front end oscillator uses a high-Q ceramic
form tank coil. It is free from spurious radiation, and
operates at high efficiency.
Adding to the superior design of the front end is
the integrated circuit balanced mixer. It is practical-
ly impossible to overload. Oscillator pulling, cross
modulation, and other types of distortion so com-
mon in ordinary transistor or FET mixers is minimiz-
ed.
PERFORMANCE LIMITS and RATINGS
TUNING RANGE:
88 to 108 MHz.
ANTENNA INPUTS:
300 ohms balanced; 75 ohms unbalanced.
INTERMEDIATE FREQUENCY:
10.7 MHz.
SENSITIVITY:
2mV for 35 dB quieting; 2.5 mV at 100% modulation
(± 75 kHz deviation) for 3% total noise and har-
monic distortion.
SIGNAL TO NOISE RATIO:
75 dB below 100% modulation.
HARMONIC DISTORTION:
0.2% mono or stereo at 100% modulation, 20 Hz
to 15,000 Hz. Typically, 0.05% at 1,000 Hz.
DRIFT:
25,000 Hz for the first two minutes; thereafter
5,000 Hz at 25° C in 24 hours.
FREQUENCY RESPONSE:
Mono: ± 1 dB 20 Hz to 20,000 Hz with standard
de-emphasis, (75 mS); Stereo: ± 1 dB 20 Hz to
15,000 Hz with standard de-emphasis (75 mS).
CAPTURE RATIO:
0.25 dB detector only; 2.5 dB complete tuner.
SELECTIVITY: Switch Setting:
NORMAL NARROW SUPER NARROW
Adjacent Channel 7 dB 22 dB 55 dB
Alternate Channel 55 dB >90 dB >>90 dB
SPURIOUS REJECTION:
100 dB IHF.
IMAGE REJECTION:
100 dB at 88 to 108 MHz (IHF).
INTERMODULATION DISTORTION:
0.2% mono or stereo for any combination of fre-
quencies from 20 Hz to 15,000 Hz with peak
modulation equal to 100% or less. Typically 0.1 %.
MAXIMUM SIGNAL INPUT:
12 volts across 300 ohm antenna terminals will
not increase harmonic or intermodulation distor-
tion.
AUDIO HUM:
75 dB down from 100% modulation..
MUTING:
70 dB noise reduction between stations.
MUTING THRESHOLD (Typical):
DISTANT 5 mV; LOCAL 20 mV
SCA FILTER:
50 dB down from 67 kHz to 74 kHz; 275 dB per oc-
tave slope.
STEREO SEPARATION:
40 dB at 1,000 Hz.
STEREO FILTER (Typical):
10 dB noise reduction in Position 1.
20 dB noise reduction in Position 2.
AUDIO OUTPUT:
Front Panel Controlled: 2.5 volts into 47,000 ohms;
Fixed Output: 2.5 volts into 47,000 ohms, 1.0 volts
into 600 ohms.
All tuner performance limits were measured with SELECTIVITY
switch set at NORMAL, unless otherwise slated.
GENERAL
POWER REQUIREMENT:
120 volts, 50/60 Hz 35 watts.
SEMICONDUCTOR COMPLEMENT:
3 JFET, 2 MOSFET, 17 Bipolar Transistors, 43
Diodes, 4 Integrated Circuits.
MECHANICAL
SIZE:
Front panel: 16 inches wide (40.64 cm) by 5-7/16 in-
ches high (13.81 cm); Chassis: 15 inches wide
(38.1 cm) by 13 inches deep (33.02 cm), including
PANLOC shelf and back panel connectors; Knob
Clearance: 1-1/2 inches (3.81 cm) in front of moun-
ting panel.
FINISH:
Front panel: Anodized gold and black with special
gold/teal panel nomenclature illumination;
Chassis: Chrome and black.
MOUNTING:
Mclntosh developed professional PANLOC.
WEIGHT:
27 pounds (12.25 kg) net, 39 pounds (17.69 kg) in
shipping carton.
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