Specifications

illuminated over a portion of its length. Also located within
the dial opening are symmetrically positioned center-of-
channel and signal-strength meters. To the left of the dial
area are two rectangular areas denoted as "function" and
"selectivity". Illuminated words "stereo", "filter", and
"muting" appear (when selected) in the upper area, while
the lower area is illuminated with either the notation
"-7 dB", "-22 dB" or "-55 dB". These correspond to the
three selectivity settings of the selectivity switch just
below. The dB numbers represent adjacent channel selec-
tivity and should not be confused with the more familiar
alternate channel selectivity values which are, of course,
much greater. A large tuning knob, coupled to a blacklash-
free flywheel and dial string arrangement, is located to the
right of the dial area opening.
In addition to the selectivity switch, other rotary controls
along the bottom section of the front panel include a meter
selector switch which chooses either the signal strength or
multipath indication function of the signal-strength meter,
a filter switch which, in addition to having an "off" posi-
tion, has two settings for noise reduction during weak-signal
stereo FM reception, a muting switch, with positions for
local and distant reception (which varies the muting thres-
hold), a mode switch with "stereo only", mono, and
"stereo auto" positions and a volume control which also
turns power on and off to the tuner.
The rear panel of the MR 78 has a pair of test points,
as illustrated in Fig. 2., which are intended for use with
the Mclntosh Maximum Performance Indicator, an acces-
sory product marketed by that company. A fuesholder cap
is also accessible from the rear for replacement, if necessary,
of the tuner's 0.5 ampere slo-blow line fuse. Push-type
Fig. 2 — View of back panel
terminals are supplied for 300-ohm antenna transmission
line connection, while a 75-ohm coaxial connector is offered
for connection of that type of transmission line. Two sets
of audio output jacks (one pair for fixed output, the other
controlled by the front-panel volume control) and one
convenience AC receptacle complete the rear panel layout.
Internal Construction and Circuit Highlights
As can be seen in the internal view of the chassis, Fig. 3,
the Mclntosh MR 78 is extensively shielded. Six major
circuit board modules are used in addition to the RF front
end. These include the IF circuit board, the selectivity selec-
tion junction board, the multiplex section, the detector cir-
cuit board, the output amplifier board and the power supply
circuit board. The front end utilizes a 5-section tuning
capacitor. A junction FET is used as impedance converter
to drive a 5-watt power transistor in the RF stage for high
overload capability. An antenna matching circuit at the RF
input has a two-position selectable switch which alters the
RF bandpass characteristics of the input circuit. The switch
positions are labelled "Lo Gain" and "Hi Gain", a bit con-
fusing unless the user reads the manual carefully and dis-
covers that the gain references refer to the antennas which
might be used with the tuner. Thus, the Lo Gain position is
intended for use in seeking out distant or weak signals
Fig. 3. — View of chassis layout
while, in most applications, the "Hi Gain" position should
be used since it yields the lesser amount of RF signal at the
input to the RF amplifier stage, as well as a sharper bandpass
characteristic.
A block diagram of the RF and IF sections of the MR 78
is reproduced in Fig. 4. A balanced integrated circuit mixer
stage is used as a further safeguard against possible overload
and to reduce oscillator pulling. Its 0.7 MHz output is
coupled to the IF amplifier section by means of a bifilar
transformer. Linear phase filters are used in both the
NORMAL and NARROW selectivity settings of the MR 78,
while for the SUPER-NARROW selectivity setting, a
4-pole, 4-zero crystal filter having constant delay, is intro-
duced into the IF chain. Mclntosh developed a new type of
FM detector for this tuner which they call a linear phase
bridge discriminator. It uses a balanced transmission line
bridge in conjunction with a differential voltage doubling
circuit to achieve ultra-low orders of distortion in the
demodulated FM audio signals.
A block diagram of the Multiplex and Audio Sections is
illustrated in Fig. 5. An IC audio amplifier with over 120
dB of negative feedback in two feedback loops is used to
drive the stereo demodulator. Muting, "stereo only" tuning
and automatic mono/stereo switching are all functions
which are performed within the stereo decoder circuitry.
A stereo filter circuit follows the matrix decoder and de-
emphasis networks to reduce noise when tuning to weak
stereo stations. This filter employs a twin-T bandpass
filter design to suppress noise while only minimally affecting
stereo separation and imaging. The audio amplifier section
consists of a pair of two-transistor amplifiers, one for each
channel. Each amplifier delivers 2.5 volts to the fixed output
jacks and to the volume control sections which feed the
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