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cable company is not on its correct frequency. This
is done by the use of two operational amplifiers. The
lock circuit will track a station even if it drifts 1 MHZ.
The mixer is a balanced matched dual J-FET and
bipolar transistor circuit. A low loss toroidal phase
splitting transformer is used as an impedance mat-
ching network in the gate circuit of the mixer
eliminating even order spurious responses.
The MR 80 has the narrowest IF bandwidth ever
used in a stereo tuner. It is the correct width to let
just one FM station through. The excellent selectivi-
ty of the MR 80 (210 kHz wide at 60 dB down) permits
tuning to stations that are impossible to receive on
ordinary tuners.
After the mixer the signal is electronically switch-
ed to go either directly to the IF amplifier or to go
through a quartz crystal filter. The SUPER NARROW
selectivity position adds a 4 pole - 4 zero crystal filter
to the other 5 IF filters. SUPER NARROW permits
receiving stations never heard before with most
other FM tuners. The 5 stages of IF amplification
provide the necessary gain to reduce noise in the
signal and interference from other stations. The 5 IF
stages use piezoelectric fixed frequency filters in
place of normal tuned circuits so the IF stages will
always stay in alignment, even with age.
The signal strength indicator column is summed
off of all 5 IF stages instead of just 1 so it is looking
at the signal strength throughout the entire IF
system.
Following the selectivity section of the IF
amplifier is the LIMITER. A total gain of 80 dB is
used in this circuit. The use of very high gain in the
limiter circuit produces hard limiting with very good
impulse noise rejection. Limiter bandwidth is
greater than 50 MHz, producing excellent detector
capture characteristics.
A broadband Foster-Seeley discriminator is used
as the detector. This detector coupled with the
broadband limiter produces unmeasurable noise
and distortion.
The heart of the multiplex section is a new third
generation phase locked loop (PLL) stereo decoder
integrated circuit (IC). This PLL IC incorporates two
special systems, an automatic variable separation
control circuit to reduce background noise when
receiving weak stereo stations, and tri-level digital
waveform generation which eliminates interference
from SCA signals and from the sidebands of adja-
cent channel FM signals.
The variable separation control is operated from
the IF amplifier's signal strength detector. A smooth
transition is provided from mono to stereo or from
stereo to mono at weak signal levels to provide the
optimum signal to noise ratio and best stereo
separation for the prevailing signal conditions. The
circuit operates only during stereo reception. It swit-
ches automatically to monophonic if the 19 kHz pilot
tone is absent.
In the PLL the internal oscillator operates at
228kHz, locked onto the 19kHz pilot tone. The
228kHz, feeds a 3 stage Johnson counter via a binary
divider to generate a series of square waves.
Suitably connected NAND gates and exclusive OR
gates produce the tri-level drive waveform for the
various demodulators in the circuit. The usual
square waveforms have been replaced in the PLL
and decoder sections by tri-level waveforms. These
tri-level forms contain no harmonics which are
multiples of 2 or 3. This eliminates frequency
translation and detection of interference from the
side-bands of adjacent stations since the third har-
monic of the sub-carrier (114kHz) is excluded. It also
eliminates interference from SCA broadcasts since
the third harmonic of the pilot tone (57kHz) is exclud-
ed. Unwanted spurious audible components and
phase jitter in the PLL are Inherently eliminated by
this technique.
Additional advantages of the phase locked loop
stereo demodulation are the elimination of induc-
tors to minimize drift, integral lamp driving capabili-
ty to indicate the presence of the 19kHz pilot carrier,
excellent channel separation over the entire audio
frequency range, extremely low distortion, low out-
put impedance, and transient-free mono/stereo
switching.
Following the MPX decoder is the three position
de-emphasis switch. The three different positions
allow the MR 80 to be used in North America with
standard 75ms de-emphasis and in Western Europe
and the Far East with 50ms de-emphasis. The 25ms
position is for use with an external noise reduction
adapter.
An electronic blend filter circuit, implemented
with two J-FETs of a quad J-FET package, is used to
reduce out of phase noise when in the stereo mode
and tuned to a weak station. This filter is actually a
twin-T bandpass that blends the high and low fre-
quencies, leaving separation unaffected at mid-
frequencies. This results in a greatly improved
stereo image when the filter is required.
Special design attention has been given to the
power supply section. Nine separate power circuits
are used. Six of these are regulated to prevent loss
of performance during a brown out. The - 15, - 5.2,
5, 15 and 30v use integrated circuit 3 terminal
regulators, while the 3ma current source is made
with discrete transistors because of the high voltage
on the input terminal. The remaining voltages are
used for the headphone amplifier and the touch con-
trol reference signal driver.
Mclntosh, recognizing the need for improved
reliability, has designed a new circuit to drive in-
candecent lamps. This new circuit prevents the
filaments from failing due to notching when
operated on direct current. This failure mode can
reduce lamp life from one half to one tenth of the
data sheet value. In the MR 80 the three lamps that
are used for STEREO, LOCK and FILTER indicators
are operated on AC at lower than rated voltage to ex-
tend the useful life to well in excess of 15 years.
Only Mclntosh brings you this feature.
Your Best
Investment is
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The Frequency Display indicates the
received frequency to the nearest 100 kHz.
A column of LED lamps respond to the
signal strength of the station being receiv-
ed. The stronger the signal the higher the
column reaches.
These touch pads select one of the four
preset stations. Just touch to operate. A
small rectangle will light above the pad in-
dicating which preset has been selected.
AUTO SCAN is provided by two touch
pads. These two touch pads start the tuner
searching for a new station. It will scan un-
til it stops at a station or reaches the end of
the band. At the band end the tuner will
reverse its scan direction.
Three indicator lights are provided. The
uppermost STEREO indicates when a
stereo broadcast is being received. The se-
cond, LOCK, indicates when the tuner is
locked to the station. The third indicator
light, FILTER, indicates when the stereo
noise filter is in operation.
When the MR 80 is turned on the circuitry
automatically selects the manual or knob
tuning control mode.
Two degrees of IF selectivity are select-
able by the SELECTIVITY switch. The NAR-
ROW or normal position provides adequate
selectivity for stereo reception even under
severe receiving conditions.
Setting the SELECTIVITY switch to
SUPER NARROW adds an additional 4-pole
4-zero quartz crystal filter. Use the SUPER
NARROW position to receive stations from
distant cities which are on channels adja-
cent to local stations.
The VOLUME control has been precision
tracked throughout the listening range (0 to
- 65 dB) for accurate stereo balance.
MUTING suppresses the background
noise and hiss normally heard between
stations.
The MR 80 FILTER switch provides auto-
matic stereo noise reduction on weak
stereo stations.
The SCAN control adjusts the speed at
which the tuner searches for the next station.
A two position MODE switch is provided
so that you may select either automatic
stereo or mono reception.
A HEADPHONE jack is provided to drive
low impedance dynamic headphones from
an internal power amplifier.
