Specifications

separate closely spaced signals in our listening area.
We have been told by some of our friends and col-
leagues, however, that there are some areas of the
country in which high adjacent selectivity is a must if
listeners are to be able to tune to distant weak
signals without being swamped by local strong sta-
tion signals separated in frequency from the remote
desired signal by only one channel width, or 200 kHz.
The next rotary switch activates the stereo
multiplex filter circuitry, either permanently, when
set to "in" or, when set to "auto" automatically
whenever stereo signal strength falls below 100
microvolts or so. Since activation of the filter is
always accompanied by an indicator light mentioned
earlier, the listener is always aware of the filter being
turned on, even if turn-on occurs automatically.
A stereo/mono mode switch follows, and to its
right, beyond the headphone jack, is a continuously
variable control called "scan" which determines the
speed of tuning when in the scan tuning mode. A
variable muting control comes next, which is used to
set muting threshold and also determines scanning
sensitivity, or how strong an incoming signal must
be in order for the auto-scan tuning system to stop
on that signal. Finally, the rightmost control located
just below the tuning knob is a master output level
control which determines audio level at the variable
output jacks on the rear panel and which, when
rotated fully counterclockwise, disconnects power
to the tuner.
The rear panel (Fig. 2) of the MR 80 is equipped
with two 75-ohm coaxial antenna connectors (one
of which is intended for commercial cable input),
300-ohm antenna terminals and a ground terminal.
A jack nearby accepts a supplied plug which is on
the end of a long remote push-button cable that
allows the user either to scan or to call up the preset
stations, one by one, from a remote location, depen-
ding upon the setting of a nearby slide switch.
Variable and fixed level output jack pairs are at the
lower right of the rear panel, while nearby are a pair
of scope jacks (horizontal and vertical) intended for
connection to an oscilloscope for observation of
multipath (reflections) problems. A line fuseholder
and an uswitched convenience AC power receptacle
complete the rear panel layout.
Additional adjustment and controls are located
along the top surface of the tuner. These include
four continuously variable rotary controls which are
used to set up the frequencies of the four desired pre-
set stations, a rotary control which adjusts the sens-
itivity of the signal-strength LED column display (you
can set it so that the strongest station in your area
will cause full-scale readings) and five small push-
button switches. An RF preselect circuit is activated
by the first of these buttons and adds a tuned circuit
between the antenna and the first RF stage to immun-
ize against strong signal overload. The next button
selects the Cable or your own antenna input. The
third button can be used to disable the locking cir-
cuit. The last two buttons provide selection of either
25, 50 or 75 microsecond de-emphasis characteristics.
Circuit Highlights
An electronic antenna switch selects signal inputs
either from the cable input or from one of the direct
antenna inputs and feeds the signal to the first RF
amplifier which consists of a low-noise junction FET
and a high-power bipolar transistor arranged in
cascode configuration. Two PIN diodes are used to
insert a second preselector stage during strong-
signal reception. Tuned circuits are tuned by a series
parallel connection of four matched varactor diodes
which are tuned by relatively high voltage (5 to 26
volts) to eliminate diode non-linearities and possible
IM distortion. Two parallel tuned circuits follow the
RF amplifier to improve image rejection and increase
RF selectivity.
The balanced mixer stage is a matched dual FET
and bipolar transistor circuit. A low loss toroidal
phase-splitting transformer is used as an impedance
matching network in the gate circuit of the mixer. A
bipolar transistor is used as an oscillator buffer to
prevent oscillator pulling on strong signals, and as
the constant current source for the dual J-FET mixer.
Four differential amplifiers, coupled with linear-
phase monolithic filters, comprise the narrow selec-
tivity and signal-strength sections of the IF amplifier.
A 4-pole, 4-zero crystal filter is inserted in the signal
path as well when the Super Narrow selectivity set-
ting is selected. A solid state signal strength meter is
used as a front panel indicator of incoming RF signal
strength. This meter can be user-set to give a full
scale indication on a signal as low as 2 microvolts or
as high as 1 00,000 microvolts. Signal strength vol-
tage is also used to control mono-stereo switching,
automatic stereo filter insertion, muting, and
automatic scan stop. The control voltage also ad-
justs stereo separation at low RF signals, so that
unlike many other tuners, there is no abrupt change
from mono to stereo in the presence of marginally
weak signals and reduced separation also produces
the best possible signal to noise ratio at weaker
signal strength levels.
The limiter following the selectivity section of the IF
amplifier has a total gain of 80 dB for extremely hard
limiting with good impulse noise rejection. A broad-
band Foster-Seeley discriminator is used as the FM de-
modulator, and output of the detector is fed to a buf-
fer stage for isolation from variations in load impedance.
The phase-locked loop stereo decoder IC incor-
porates two special new systems: the automatic var-
iable separation control circuit mentioned earlier and
tri-level digital waveform generation which helps to
eliminate interference from SCA signals and from the
sidebands of adjacent channel FM signals. Following
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Fig. 2 Rear Panel view of Mclntosh MR 80 Tuner.