McIntosh MR80 Owner's Manual page 17

sor reacts in a small fraction of a second so the
tuner appears to change stations instantaneously,
without any interstation noise even with the muting
turned off.
When either of the scan sensors is touched the
tuner is muted and the display blanked for two
seconds, this allows the tuner to reach the next sta-
tion under normal listening conditions without any
interstation noise. If the scan speed is set to a slow
setting and the muting is turned off, interstation
noise and very weak stations will be heard after the
two second delay, which is very useful when remote
tuning is used.
Driven from the tuning control logic are six LED
lamps. Four are used to indicate which of the
presets are in use. The other two lamps indicate the
direction of scan. These six lamps and the frequency
display indicate at a glance how the tuner is
operating and what preset or scan direction has
been chosen.
LOCK CIRCUIT
An innovative new circuit was developed for use in
the MR 80. This new circuit allows correct tuning
without the use of a center tune meter. The MR 80
will be correctly tuned even if the station or the
cable company is not on its correct frequency. This
is done by the use of two operational amplifiers,
A deviation signal from the detector is fed into the
first amplifier. This circuit is a log amplifier that pro-
duces an output voltage proportional to the log of
the DC component in the detector output. Because
the output voltage contains audio signals, a second
amplifier, connected as a switched gain low pass
filter, removes the audio signals. This amplifier has
a very high gain when the lock circuit is on and unity
gain when the lock circuit is off. An N-channel J-FET
is used as the gain switching element in the
negative feedback path. The filter output, now called
the correction voltage, is fed into a scaling circuit
that compensates for the tuning diodes nonlinear
frequency to voltage response. Both of the
amplifiers operate with greater than 50dB of gain at
DC. With a closed loop gain of greater than 100dB
the tuning error when locked is less than 1kHz at
100MHz, or 1 part in 100,000. This error is the same
as a crystal controlled synthesizer with the addi-
tional benefit of being correctly tuned even if the
station or the cable signal is not on the proper
frequency.
Because the lock circuit will track a station even if
it drifts 1MHz, the lock circuit must be turned off for
the operator to be able to tune the MR 80. This is the
reason for the touch sensor on the main tuning
knob. To prevent the tuner from locking onto a
strong signal next to a weak signal, a circuit is used
to sense the strong adjacent channel signal and in-
hibit the lock circuit under these conditions. On the
top cover control panel is a manual lock ON/OFF
switch that inhibits the lock circuit only if the tuner
is in the main tuning knob mode. This provision is
made for testing the tuner and for cases where the
station being received has a deep fade to the point
of losing lock. If the switch is set to the off position
the lock circuit will work normally for all the presets
and the scan circuit, but not for the tuning knob.
SCAN CIRCUIT
With the voltage tuned RF section, automatic
scanning is implemented without any moving parts
such as motors, flywheels, or strings. By the elimina-
tion of all moving parts, a much greater degree of
reliability is achieved and tuning convenience im-
proves.
The MR 80 scan circuit consists of five sections:
1) switching circuit; 2) bipolar current source; 3) in-
tegrator; 4) band limit detector; and 5} the auto-track
circuit. The scan switching circuit controls the scan
function by switching two flip-flops, the scan on/off
flip-flop and the scan run/stop flip-flop. When either
of the scan buttons or the remote control starts a
scan cycle, the preset buttons and tuning knob cir-
cuits are switched off, the lock circuit is inhibited,
the display is blanked, the audio is muted, and the
scan on/off flip-flop turns on. The display is blanked
and the audio is muted for two seconds. The scan
on/off flip-flop switches the RF tuning control to the
scan ramp voltage.
Depending on the scan sensor touched, the
bipolar current source is set to the proper value
(negative for scan up, positive for scan down). This
current is adjusted by the front panel SCAN SPEED
control for the proper search speed. The run/stop
flip-flop stops or starts the scan sequence. This flip-
flop is set whenever a scan stop signal is sent from
the multiplex. To reset the flip-flop one of the scan
sensors must be touched.
In the integrator portion of the scan circuit the cur-
rent is converted into a time dependent ramp voltage
until a stop signal is received from the multiplex.
When the tuner has stopped on a station the tuning
voltage is stored on a special low leakage capacitor.
To prevent drift due to leakage currents, guard
tracks were employed around the negative input to
the integrator operational amplifier. This amplifier is
a special type with MOS-FET input transistors that
have an input resistance of greater than 100 million
megohms. With special circuit components and
printed circuit layout, drift in the scan circuit has
been eliminated. The positive input to the opera-
tional amplifier is used as the input for the lock cor-
rection voltage. The correction voltage is only ap-
plied after the tuner has stopped scanning and fine
tunes the tuner to the center of the station.
Two comparators connected to the tuning voltage
input in the RF section are used for band limit detec-
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