McIntosh MR80 Owner's Manual page 18

tion. The comparators sense when the tuning
voltage exceeds the value needed for 87.5 or
108.1MHz and generates a pulse that toggles the
flip-flop controlling the bipolar current source,
changing the direction of scan. If a person attempts
to scan past the band ends, both scan indicators will
light and the tuner will hang at the band limit until
the sensor is released.
When any of the presets or the tuning knob are ac-
tive the auto track circuit is used to force the output
of the integrator to a value equal to the current tun-
ing voltage. During the turn on delay the auto track
ensures that the integrator's output voltage is within
the band limits. This circuit allows the tuner to start
searching for the next station from the current loca-
tion.
FREQUENCY COUNTER
The counter and display form the digital
equivalent to the slide rule dial and pointer, but with
greater accuracy and readability. In the RF section a
small portion of the oscillator signal is coupled into
the buffer amplifier on the counter circuit board.
This buffer amplifier is made with emitter coupled
logic circuits and is used to change the small
oscillator signal into a level acceptable by the high
speed ECL divide-by-four circuit. The output of the
divider is level shifted to low power Schottky levels
for use in the rest of the counter circuit. The divided
local oscillator signal is sampled for 40.96mS by a
synchronous gate, made from a J-K flip-flop to
eliminate the ± 1 count error. The actual counting is
done by low power Schottky integrated decade
counters. IF frequency offset is accomplished by
presetting the counters to the complement of the IF
frequency, 893. This allows subtracting the IF offset
by adding the negative of the IF frequency.
The gate signal is derived from a stable 200kHz
crystal oscillator and a binary divider made from LS
integrated circuits. Two other signals are derived
from the divider chain. The first is used to transfer
the information in the decade counters to the
display buffer. The second is used to reset the
counter to the complement of the IF frequency.
In the display portion, three TTL integrated cir-
cuits are used as a latch to store the frequency infor-
mation and as constant current sources to drive the
LED frequency displays. By using the latches, a
flicker free display is assured even when the display
is changing. The constant current sources improve
reliability by reducing the number of components.
LED displays were used because of their visibility,
character font, and very long life.
POWER SUPPLY
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 us-
ed for the headphone amplifier and the touch control
reference signal driver.
LAMP DRIVERS
Mclntosh, recognizing the need for improved relia-
bility, has designed a new circuit to drive incande-
cent 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 extend the useful life to
well in excess of 15 years. Only Mclntosh brings you
this feature.
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