Technical Description
TUNING MECHANISM AND DIAL DRIVE
In the MR 78, the unique design and careful manu-
facture of the mechanical dial drive assembly gives
smooth flywheel tuning.
By controlling the relationship of mass and mechan-
ical resistance, and by dividing the workloads in the
dial drive system, it becomes nearly impossible to
detect any backlash. Yet, the entire dial drive is a
model of mechanical stability.
For added ease and increased tuning accuracy, a
section of the dial pointer is illuminated.
FRONT END
FM signals enter the tuner through the antenna ter-
minals. A coaxial cable conducts the signal to the RF
amplifier. In the amplifier the stations are separated,
amplified, and converted to a common 10.7 MHz IF
frequency. Instead of using a conventional easily
overloaded transistor or FET as a straight RF ampli-
fier, the MR 78 uses a rugged Junction Field Effect
Transistor as an impedance converter to drive a 5-watt
power transistor. This combination (a cascode cir-
cuit) makes the RF amplifier virtually impossible to
overload or cross-modulate. As an example, if you
are tuned to a 3 microvolt signal at 96.3 MHz, the MR
78 will reject signals elsewhere on the dial which are
at least 4,000,000 times stronger. Thus, a 12 volt signal
received at 104.3 MHz would not bother the signal at
96.3 MHz. When tuned to 104.3 MHz, the tuner will
not overload. All MR 78 tuners must pass the 12 volt
overload test.
The MR 78 has an antenna matching circuit in the
RF input. In fringe areas a broad low-loss RF circuit
provides maximum sensitivity for a low-gain antenna.
In metropolitan areas crowded with many strong sig-
nals, a sharp RF input circuit greatly reduces spurious
responses and achieves maximum sensitivity when
connected to a high-gain antenna.
The RF bandpass of the antenna matching circuit
can be modified by switching the resonator L1 C1 in
or out of the circuit. Broad band pass is obtained by set-
ting the ANTENNA MATCHING switch to the LOW
GAIN ANT. position. This c24 volts to the cir-
cuit turning diode D1 off and diodes D2, D3 on. In this
configuration the antenna is connected directly to the
RF amplifier input resonator L2 C2. Diode D3 is tapped
on L2 to yield optimum noise, while D2 shorts reson-
ator L1 C1 so its loss is not coupled into the circuit.
Sharp bandpass is obtained by setting the ANTENNA
MATCHING switch to the HIGH GAIN ANT. position.
This connects -12 volts to the circuit, turning diode
D1 on and diodes D2, D3 off. The RF input circuit
becomes a double tuned filter with high Q in both
resonators, which provides sharp bandpass. Diode D1
is tapped on L1 to yield optimum selectivity. The oscill-
ograph below shows the two bandpass curves of the
RF input circuit.
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