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4.7
METER AMPLIFIER
This circuit receives its signal from the line amplifier
.
Attenuator
R50, R51
reduces the level, and current sense resistor
R27 allows the
amplifier
to
compensate the meter reading
for
differences
in
output loading.
If
switched to the average reading
characteristic, the meter
is
driven directly from
the output of
IC
5A.
Resistors R69 and R70 are selected to indicate
0
VU
on
the meter
when
the actual output level
is
+4
dBm
(1.23 V) or
+8
dBm
(1.95
V
)
.
The signal for the meter's peak reading characteristic
is
de-
veloped
in
rectifier circuit
IC
5D and
amplifier stage
IC 5C
and
IC 5B.
The gain of the circuit
is
designed
to indicate
a
6
dB lower
reading when compared to the "average" characteristic.
The time
constant
for the
rise
and fall of
the meter
is
determined by com-
ponents
C51
,
R60, and R59.
When
the
meter
is
switched to gain reduction
(GR),
one side of
it is
biased with network
R61 and R62 to read
zero when no limiting
occurs.
The positive GR control voltage, buffered
in IC
7C,
causes
a
reduction
in
potential difference across the meter terminals, thus
deflecting the needle of the meter downscale
(left hand side) in
proportion
to the
amount of limiting.
Calibration (tracking)
is
adjusted with
R65.
Components
R67, R68, and
CR15 ensure linearity
and
correct meter ballistics.
4.8
POWER SUPPLY
The power supply
is
bipolar employing two integrated circuit
voltage regulators, VRl
and VR2, to
provide low-ripple,
±18 volt
DC.
Additional filter capacitors assure power supply stability
and
low noise.
The pilot LED
is
connected
to the
positive
and
negative sides
of the power
supply to indicated
a
power ON condition.
- 19
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Troubleshooting
