(typical)
when compared to its reading on
a steady-state tone. It thus provides a
true peak-reading capability
rather
than a quasi-peak capability like an
EBU-
standard PPM.
The
output of IC9B is
rectified by an inverting half-wave
precision rectifier ICI5B
and associated components. Twice
the
output
of
the
rectifier
is summed
(through
R34)
with
its
input
to
create a full-wave rectified signal
at
the
(+)
input
of
ICI9A.
I
C
1
9
A
and
ICI9B operate as
a
peak
detector with peak-holding
capabilities in a
manner very similar to the G/R meter driver.
The reader is referred
to
the Gain
Reduction Meter section of
(3)
(Compressor/Limiter
Control Circuitry), above, for
details.
To
achieve the very
fast
response desired,
the
peak-holding
capacitors CI2 and
CI3
were made relatively small. To achieve
a
sufficiently
slow
recovery time
with
a practical value resistor (R38=22meg), R38 is
bootstrapped
to
the
output
of ICI9B
through R36. R37 introduces enough DC offset to produce approximately 0.5
volts
across R38 at
all
times. This
multiplies
the
effective value of
R38
by about 30x,
slowing
down
the recovery time as desired.
6)
Power
Supply
Unregulated voltage
is
supplied by two pairs
of
full wave diode rectifiers CR3I,
CR32 and
CR33, CR34
operating into a
pair
of energy
storage
capacitors
C28, C29.
The power transformer Tl
is
switchable for either
115 volt or 230
volt operation;
the
two sections of the primary are paralleled for
115
volt operation and
connected
in series
for 230
volt operation.
The nominal
unregulated voltage is +22 volts DC
at
rated line
voltage. This will
vary
widely
with line voltage variations. Regulator dropout will occur if
the
unregulated
voltage
falls
below
about +17.8
volts.
Regulated voltages are supplied by a pair
of
overrated
500mA
"three terminal"
1C
regulators IC5,
IC6.
Because they are operated so conservatively,
they can be
expected
to
be extremely reliable. Therefore, before replacing the regulators, check
to
see
whether other
abnormalities in the circuitry (such as a shorted 1C)
have
caused excessive current demand which
is causing the regulator IC's to go
either
into current
limiting
or into thermal shutdown, their two built-in protective modes.
If
it becomes necessary to replace
a
regulator, be
sure to
replace its
heat
dissipator securely.
The regulators
IC5
and IC6 are frequency-compensated by
C30,
C3I at
their
outputs to prevent
high
frequency oscillations. If
C30
or
C3I
are ever replaced, be
sure
to
use low-inductance aluminum electrolytics. Tantalums can fail because
the
current-delivering capacity
of
the power
supply can cause
a
runaway condition
if
the dielectric
is
punctured momentarily;
high-inductance aluminums can fail
to
prevent
the regulators
from
oscillating.
It
is therefore necessary to check for
oscillation on
the power
bus
with
an oscilloscope
if C30 or C3I is
replaced.
In
addition,
small
0.05uF/25V
ceramic capacitors
bypass
the
power busses
to ground
locally throughout the board to prevent signal-carrying
IC's
from oscillating
due to
excessive power-lead inductance.
39
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