Orban 424A Operating Manual page 43

(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