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When
a
Synth
mode
is
chosen, the correct
FET
(Q102
or
Q103) immediately
turns
on
to
permit
a
smooth
crossfade.
This
scheme means
that
Q102
and
Q103
do
not turn
off as
soon
as
the
main
logic
chooses Bypass
mode.
Instead,
they
remain
on
as
appropriate
until
the crossfade
is
finished.
Achieving
this
requires special
logic,
which
is
realized
in
IC42.
IC42c,
IC42d form an
RS
flip-flop.
If
IC41b
is
on, this flip-flop
simply
follows the
SYNTH
LEFT/RIGHT
latch
lC39a, IC39b.
However,
if
IC41b
is
off
and IC40b
is
on,
the
User Control
Interface
inputs,
not the input latches, control
the
logic
lines.
Flip-flop
IC42c,
IC42d
has one output active
at
all
times. This assures that
at least
one
FET
(Q1Q2
or
Q103)
will
be on
to
assure
a
smooth
crossfade
to
BYPASS,
even
when
BYPASS
has
been invoked
by the
User Control
Interface
input.
"Ramp
Done"
is
indicated by
pin
11 of IC33d's going
LOW
(-15V).
This forces
both
pin
3
and
pin 4 of
IC42
HIGH
(ground).
These
pin 3
and
pin 4 signals
are inverted
by
IC34a and
IC34b,
which
both go
LOW
when
the
ramp
is
done, assuring that both
Q102
and
Q1Q3
are
OFF.
11)
Active
Status
Lamps
Lamps
are driven
from
logic
outputs
to
indicate that the
275A's actual
status
is
SYNTH
FROM
L,
SYNTH
FROM
R,
or
BYPASS,
and
to
indicate
AUTO
Mode.
An
examination
of the
schematic, combined
with an understanding
of the logic
(as
explained
above), should
be
sufficient to
allow understanding
of
the
operation
of
these lamps.
12)
Automation Lockout
Circuit
IC35b
is
configured
as
a
buffer with strong
hysteresis,
achieved
by
means
of
positive
feedback through R340.
This
circuit
de-bounces
S3
(AUTOMATION
LOCKOUT).
Feedback between
IC35a's inverted
output and
its
D
input
causes
the circuit
to
change
state
every time
it
receives a positive-going
edge from
IC35b, thus
toggling
the
Automation Lockout
function
between
ON
and
OFF
every time S5
is
operated.
When
Automation Lockout
is
enabled,
it
turns
IC40a and IC40b
OFF
by
pulling pins
1
and
19
to
ground,
entirely
disconnecting the
SYNTH
L,
SYNTH
R,
BYPASS,
and
AUTO
User Control
Interface inputs
from
the
275
A
logic
system.
7.
REMOTE CONTROL
The
optional
Orban
Remote
Control Panel
275A/RC
is
a small,
passive chassis
which
is
driven
entirely
from
the
275A's
power
supply.
To
avoid wasting
power
supply current,
all
remote lamps
are
placed
in
series
with
the
lamps
in
the
main
chassis.
This requires
that
an
internal plug
be
moved when
the
275A/RC
is
installed to
enable the
remote
lamps.
When
this
plug
is
in
the
Remote
position,
disconnection
of the
Remote
Control
Panel
will
interrupt
current through the
main
chassis
lamps, causing the
main
chassis
to
be dark
and
forcing the
unit into
Synthesize
mode
(because no current can flow
through the
lamps
in
optically-coupled
resistors
IC51 and
1C52).
6-11
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