Orban 674A - Circuit Description; Input Buffer Circuitry

The
unit
may
be
powered-up
for
additional
troubleshooting.
However,
ensure
that
the
foils
of
the
lowest
(input)
PC board
do
not
short
to
any
metallic
surface.
CAUTION!
FULL
MAINS
VOLTAGE
APPEARS
ON
THE
POWER
SWITCH
TERMINALS
AND
OTHER
COMPONENTS
ON
THE
CHASSIS.
USE
EXTREME
CARE
TO
AVOID
INJURY!
To
avoid
further
disassembly,
we
suggest
that
defective
passive
components
be
replaced
by
clipping
out
such
components
very
close
to
their
bodies,
and
then
tack
-soldering
the
replacement
to
the
lead
stubs.
Be
careful
not to
desolder
these
stubs
from
the
underlying
traces.
If this
operation
is
performed
carefully,
it
should
not
compromise
the
reliability
of the
unit.
4)
Reassembly:
It is
suggested
that
the
unit
be
electrically
checked
before
the
chassis
is
reattached
to
the
panel.
Note
the
CAUTION
immediately
above.
If
further
access
is
required,
refer to
the
detailed
instructions
below.
To
reassemble,
replace
the
POWER
LED in
its
lens.
Then
insert
the
four
studs
on
the
front
panel
through
the
appropriate
chassis
holes.
Use
tool (b)
to
start
the
nuts
on
the
studs,
and
tool (a)
to
tighten
the
nuts.
Check
lead
dress
to
make
sure
that
wiring
between
the
PC
boards
and
the
rear
panel
has
not
become
twisted
or
tangled,
and
that
left
and
right
channel
wiring is
not
in
close
proximity.
Replacement
of
Bandwidth
and
Tuning
Pots:
These
pots
can be
replaced
without
further
disassembly
if
careful
desoldering
technique
is
used.
It may be
helpful to
clip off
each
terminal
of
the
pot,
to
remove
the
pot,
and
then
to
unsolder
each
terminal
individually.
(See
section
Replacement
of
Components
on Printed
Circuit
Boards
below
for
further
hints.)
Instructions
for
knob
removal
are
found
immediately
below.
Disassembly
of
EQ Board
Assembly
(Sandwich
Board):
It is
necessary
to
perform
this
procedure
to
replace
any
of
the
EQ slider
controls,
or if
you
wish to
replace
a
passive
component
on
the
EQ
board
without
using the
shortcut
"clip
and
tacksolder
procedure
described
above.
1)
Knob
Removal:
With a
pointed
instrument or
knife, pop
off
the
sixteen
caps
on
the
BW and
TUNING
controls.
Turn all knobs
fully
counterclockwise,
and
note the
endstop
position
so
that
the
knobs can
be
accurately
reinstalled
later.
Using tool (c)
above,
loosen
the
collets
on
each
knob
and
pull
the
knobs
off.
Gently
pull
off
the
sixteen
slide
pot knobs.
2)
Remove
the
eight
screws
attaching
the
circuit
boards to
the
front
panel.
Pull
the
sandwich
assembly
back so
that
the
pot
shafts
clear
the
panel.
At the side
of the
EQ board
nearest
the
filter
section,
gently
disconnect
the
16-pin
interconnecting
plug.
Note
the
orientation
of
the plug
for
reassembly.
CAUTION!
The
two
boards
in the
EQ
"sandwich"
are
now
held
together
only
with
fragile
pin
and
socket
connectors
--
handle
the
assembly
carefully!