LeCroy 9450 - Page 51

The following two adjustments
have
to be
made after
the
front-end
9450-7
has been mounted into
the
scope
:
1.
9420 50
Ohm
Overload
Protection
Adjustment
Procedure
Two
different thermocouples
exist;
an
old
type wrapped in
a
heat
shrink
tube
and
a
new
type wrapped in
copper wire.
The
adjustment
procedures
are
the same
for old
and new
thermocouple,
but
they
depend on
the
Revision of the
9450-7
front-end
board:
Procedure for
9450-7
up to Rev D:
-
The front-end has
to be
in
the
DS0 with
the RF shields
(aluminum
covers)
mounted
and the
upper DS0
cover
closed. Warm
the
scope up for
20 minutes.
-
Make sure the
9450-7
front-end
board is
at ECO
1003,
MCN 1
or
up.
-
Apply
5.5
V to
CHI
set to 50 Ohm.
Wait for
30 sec. Adjust CHI
overload detection
(potentiometer
through opening
in
9450-7
aluminum
cover) such that pin
10 of A21
is
about 200 mV below
threshold on
pin 11 of A21
(about 1.6 V).
-
Apply
7.0 V
to
CHI.
Adjust
potentiometer
such that the
overload
trips within 5
to 20
seconds. Between
each
test,
allow
the
thermocouple to
cool
to
ambient
temperature. Make sure
it does not
come
on by
itself
after
power on.
It should not come
on with
a
voltge of 5.0 V applied
to the
input.
-
The same for CH2, where the
adjustment is made
at
pin
8
of A21 for
the threshold
at
pin
9
of A21.
Procedure for
9450-7
at
Rev F
and
up:
-
The front-end has to
be
in
the DS0
with the RF shields
(aluminum
covers) mounted and the
upper
DSo cover
closed.
Warm
the
scope
for
20 minutes.
-
Put jumpers on pins 1 and
2 of
J12 (CHI) and
1 and
2
of
J10
(CH2).
This
disables triggering of
the
overload system. Pins 1 and
3
off
J12
are on the side facing
the
potentiometers:
J10
pin2 pin4
pin2
pin4
J12
pinl pin3 pinl
pin3
R15
R16