Racal Instruments 9901 Maintenance Manual page 51

at IC2/13. This reset signal
is
differentiated
in
C43/R85 and
turns
off
Q27,
The
'high'
at
Q27
collector
is
inverted
in
IC12e
and
the resultant
'O'
at
IC17/4 briefly
blanks the display
while
the
instrument
is
preparing
for
a
new
measurement.
When
switch
1S7
is
set
to
HOLD
the
+5V
supply
is
disconnected
and
capacitor
C46
cannot charge up, thus
preventing
any
reset
action
.
Manual
Reset
4.40
This
is
generated
in
IC9a.
Pressing the
manual RESET
button (S52) puts
a
'O'
on
IC9a/l
.
The
resulting
'O'
at
IC9a/5
is
inverted in
IC6b
to
put a
'hold'
on
the
counter via
IC2/13. Release of
the
manual RESET allows the 10
kHz
clock signal at
IC9a/3
to
drive the 'Q'
output (IC9a/5)
to
the
'!'
state,
which
resets
the
counter via IC2/13.
At
the
same
time the transition of
IC9a/6
from
'O'
to
'!'
is
differentiated
by R68/R90/C45
and
applied
to
IC2/4
via
IC6a
and IC13e
to
momentarily unlatch the
data
stores
and
clear
the
previous reading
.
External Reset
4.41
An
external
reset signal
with a
minimum
duration of
5 ms can be applied
via
pin
7
of the rear
data connector.
This reset
is
applied
to
IC9a/1
and
operates
in
the
same
way
as
manual
reset (see previous para.).
Segment Check
4.42
This
is
obtained by
selecting
CHECK
and
holding
down
the
manual
reset
button.
In
Check
position the
switch 1S5
(via
R95) applies a
'1'
to
enable IC15a. Depressing
the
manual
reset
switch S52 applies a
'1
'
(via
IC13a/2)
to
IC15a/1
.
This
feeds
a
'O'
to
the
segment decoder IC17/3 which produces an
'all
eights'
display while the reset button
is
held
down.
FREQUENCY STANDARD
Standard Oscillator
Assembly 19-0834
4.43
In
the
standard instrument
the
reference frequency
is
generated
in
a 5
MHz
discrete
component assembly attached
to
the rear panel with access
to
the trimming
capacitor (C4), via
a rear panel aperture.
The
circuit
is
shown
in the
main p.c.b.
circuit
(Fig. 4)
and
also
in Fig. 2.
In
early models
the
5
MHz
oscillator
was mounted on
the
main
p.c.b.
and
the
output
connected
via link
LK1
.
This
early circuit
(Q17, XL1,
C35
etc.)
is
shown
in
Fig. 4.
The
circuit
is
identical
to
the
19-0834
in
Fig. 2,
but
the
component
references are
different.
Optional
Oscillator
4.44
The
fast
warm
up
oscillator
Type
9442
is
mounted
a
B7G
base
and connected
as
shown
in
the circuit
diagram
(Fig. 4).
This
a precision item.
If
a fault develops,
users
are advised
to
return the oscillator unit
to
Racal Instruments
Ltd. or
approved
agent,
for
servicing.
No
parts
list
or circuit information
is
provided
in
this
handbook.
4-8
9901 Vol
.
2