HP
5384A and
HP
5385A
Service
8-251. Transistors
Q1E
and Q1D, and
R10
and
R15
form
a
10
rnA constant
current
source. The
current
source
is
fed
through
CR8
in a
wired-OR
configu-
ration. This
trickle
charger always sources
10
rnA
of
current
to
the 6-volt sealed lead acid battery when
external
power
is
applied
to
the
counter.
8-252. The
Op
Amp
U2B
and associated resisters
form
a closed
loop
constant
current
charger
of
approximately
500
rnA. The resistor
R1
(.05
ohms)
is
the
current
sense resistor
providing
the
current
to
voltage conversion
for
feed back
in
the
loop.
The
500
rnA charger
is
turned
on and
off
by
the charge
control
circuit
through
diode
CR4.
8-253. When
the
counter
is
switched
into
the
ON
position,
the
charger may be disabled
independently
of
the charge
control
circuit
by
injecting
current
through
CR1.
When
the
instrument
is
in
Standby,
the
charge
control
circuit
will
command
the
500
rnA
current
source
to
be
on
if
the
battery voltage
is
below
6.90 volts, and
will
continue
at
the
higher
rate
of
charge
until
either
the
battery voltage reaches
7.35
volts
or
the
instrument
is
switched
from
Standby
to
ON.
Once
the
battery voltage reaches 7.35 volts,
the
battery
is
trickle
charged at a
10
rnA rate.
8-254.
If
the
battery
has
not
been charged
to
near
100% capacity by
the
charger, the
10
rnA
current
is
not
sufficient
to
keep the battery above
6.90
volts. The
high
current
charge rate will again be activated
until
the battery voltage reaches
7.35
volts. The charger
will
continue
to
cycle
from
low
to
high charge rates
as
the
battery voltage varies
between
6.90
and
7.35
volts. The
difference
between
the
two
voltages
is
a hysteresis
band
of
.45
volts.
8-255.
Potentiometer
R26
on
the
A3
battery charger
board
is
used
for
setting the
upper
hysteresis
point
to
7.35
volts. The
lower
hysteresis
point
automatically
tracks
.45
volts
below
the
upper
setting. The
upper
hysteresis level
is
set
to
7.35
volts at 25°C. Temperature
compensation
is
provided
by
CR7.
At
some
point
near
full charge,
the
10
rnA
current
will
prevent
the
battery
voltage
from
dropping
below
6.9
volts, and
the
battery
is
then
trickle
charged.
8-256. When the battery voltage drops
to
5.95
volts,
comparator U2 causes
Q1A
to
saturate. The active
low
output
of
Q1A
causes a
LO
BATT
indicator
to
appear
on
the
front
panel display. The
counter
will
continue
to
operate
for
approximately
20
minutes
longer.
8-257. When the battery voltage
drops
to
5.35
volts,
the
counter
may
not
operate reliably,
so
the
counter
8-32
power
supply
will
be automatically
turned
off. Battery
voltage
of
5.35
volts causes
comparator
U2D
to
saturate Q1C. The
output
of
Q1C
is
sourced
to
pin
9
of
U9 in
the
DC
to
DC
converter
power
supply on
the
A 1
motherboard.
The
RC
network
R7
and C2 create a
time
constant
that
delays the reference voltage
from
pin
2
of
U1
long
enough
to
prevent
power
supply
shutdown
on
initial
power
up
of
the
counter.
Diode
CR2
and capacitor
C5
integrate
the
de
input
which
holds
the
de
input
voltage
on
U2D
long
enough
to
prevent
the
power
supply
ripple
caused by
the
de-to-
de
converter
from
causing
premature
shutdown.
R16
provides hysteresis
for
the
shutdown
detection
cir-
cuitry
so
the
instrument
will
not
cycle
on
and off.
8-258. TROUBLESHOOTING
8-259. Introduction
8-260. The
HP
5384A/HP 5385A
is
a
microcomputer
based system. Part
of
the
instrument
circuitry
consists
of
digital
logic
controlled
by
the
microcomputer.
Troublshooting
is
accomplished by using
built-in
diagnostics and manual tests.
8-261. The
troubleshooting
philosophy
for
the
counter
is
to
divide
the
counter
into
five main blocks
so
the
problem
can
be
more
easily isolated.
Once
the
faulty
block
has
been isolated,
refer
to
the
appro-
priate schematic diagram.
If
the
repaired
circuit
requires adjustment after
being
repaired, refer
to
Section V and
perform
the
adjustments
as
described.
Proper static handling techniques must
be
employed when servicing semiconductor
products. The
voltage susceptiblity
of
all
IC
and transistor families are well below levels
commonly
found
in
service environments.
Exercise care and observe standard static
precautions.
8-262.
listed
below
are some special considerations
and
helpful
hints
to
keep in
mind
when
trouble-
shooting
the
5384A/5385A. They
will
help
avoid
potential
problems
when
troubleshooting.
a.
When
making measurements at IC sockets
(with
the
IC removed), avoid inserting
the
probe
tip
into
the sockets. This may cause contact
problems
when
the IC
is
re-inserted. Attach a piece
of
26-gauge
wire
to
the
probe
and insert the
wire
into
the
socket.
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