Philips pm2521 - Page 143

3
1.3.
LEVEL
CONVERTORS
When the
battery
is
being
discharged,
(power switch
in
position
"ON",
PM2521
disconnected
from
the
mains)
the battery
voltage is
supplied to V501 .
Due to this
the
circuit
which
is
made
by
T501
,
A401
(Q3),
R502
begins
to
oscillate.
This
oscillation
produces
a
positive
voltage (+13V)
on point 3
of X101
and
a
negative
voltage
(-13V)
on
point
6
of
XI
01
(with
respect to pin
4 of XI 01).
The
centre
pin
of T501 is
fed to
the
base of
Q1
(A401)
and
this
provides a
base
current
for V401
and
V402.
The result
is a
voltage
of +5V
on pin 5
of
X101
(with
respect to
pin
8
of X101 ).
1.4.
SCHMITT
TRIGGER
The
Schmitt
trigger
circuit
consists of two
transistors
Q4
and
Q5
(A401
).
When the
voltage
of the
battery drops
below 5,5V,
the
Schmitt
trigger
switches the
output
(pin
7 of X101
)
from +5V to
OV
(with
respect to
pin
8
of
X101).
2
ACCESS
Dismantling
the
battery power
supply:
—
Remove the top
cover as
described in
Chapter 2
of the
PIV12521
Service
Manual.
—
Disconnect
the connector
XI 01
.
(Fig.
2,
item 1
.).
—
Remove
the two
screws from
the battery
power
supply
cover. (Fig.
2,
item 2.).
—
Lever up
the
cover and
remove it.
—
Remove
the two
screws. (Fig.
2,
item
3.).
—
The battery
and
the
printed-circuit
board can now
be
removed.
Fig. 2.
Dismantling
the
battery
power
supply