Philips PM 5715 - Page 48

Attenuator
and d.c. offset
circuit
In
the circuit diagram,
the attenuator is drawn
in
position "10V",
that is, switch SK11
is
depressed. This
means
that the reed
switch
RE201
disconnects
the
50D
internal
load
R262, R263,
that is terminating
the output
stage
at all remaining
amplitude
settings.
A d.c.
offset from
+2.5V
to
—2.5V
can be in-
troduced
by adding
a positive
or negative
current
to
the output line via L201.
This
function
is performed
by transistors
TS226...TS229
that are controlled
by front
panel
potentiometer DC
OFFSET,
R4.
In
the 10V
position,
the offset current
is reduced
by one
half,
because switch SK1
1 is
connecting
resistor
R275 in series
with R274.
POWER SUPPLY, UNIT
PM
5715-3
Refer
to
the circuit diagram
Fig. 12
—
10
or
Fig.
12
-
12.
This unit provides three stabilized and overload
protected supply voltages:
+
22.5V, +6V
and
-22.5V.
The
+
22.5V,
that is presettable
with
poten-
tiometer R313, also serves as the
reference
voltage for the other two stabilizing circuits.
Since the principles of function is the same for
all three circuits only the
+
22.5V
portion will
be described here.
The
d.c.
voltage from rectifier bridge GR301 is
fed
to the
collectors of series regulator TS302
and its
current driver TS301
.
Operational
amplifier IC301 controls
the
current
driver TS301 via zener diode GR304. Zener
diode GR305 provides the reference voltage for
the operational amplifier,which senses and
counteracts the variations of the supply
voltage.
Overload and short-circuit protection is
achieved with resistor R308 and transistor
TS305, that is normally cut off.
An increased load current will increase
the
voltage drop across R308
and
bias TS305 into
conduction. This will reduce the current output
of series regulator TS302.