Philips PM 3266 - Page 134

139
Switching-on
protection
If the
instrument
is
switched-on
and
no
convertor
voltage
would
appear
(due
to
a
possible
defect)
the
PTC
resistor
R1806
will warm
up,
reducing
the
current
through
VI
803 to
a
safe low
value.
Output
circuits
The
output
rectifiers
are of
the
coil-input
types
delivering
the mean
value
of
the
sine-wave
transformer
voltages
across
the
output
capacitor.
Except
the
d.c.
voltages
the
convertor
transformer
delivers
also:
-
6,3
V
-
1 kV
-
1,5
kV for the
focus
and
high
tension
circuits
-
1
20 V for
the
additional
power
supply
unit.
Photo-coupier
circuit
This
circuit
delivers
a sine-wave
voltage
(derived
from
the
mains
voltage) used
for
mains
triggering
or
mains
deflection.
The
photo-coupler
VI
842 which
provides
isolation
between
the mains
voltage
and
the oscilloscope's
circuitry
drives
the VI
843-circuit
in
saturation,
so that
the square-wave
voltage
at the
collector
of VI 843
has the
same
value
for
all mains
voltages.
With
an integration
network
R 1
851
,
1852,
1853 andC1836,
1837,1838
the original sine-wave
is
obtained.
Via V1844
and
V1846
this signal is applied
to the trigger source
selection
Calibration
circuit
The
calibration
unit
is
a
square-wave
generator
consisting of
an operational
amplifier
D1901
with feedback.
The
oscillator
frequency is
determined
by resistor
R1909 and capacitor Cl 903.
Capacitor Cl
902 keeps point
3 of
the 1C constantly
equal
to the
average output
voltage.
Consequently,
the generator
is independent
of
fluctuations
in the
supply
voltage. The
square-wave
amplitude is
determined
by zenerdiode
VI 901.
Potentiometer
R1906
allows accurate
adjustment
of
the output
voltage and output
current.
This
output voltage
is fed
to socket XI and
the output current
flows through
current
loop X2. This is
the
front
panel
CAL terminal.
The calibrator
output signal can
be used for
probe compensation
and/or checking the
vertical deflection
accuracy.