Technical Description - Commodore 1901 Service Manual

TECHNrCAL DESCRIPTION
SWITCH MODE POWER
SUPPLY
The SMPS works from 220 AC to 240 AC at both
50 Hz and 60 Hz frequency with an output power of
50 watts. The output voltages available are 114V and
24V. The 114V is used for the line output stage whereas
the 24V is used for the oscillators, line driver stage and
video processing stage. The supplies for sound, heater
and video output amplifier are then tap from the out¬
put line transformer.
The AC mains voltage is applied to line filter LP01 via
an on-off switch and a fuse which provides overload
protection. This voltage is then rectified by DP01 to
DP04 and partially filtered by CP04. The resulting
DC voltage then acts as a supply voltage for switching
transistor TP01 which is controlled by 1C TDA 4601.
During start-up time, the 1C 4601 is supplied from the
mains through RP12. However, in normal operation,
the voltage will be tap from windings 11/9 of the switch
mode transformer. For low input supply voltage, the
switch on time will be delayed by resistor RP12, hence
a speed up circuit is necessary. This comprised of tran¬
sistor TP02 and its surrounding components.
The variation of output voltage is simulated by the
feedback control winding 1/9. This voltage is rectified
by DP05 and applied to the amplifier input pin 3 of
control 1C. It will be compared with the reference
voltage of 4V at pin 1 and the error voltage produced
is amplified and converted to pulse width modulation
in the 1C. This will in turn drive the switching transistor
TP01 at pin 7. The amount of base current is deter¬
mine by RP11 between pin 7 and pin 8.
The divider RP08, RP09, RP19 at pin 5 monitors the
unregulated DC voltage and hence the AC input. If the
input is lower than 80V, the voltage at pin 5 will be
lower than 2 volt and the 1C will subsequently go into
standby operation. This is to avoid operation at too low
a supply voltage.
The collector current is simulated by an external RC
combination of CP07, RP07, flP20 at pin 4. The voltage
is clamped at 2V and the sawtooth rising AC voltage can
vary up to 4V. This is the maximum limit of collector
current which will trigger the 1C for safety operation.
The purpose of this is to prevent overloading of collector
current At pin 2, the zero crossing of the output AC is
monitored for setting the reference of the base drive
pulse of TP01.
The secondary pulses from winding 2/6 and 4/6 are
then rectified and filtered before supplying to the other
stages of the set.
HORIZONTAL AND
VERTICAL SCAN CIRCUITS
The 1C used for this function is IL01 TEA 2017. 2
supply voltages are required here; the higher 24V for the
frame'output drive and the lower 12V derived from the
RC filter RP17, CP18 and regulator IL02 7812 for the
oscillators circuits.
The input to the 1C is composite signal via RL02, CL02
to pin 7. Provision for grounding the input is by 8AOS
(used when aligning the free running oscillator fre¬
quencies). The DC level at pin 7 is clamped by RL03
and RL04. Internally, the sync, separator will separate
the video information and synchronization pulses,
the slicing level of which is determine by RL05 and
RL06.
Line flyback pulse is integrated by RL07, CL05 and
subsequently coupled through CL06 to pin 3. The pur¬
pose is to compare with the input synchronizing pulse
to obtain the correct triggering time for the line
oscillation. The DC level at pin 3 is adjustable by PL01
to allow a small range of shifting in the picture. RL11
and CL09 determines the frequency of the free-running
line oscillator and can be adjusted by PL02. RL10
controls the lock-in range.
The frame oscillator frequency is determine by RF01,
CF01 and PF01 (adjustable). The components DF01,
CF02, DF02 are used to double the supply voltage
necessary for the frame deflection. Pin 15 is the frame
output signal to drive the deflection coils, BA02. The
S-correction required is achieved by CF06 and the DC
feedback is obtained by RF04 and RF05 back into pin
10. PF02 controls the amplitude of the picture.
Pin 1 of the 1C is the line pulse to drive transistor
TL01, The collector voltage is step down via UL01 to
provide sufficient base drive current for TL02 —
switching transistor for the flyback transformer, UL02.
The collector of TL02 is connected to the deflection
coils, BA03, which is driven by a linear sawtooth current
to produce the necessary horizontal scan. Linearity
of the horizontal scan is by LL01 and S-correction
by CL12. CL11 is the tunning capacitor and determines
the scanwidth.
The supply to the flyback transformer is the 114V rail
from the switch mode power supply. A damping net¬
work of RL17, CL13 and LL02 is provided to reduce
ringing on the picture. The secondary of the flyback
transformer is tap and rectified, before going to the
other stages of the set.
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