CTX CMS-3435 - Circuit Descriptions; Power Supply Circuit; Video Circuit

DESCRIPTION OF CIRCUITS
Power Supply Circuit
The power supply consists of 2 regulators, one
is a non-synchronous self-excited power con-
verter which supplies 200V, 95V, 21V, 12V, 6V,
the control loop is to keep 95V regulated, Q103
serves as an error amplifier, |C101 (4N35 optical
coupler) couples back the error to the primary
side to control the on time of the driver Q102,
thus controls the off time of the switcher Q101,
this loop tends to regulate the 95V output, once
the 95V is reached, Q102 turns on and Q101
turns off, then the energy stored in the trans-
former transfers to the secondary side. When
the energy is dried out, an induced magnetic
force turns Q101 on, the current flows through
Q101 to the current limiter R110, a sawtooth
voltage is thus set up across R110, this voltage
couples through R109 and C110 to the base of
Q102, imposes on a DC level controlled by the
error amplifier Q103. When the 95V tends to
drop due to a heavy load, the DC voltage of Q102
base drops too, so it takes longer time for the
sawtooth imposed on this DC to reach the on-
threshold of Q102 base, the result is that Q101
will switch on for a longer time, and output more
energy to make the 95V output back to its level.
When the load is getting lighter, the 95V outputs
tends to rise, this makes the DC in Q102 base
higher, so the sawtooth takes less time to turn
Q102 on, and cut Q101 off, so Q101 will turn on
for a shorter period, the energy being transferred
is less and the 95V output is kept constant.
The multiple secondary output DC voltages are
as follows:
200V to be further regulated to supply
horizontal deflection output
stage.
95V mainly for video output amplifier
and horizontal drive circuit.
21V _ is for vertical deflection output
amplifier and frequency voltage
conversion circuit.
12V __is for deflection processing IC
and sync separation circuit.
6V _ isfor CRT heater and logic circuit
on interface board.
The 200V output is further regulated by a
synchronous step down regulator to a voltage
ranging from 50V up to 120V for the horizontal
deflection output circuit and maintains the CRT
anode voltage fairly stable in spite of various
Scanning frequencies.
10
The step down converter is synchronized with
horizontal scanning frequency. The horizontal
drive pulse is taken from IC301(LA7850), ampli-
fied by Q104, then differentiated through C122,
and the voltage sensed for comparing is taken
from the FBT secondary voltage, error amplifier
Q108 amplifies the difference between reference
voltage (ZD104 + D114’s forward voltage) and
the sensed voltage for comparing, the result is a
DC level variation in the base of Q105. The diffe-
rentiated horizontal pulse from C122 is imposed
on this DC, thus the DC level variation can deter-
mine the on / off time of Q105. Q105 will turn
Q106 on / off, and Q106 drives the output Q107
through the drive transformer. When Q107 is on,
current flows through choke L104 and charges
C128 to the desired level, when Q107 is off, the
energy stored in L104 will charge C128 through
the fly-wheel diode D112 to maintain the output
voltage. This output voltage applies to the FBT
and generates the desired H.V.. Since the sens-
ed voltage for comparing is taken from FBT se-
condary side, it is propotional to the H.V., any
variation in H.V. will reflect on the sensed volt-
age, through the control loop, the supplied DC
voltage for FBT always tends to regulate this
variation and generates a fairly stable H.V.
Video Circuit
The input signals from 9 Pin D type connector
are first selected by the TTL / Analog Switch
(SW401). If the video signals are analog RGB
signals, it will be sent directly to the video
preamplifier 1C501 (M51387P), and the signals’
level is controlled by Q403 and Q404 for users
to adjust picture contrast to their preference. If
the video signals are TTL RGB signals, before
being sent to the video preamplifier |C501, the
signals are first processed with color mapping
and mixing circuit.
TTL RGB signals are sent to 1C401(82S147)
through buffer 1C402(7407). 1C401 is a PROM
(Programmable Read Only Memory) served as
‘color mapping table for different colors combi-
nation. Pin 17 and Pin 18 of this IC are mode
control pins for selecting 8, 16, 64 colors or mo-
no text. When the Auto / Manual Switch (SW1)
located on the rear panel is set at Auto mode,
1C401 will automatically output 8, 16 or 64 colors
signals according to IBM CGA / EGA standard.
But if the input signals are from computers not
compatible with IBM PC, then SW1 should be set
at Manual mode, and 8, 16 or 64 colors can be
displayed by setting the Dip Switch (SW3) manu-
ally. When Text Switch is ON, 1C401 will output
signals to display single color, and users can
select Red, Blue, Green and Amber by setting
Dip Switch (SW3).