Samsung SP43T7HF1X/BOB Service Manual page 20

Alignment and Adjustments
1-6 High Voltage Part
1-6-1 PWM REG Circuit
For the existing high voltage REG circuit (input
voltage variation type), a dynamic REG response
is not provided. So it is difficult for both beam
linearity and uniformity in screen size to be
maintained on the screen with rapidly changing
beams.
A PWM (Pulse Width Modulation) type of high
voltage, however, provides the maintenance of
beam linearity and uniformity in screen size via a
quick response to beam change by performing
sync lock every 1H line, and detecting beam
fluctuation at 1H line, and then controlling the IC
current of high voltage output circuit.
1. High Voltage Fluctuation Detect (DC Detect)
FBT pin 11 detects DC high voltage fluctuation.
The detected DC high voltage value is input to
PWM IC471 pin1 through R473, VR471, R471,
and then it is input to a differential AMP circuit
that differentiates the gap after comparing with
the reference voltage input to pin2.
2. High Voltage Fluctuation Detect (AC Detect)
To check AC high voltage fluctuation, the
output from FBT is detected by using a
capacitor inside the high voltage distributor. The
detection of AC high voltage fluctuation,
a detection of dynamic beam current change is
required in order to keep beam linearity and
uniformity in size.
Regarding the capacitor, a capacity of less than
3000P should be applied to a PWM type. (The
existing type needs a capacity of about 6000P.)
AC detect circuit eliminates unnecessary high
frequency by using C476, D472. Also, AC gain is
limited to + / - 0.7V (D472). This AC gain is
combined with the detection value of DC high
voltage fluctuation by using C478.
1-18
3. PWM IC OSC Sync Lock
A PWM type IC needs sync lock for PWM pulse
and horizontal scan line.
The standard time constant of OSC circuit is
determined by C487, R475 (PWM IC pins 5 and 6).
And the standard OSC frequency is about 27
kHz . The horizontal frequency of scan line is
31.5kHz(NT), 31.25kHz(PAL), so sync lock for this
horizontal frequency should be performed using
sync lock circuit. The sync lock circuit consists
of Q481(Tr KSC815-Y), D479, D478, and C492.
The input AFC signal is connected to PWM IC
pin 5 through D479 so that it can be negative
Trig.
4. Dead Time (HV Protect)
Dead Time (PWM IN pin4) consists of C481,
delays high voltage for a certain time to soft
start in power on, a x-ray protection circuit.
The voltage of Dead Time is detected by FBT
pin7 and through DC Feedback. The normal
voltage of Dead Time is +27V. When high
voltage increases, however, detected voltage is
in proportion to high voltage. Then, the detected
voltage is applied to ICR01S(TL431).
If the voltage is over 2.5V (normal:about 2.25V),
TL431 turns ON, the base port of QR401S
becomes low, and then an emitter current flows.
At this time, a high voltage protection point is
set. When QR401S turns ON, high voltage is
applied to PWM IC pin4 and then muted.
OSC : 27KHz
AFC Waveform : 31.5KHz(NT)
Locked OSC Waveform : 31.5KHz
Samsung Electronics