Theory Of Operation - Samsung MP5671 Service Manual

• THEORY OF OPERATION
1. POWER SECTION
This design utilizes a discontinuous flyback topology
operating in current-mode. This results in a multiple
output switcher who's output track well. No output
filter chokes are needed and slower diodes can be
used. The fast transit response maintains picture
integrity. Cycle by cycle current limiting makes the
switcher short circuit proof.
Output Voltages:
LOCA NO.: T603
TRANS NO. OUTPUT VOLTAGE REMARK
#3 +75V DC
#4
+50V DC
#6
+25V DC
#2-
+8V DC
#12 -8VDC NC
2. MAIl\! SECTION
1) Horizontal
A phase lock loop is used to synchronize the
horizontal switch to the timing pulses from the
computer. Storage delay in 0303 2SC3636 (MJH
16006) is compensated by the P L L.
Incoming H-sync level is translated by 0301 then
fed into the input of the phase comparator.
Transistor 0304 watches the falling edge of the
flyback pulse falls to within a few volts of ground
a raising edge is sent to the other input of the PLL
phase comparator.
The phase comparator is used in an edge look
mode. Integrated circuit IC301 MC14046 pin 13 is
in a high impedance mode most of the time. If the
raising edges fed into pins 3 and 14 do not coin-
cide, pin 13 goes into a low impedance state for
the time difference. Depending on which edge
leads the other, current will be fed into or pulled
out of the filter capacitors C302 and C303.
Voltage controlled oscillator input pin 9 deter-
mines the freq uency and phase of the output sq uare
wave on pin 4. The free running frequency and
look range is set by C301, R306, R305 and
VR301 and should be preset below the operating
rate.
2) Vertical Deflection
The I C201 TDA 1670 incorporates all of the neces-
sary functions for providing the yoke of a monitor
4
or television receiver" with the current required for
vertical deflection. Incorporated in silicon are a
synchronization oscillator, ramp generator, voltage
regulator, voltage doubler and power amplifier.
3) Focus Modulation
Each part of a cathode ray tube focuses at a
different voltage. The focus voltage at the center
may be several hundred volts below the top and
bottom with the right and left sides being at a
different level. To get the best looking picture
over the entire tube, the focus voltage is modu-
lated with both a vertical and horizontal signal.
The vertical parabola is fed into a high voltage
amplifier 0401. 0401 swings the focus voltage
at the vertical rate. Mixed in with this signal is
another par.abola at the horizontal rate which
comes from L401. The focus adjustment VR401
sets the DC focus voltage and bias on 0401.
3. H/V REGULATOR SECTION
The patented high voltage regulator sits between the
supply and the top of the primary of the flyback
transformer. An onboard switching regulator is
powered from the 12 volt supply. It watched the
second anode voltage through a 300 mega ohm
resisto in the flyback transformer. During flyback
the regulator pulls the top of the flyback transformer
negative some amount at the same time the flyback
pulse is raising the other end. In this way the size of
the pulse as seen by the flyback transformer can be
varied to keep the second anode voltage constant
under load.
4. VIDEO SECTION
1) Sync Processing
To help understand the complex sync processing
please refer to both the schematic and the timing
diagrams at the end of this document.
2) Vertical
Vertical sync is detected by a low pass filter and
an 'XOR' gate. Pin 2 of P4 contains Vertical sync.
3) Horizontal
Horizontal sync undergoes several stages of pro-
cessing.
4)
Data Centering
The second half of the IC102 (74LS221) is used
for data centering. The one shot triggers with the
leading edge of sync and time out with the back
edge of the flyback pulse. The data centering
potentiometer should be set to center the picture
within the raster.