ViewSonic A90f+-1 VCDTS23307-2R Service Manual page 19

6-3-2 HORIZONTAL OUTPUT STAGE
The Horizontal Drive Pulse output at pin 26 is pulled up to 12V to drive Q404 and then coupled via T401
to output transistor Q433 for on-off control. Thus saw-tooth current through Horizontal Deflection Yoke is
obtained.
The function of C-correction is to correct the asymmetrical non-linearity of picture. L402 and horizontal
Yoke are in series for C-correction. Pin36 of IA01 provides a voltage which depends on Horizontal
Frequency to control L402 via Q405, thus the inductance of L402 changes according to the Horizontal
Frequency to correct the linearity.
The function of S-correction is to correct the symmetrical non-linear distortion equidistant lines.
C416, C423, C419, C420, C421 are S-correction capacitors, also they block the DC voltage to the Yoke.
C416 is a fixed one; the others are controlled by IA01 via Q417, Q411, Q413, Q415 switches. When any
switch turns on, the accordingly capacitor becomes in parallel with C416. The switches on-off depend on
the Horizontal Frequency. They are:
Horiz. Freq.
70 < fh 87kHz
65 < fh 70kHz
58 < fh 65kHz
49.5 < fh
45 < fh 49.5kHz
40 < fh 45kHz
36 < fh 40kHz
32 < fh 36kHz
32 kHz
6-3-3 SUPPLY VOLTAGE CONTROL FOR HORIZONTAL DEFLECTION
The B+ control function block is included in I301 which consists of an operational trans-conductance
amplifier, a voltage comparator, a flip-flop and a discharge circuit. Pin 28 of I301 is a drive voltage which
consists of horizontal width, pincushion, and trapezium and pin corner correction information. This
voltage is controlled by IA01 via I
frequency of the B+ control block is the same as horizontal frequency. The B+ drive output of the control
block is at pin 28(B.DRV.), this drive voltage switch Q402 on-off via a buffer build by Q400 and Q401.
Q402 act as a step down DC converter to drive horizontal deflection output Q433 via L401. The width,
pincushion, trapezium information are modulated on the DC converter by changing the duty of the B+
drive.
Moreover, the B+ duty is controlled with a charging circuit at same time. When the Q302 is OFF, the
voltage at pin16 of I301 will linearly increase with a charge time of determining by R326 and C308 to
modulate B+ duty.
Horizontal DC centering circuit is build by Q423, Q424. By controlling the small current flow in or out of
L405, it will provide DC center compensation for CDT.
ViewSonic Corporation
C416
controlled
Fixed
by Q416
0.24
0.1(CS0)
ON
ON
ON
ON
58kHz
ON
ON
ON
ON
ON
2
C bus. The drive voltage feed into pin15 (BIN) of I301. The operation
C423 C419
controlled controlled
by Q410 by Q412
0.22(CS1) 0.56(CS2)
ON OFF
OFF ON
ON ON
OFF OFF
OFF ON
ON ON
OFF ON
ON OFF
ON ON
16
Confidential – Do Not Copy
C421
C420
controlled
by Q414
1.2(CS3)
OFF OFF
OFF OFF
OFF OFF
ON OFF
ON OFF
ON OFF
OFF ON
ON ON
ON ON
A90f+-1