NEC MultiSync FE001SB, Service Manual

Page: 117 / 178

7-8
3.2 Horizontal Deflection Circuit
3.2.1 Horizontal Oscillator Circuit
The outputs of the horizontal oscillator drive pulses (Pin 8 H_DRV) and the Power B drive pulses (Pin 6
B_DRIV) are generated from the U501 (TDA4856).
The horizontal drive pulses of about 10Vp-p are output from Pin 8 (H_DRV) in the open collector mode. The
duty cycle is about 50% and its frequency is dependent on the input horizontal sync frequency. Even though
the input frequency varies, the duty cycle is kept almost constant (about 50%).
The AFC feedback signal from the horizontal deflection output circuit is input to Pin 5 (B_IN) and an error
amplifier of R515, R516, C510, and C511 is established between Pin 5 (B_IN) and Pin 3 (B_OUT).
Based on the error-guaranteed voltage at Pin 3 (B_OUT), a voltage B drive pulse output is generated in the
open collector mode from Pin 6 (B_DRIV) with the use of the IC’s internal comparator and the flip-flop circuit.
These pulses are output in synchronization with the horizontal drive pulses.
3.2.2 Step-up Horizontal Deflection source (+B)
This is a step-up type chopper circuit consisting of Q504, T601, D505, etc.
Based on the input power supply of 26V, an output of about 30 to 40V (horizontal size MIN to MAX) is
generated when fh = 31kHz and another output of about 110 to 130V is generated when fh = 93kHz.
To produce this circuit input, the drive pulse output from U501 (TDA4856) Pin 6 (B_DRIV) and the 26V power
is stepped up to obtain a B power source according to this pulse duty.
The deflection source B is obtained from the input pulse duty when the power input is assumed to be Vi,
using the formula given below.
D = 1 - (Vi/B)
B = Vi/(1 - D)
For horizontal size and distortion adjustment, a signal is output from U501 Pin 11 (EW_DRV) to Pin 5 (B_IN)
which is superposed on the drive pulses output from Pin 6 (B_DRIV).
[B_SENS operation waveform]
When the current of R525 reaches about 3.5A peak (B_SENS voltage at U501 pin 4 = 0.27
Ω
* 3.5A =
0.945V; TYP 1.0V for IC specifications), the B_DRV signal at U501 Pin 6 turns OFF and the FET Q504 is
turned OFF. At that time, the drain voltage of Q504 is raised by the counter electromotive force and the
resultant voltage is used as a B power supply for the deflection circuit. The duration of the counter
electromotive force pulse for the B power supply is dependent on the load that is required by the deflection
circuit. When the required amount of current is flows through D505, the drain voltage of Q504 settles down to
the source voltage (approx. 26V) through the inductance of T501.
(Fig. 3.2.2) Step-up Horizontal Deflection Source (+B)
26V
B_DRIVE
(From U501 6pin)
B_SENS
(From U501 4pin)
H_OUT Circuit