Eastern Digital Co V700P, Руководство по обслуживанию

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5. CIRCUIT DESCRIPTION
5-1 MICRO CIRCUIT
IC301 is CPU, This CPU has the following functions.
1. Detect timing mode by sensing the horizontal frequency, vertical frequency, the polarity of Hor. Sync and Ver.
Sync.
2. Keyboard scan control.
3. Geometry control internal D/A converters and I
²
C bus control.
4. Cs capacitor switch control.
5. Power saving control.
When CPU detects timing, it takes data from E
²
PROM (IC302), then output voltage to control the geometry of this
monitor.
If key is pressed, the CPU will do some job according to the key function. For example, if OSD key is pressed, it can
change different value to control screen geometry (H-SIZE, V-SIZE...etc.)
5-2 DEFLECTION
CIRCUIT
Hor. sync. and Ver. sync. come from PC, go into the CPU (IC301). The output goes to the Hor. oscillation and Ver.
oscillation processor (IC601). The IC601 treats sync. Signal and output the drive signal to horizontal and vertical
output circuit. IC601 also generates some functions for geometry use, like, horizontal center, vertical size, by I
²
C bus
control, the geometry can be controlled. IC501 is a vertical output IC to supply the vertical scan. Q605, Q606, Q607
and L604 are the horizontal size controls. Q604 is the horizontal deflection output, supply the horizontal scan of the
monitor. Q623, Q624 and Q625 generate the Blanking signal output to G1 of CRT.
Q619,Q620,Q621 and Q622 are mute control, brightness control and G1 DC voltage output.
5-3 VIDEO
CIRCUIT
IC801 is a video amplifier, clamping signal input from pin No. 11 to restore the DC voltage of video signal, the signal
output from IC801 pass through IC802 Video package amplifier stage LM2439T, then go to the cut off DC restore
stage, The video output signal is about 40Vpp.
5-4 POWER
SUPPLY
The design uses a discontinuous flyback topology operating in current-mode resulting in a multiple output switcher
with stack well. Faster diodes are used. The fast transient response of the control loop maintains picture integrity. Very
fast current limiting protects the switcher agains short circuits.
UC3842 (IC701) is the current mode controller selected. It offers feed forward compensation, feedback error amplifier,
and low voltage lock out features. The 3842 draws very little current is start up mode. There is enough power from the
line bleeder to slowly charge a capacitor to the 16 volts needed to start the switcher.
The FET starts a cycle by allowing current to flow into the primary of the power transformer. As current ramps up
with time, the voltage across the current sense resistor (R706) also ramps to a point where the 3842 determines that
enough power is stored and turns off the FET. As the voltage on the transformer reverses, power is dumped from the
main power transformer through diodes into the different supplies. To keep RFI to a minimum and reduce transistor
heating, a turn-off snubber network is placed across the FET. Current from the secondary windings are rectified and
filtered to freate the desired voltages. Small high current capacitors quickly return charging current to the source. Filter
inductors remove high frequency noise.