DEC VT220 - 9.2.3 DC OK Circuit

The pulse width modulator circuit generates two outputs: Biasing
to the switching transistors in the input rectification circuit
(via T504) and +5.1 V reference voltage to the overcurrent protect
circuit.
The input to the
processor in the
width modulator's
modulator operation
pulse width modulator from the horizontal
monitor circuits is a sync input to the pulse
internal oscillator. This input synchronizes
to horizontal retrace time.
The switching transistor bias output is directly affected by the
biasing supplied to the pulse width modulator device by the
overcurrent protect circuit (pin 8 input to modulator), summing
network input (developed from +12 V potential summed with input
from +5 V output circuit), and the power sensing components.
The power sensing components provide for operational voltage input
from either the +12 V start up circuit (during power up
situations), or from the +12 V dc output circuit (when fully
operational following a power up). C5l7 inhibits operation during
the power up sequence by holding pin 8 of E505 low until this
capacitor can be charged up. This prevents operation of the pulse
width modulator during the time that initial surge conditions may
occur.
9.2.3 DC OK Circuit
The dc ok circuit provides visual indication of the presence of dc
in the monitor module. The dc ok circuit (Figure 9-10) consists of
the following components.
• LED indicator (D400)
• Connector (J7)
• Drive transistor (Q400)
• Biasing components (R400, R40l, and D40l)
+5V +12V
R400
12K
-12V
Figure 9-10
9-13
DC OK Circuit Diagram