Monitor/Power Supply Components Functional Description - Wyse WY-60 Maintenance Manual

UART clock (U22)--The UART clock generator consists of a 4.9152
MHz crystal connected to the XTAL inputs of the UART. The UART
has its own internal clock circuitry that divides the basic
crystal into baud rates for the UART. The user programs the DART
in setup for the appropriate baud rate.
MONITOR/POWER SUPPLY COMPONENTS FUNCTIONAL DESCRIPTION
The terminal PCB also contains both monitor and power supply
circuitry. The power supply provides the voltages for all the
logic. The monitor circuitry controls the video image on the CRT
screen. Figure 6-2 is a functional block diagram of the monitor
and power supply circuitry. Note that each block is numbered.
This number corresponds with the item number in the following
descriptions.
Line filter (l)--The line filter attenuates noise from the
external power source (the AC power cord and wall socket). It
also filters any noise produced by the switching power supply to
comply with FCC regulations.
Voltage doubler (2)--The terminal operates with either a 115 VAC
or 230 VAC input. The voltage doubler circuit provides 270 to
300 VDC for the oscillator circuit when 115 VAC is applied, and
is passive when the 230 VAC input strap is installed. This
assures that the oscillator (DC to AC converter) always has a 270
to 300 VDC input regardless of the AC input voltage.
Pulse width modulator(PWM) (3)--The PWM consists of a pair of
switching transistors. It sends AC input voltage to the main
power transformer. The PWM is synchronized with the horizontal
synchronization pulses to eliminate beat frequencies.
Main switching power transformer (4)--The main power transformer
provides taps for each power supply.
Horizontal driver (S)--The horizontal driver makes sure that the
yoke is driven at the correct frequency so that the electron beam
sweeps from left to right at the correct rate. It also provides
the signal to drive the flyback transformer.
Regulation circuitry (6)--The regulation circuitry measures the
+5 and +12 volt supplies. It generates an error voltage
proportional to any error it may find when measuring the
supplies. Optically coupling the error signal with the pulse
width modulator which controls its duty cycle regulates the
output voltage of the main switching power transformer.
+5 volt supply (7)--The +5V is derived directly from one of the
main transformer taps which provides low vol tage
AC.
The output
of this tap is rectified and regulated.
6-6