SEA 222 Operator's Manual page 61

7.1.3
7 .1.4
7 .1.5
Volts. Thus, the regulator will provide excellent 10 Volt buss sta-
bility, even when the input line voltage has dropped to approximately
10.5 Volts.
Transistors QlO and Qll generate the non-overlapping +10 Rx and +10
Tx busses, respectively. These devices are operated as inverted
transistor switches and are controlled by the main control computer
through the serial relay driver device, Ul. Ul is located on the
PA/Filter printed circuit board. The use of transistor switches to
generate the Tx and Rx buss voltages eliminates any problems with relay
contacts or T-R buss timing.
+8 VOLT AND +5 VOLT REGULATORS
In the SEA 222, the synthesizer circuitry operates from a regulated
+8 Volt buss, while the control computer operates from a regulated +5
Volt buss. These voltages are provided by standard three terminal
voltage regulator integrated circuits, U29 and U30. U29 is a 7808 and
provides the +8 Volt buss, while IJ30 is a 7805 and provides the +5 Volt
buss.
VF DISPLAY/DISPLAY DRIVER -25 VOLT SUPPLY
The eight character alphanumeric VF display used in the SEA 222
requires a well regulated negative power supply for correct operation.
This voltage is provided by the components associated with switching
regulator integrated circuit, U26. Ll7 is the "ringing choke" in a
standard switching inverter/regulator, with the output voltage being
"dumped" into Cl56 through diode CR28. Resistors Rl60 and R161 set the
output voltage at approximately -25 Volts DC. Zener diodes CR27 and
CR32 are used to set the correct operating voltage levels on the
display and display driver chip, U24. The -25 Volt inverter operates
from the +13.6 Volt switched buss.
VF DISPLAY FILAMENT OSCILLATOR
VF displays require an alternating current filament supply in order
to· avoid "shading" effects. In the SEA 222 this need is supplied by
the quad CMOS buffer, U25. This chip operates as a free running
oscillator/buffer, and supplies a 5 Volt P-P square wave output which
is capacitively coupled to the display filament circuit through Cl60
and Cl61. The frequency of oscillation is controlled by R168 and C159.
The frequency of osci
11
at ion is chosen to prevent 11 strobi ng" of the
display. Strobing is often caused by interaction of the filament
supply voltage with the natural refresh rate of the display.
!