Heathkit H9 - HORIZONTAL SWEEP CIRCUITS; HIGH VOLTAGE SUPPLY; VIDEO AMPLIFIER

CIRCUIT
DESCRIPTION
Refer to the Schematic
Diagrams and to the
Block
Diagram
(Illustration
Booklet, Page
7)
while
you
read
this
"Circuit
Description."
To
help
you
locate
specific
parts
in
the Terminal
or
on
the
Schematic,
the circuit
component
numbers
(R1,
C101, L301,
etc.) for resistors,
capacitors,
coils, trans-
istors,
and
integrated
circuits
are in
the following
Sroups:
1-99
Parts
on the
chassis.
100-199
Parts
on the
power
supply
circuit board.
200-299
300-399
400-499
500-599
600-699
700-7gg
Each
of the
separately.
BASIC OVERVIEW
Parts
on the character
generator
circuit
board.
Parts on
the
video
circuit board.
Parts
on the keyboard
circuit board.
Parts
on the RAM
and counter
circuit board.
Parts
on
the I/O
circuit board
Parts
on the TPU
circuit board.
circuits listed
above will
be described
The Block
Diagram shows
how
the
Video Terminal
can
be
divided
into a series of
functional blocks that
coordinate
the movement of data on an 8-bit
ASCII
bus. These blocks,
which
include
the character
generator,
the RAM
and
counter,
the keyboard, and
the
I/O
(Input/Output),
are connected directly
to the
ASCII bus. Data
is
transferred
from
one
block to
another and to the external devices
over the
ASCII bus
under the control of
the timing and
processing
unit
(TPU).
Other
functional
blocks
include the
video
cir-
cuits, a line operated
power
supply, and a
high vol-
tage
power
supply, or
flyback
system,
which is
oper-
ated
from the video circuits.
The
character
generator
has a master oscillator
and
dividers that
generate
the
timing
signals
for all of the
functional
modules within the Terminal.
Basically,
the character
generator
takes data
from the RAM
(Random
Access MemoryJ
and converts
it from B-bit
parallel
ASCII information
to
serial
dot information.
The serial dot information
is
then
used to
modulate
the raster scan on
the
CRT
screen.
The video
module contains
a video amplifier
that
converts
the
TTL dot data
from the
character
generator
into
voltage levels that
drive the cathode
of
the CRT.
The
video circuits
also have
a vertical oscil-
lator and amplifier
that
generates
the
vertical sweep
and drives
the
vertical
portion
of
the CRT
yoke. It
also
contains
the
horizontal oscillator
and driver
that
drives
the
horizontal flyback
and sweep
systems.
The
vertical and horizontal
oscillators
are
synchronized
by sync
pulses
generated
by
the character
generator.
The Ram and counter
board contains
the
memory
that
stores
the
information
to
be displayed
on the
screen.
Everything
that appears
on
the
face of
the
CRT
is
stored
in the
RAM.
The
RAM
and
counter
circuit
board
also
contains
the
RAM address
counter,
a cur-
sor
latch,
which
remembers
where
the cursor
is sup-
posed
to appear on
the
screen,
and scroll
counters that
remember
the start of
page,
or scroll
information.
The
keyboard
contains
a
standard
ASCII
keyboard
and
a series
of
other
special
function
keys that
control