Tandy CM-4 User Manual

THEORY
OF
OPERATION
1.
RGB
Drive
Circuit
The
RGBI
signals
are applied to
HEX inverter
IC491
used
as
a
buffer). The
outputs of
IC491 are then
ap-
alied
to
IC492
which
is
an
open
collector HEX invert-
er.
The
resistors
tied
to
the
outputs of IC492 are
used
:o
set
the
appropriate
voltage
level
for the color
sig-
nals.
The
signals
are
then applied to
the base
of RGB-
Amp
transistors
Q451,
Q461 and Q471.
The
brightness
and
sub
brightness controls
(R41
7
and
341
6)
are used
to
adjust the
bias of
the
RGB-Amp
tran-
sistors.
These
controls are
connected
to the
base of
Q401
and
Q402.
When the intensity
signal
is
LOW,
transistor
Q421
is
turned
ON.
The
contrast
control
(R422) is used
to
adjust
the con-
trast
between
high
and low intensity color
signals.
2. Video
(RGB) Output {Fig.
2)
An RGB
drive system
is
utilized
in
the
video output
cir-
cuit of
this
unit.
The
function
of
this
circuit
is to
com-
oine
the
color signals and the brightness
signal,
and
amplify them
sufficiently
to
drive
the
cathodes. 1
45V
DC
must
be applied
to
the collectors
of the
output tran-
sistors
(Q851, Q861,
Q871).
When
the horizontal
output circuit is
operating, pulses are
developed
and
fed
to
the
1
16V supply
where they are applied
to
a
winding
of the
horizontal output transformer (T602).
This pulsed
DC
voltage
is
then
taken
from terminal (s)
of
T602 and applied
through
D71
7,
and
R865
or
R866
or
R867 to
the
collectors of
Q851,
Q861
andQ871,
respectively.
The brightness
signal
from the
Blanking
(Q402)
is
ap-
plied
to
the emitters of
Q851,
Q861 and
Q871.
C853
and C855
are
peaking capacitors.
Color signals from
the
outputs
of
Q451,
Q461 and
Q471
are
applied
to
the bases of
Q851,
Q861 and
Q871 . The picture
tube
used
in
this
unit
is
a
precision,
inline
gun-type.
The control grid
(G1)
and the
screen
grid
(G2)
are common
with
respect to the red, green
and blue cathodes. Consequently, the
emitter circuits
of Q85
1
, Q86 1
and
Q87
1
are
provided
with bias con-
trols
(R862, R863 and
R864,
respectively)
for picture
tube
cut-off
adjustment. Drive
controls
(R856
and
R858) are provided
in
the emitter circuits
of Q851 and
Q871 for white balance
adjustment.
FROM
EMITTER
>
OF Q451
Q851
RED
OUTPUT
FROM
EMITTER
>
OFQ461
FROM
EMITTER
>
OFQ471
FROM
VIDEO
DRIVE
>
CIRCUIT
<
+145
Figure
2.
Video
Output
Circuit

Tandy CM-4 Specifications

General

Brand	Tandy
Model	CM-4
Category	Monitor
Language	English