Philips PM3208 - Page 24

4 CIRCUIT DESCRIPTION 4-3
4.2.3 Main time-base (MTB) sweep generator
The function of the time-base generator is to produce a ramp-voltage, to drive the
horizontal amplifier. |C603 together with Q611 act as a current source. The magnitude of
the current can be adjusted with S602a, the TIME/DIV switch and with RV614, the
variable control MTB VAR. The ramp-voltage is made in the time-base capacitors
C606-607 by charging these with the current from the current source. As.in the range
iu s...5004s the same currents are used as in the range 1ms...500ms, the capacitor is, at
the lower speeds, increased by a factor of 1000 by shunting C607 with C608. This is
realised with Q607 which acts as a switch, controlled by Q609, D611 and a section of
$602. The ramp-voltage is available at Q606-drain and via Q612 sent to the horizontal
switching circuit. In position "X DEFL" of the TIME/DIV switch, the current source is
switched off. Thus, the time-base capacitor can not be charged and there will be no
ramp-voltage.
The
time-base
is
started
by
a
trigger
pulse
at
JK
flip-flop
1C601
pin
1.
At
this
moment,
IC601
pin 6
becomes
LOW
and
Q605
is cut
off.
From
now,
the
current
to
the
current
source
Q611
will
flow
through
the time-base
capacitors.
The ramp-voltage
at
the
time-base
capacitor
is
measured
by
Q616.
When
the
ramp
reaches
about
+9
V,
Q616
turns
on
and
resets
one
flip-flop
of
|C601
at
pin
14.
Due
to
this,
pin
9
will
become
LOW
and
also
pin
15
does.
As
a
result
of this,
also the
other
flip-flop
at
pin 15
is
reset.
Terminal
6
will
become
HIGH
and
the
current
from
Q603
is
transferred
from
D601
to
D602
which
will
cause
Q605
to
turn on. This
is
the
end
of
the
sweep
and
now the
time-base
capacitors
are
discharged
by
Q605.
At
the
end
of
the
discharge-period,
Q604
will
turn
on.
After
some
time,
depending
on
the
current
through
the
hold-off
transistor
Q613,
transistor
Q615
will
turn
on
and
set
flip-flop
IC601
at
pin
10.
Now,
IC601
pin
15
becomes
HIGH,
and
the
time-base
circuit
can
be
triggered
again.
During
the
hold-off
period,
a
trigger
pulse
can
not
start
the
time-base,
as
IC601
pin
15
is
still
LOW.
The
current
through
Q613
is
controlled
by the
potmeter
RV441b,
situated
in the
trigger
generator.
-
In
position
"TRIG"
of
the
sweep-mode
switch,
the
time-base
will
only
start
if
there
are
trigger
pulses.
-
In
position
"AUTO",
the
time-base
is
restarted
immediately
after
flip-flop
|C601
at
pin
10
has
been
set.
At
this
moment,
from
pin
9,
1C601
pin
15
will
become
HIGH
and
pin
4
LOW.
As
a
consequence,
the
flip-flop
at
pin
4
will
be
set
and
the
time-base
starts
again.
If
in
position
"AUTO"
trigger
pulses
occur,
they
will
overrule
the
function
of
the
auto
circuit.
In
that
case,
the
trigger
pulses
turn-on
the
thyristor
circuit
Q601-Q602,
and
1C401
pin
10
is
kept
LOW.
Now,
the
flip-flop
can
only
be
set
by
trigger
pulses
at
pin
1.
-
In
position
"SINGLE",
the
input
of
Q615
is
kept
LOW
via
D633.
Now,
setting
flip-flop
IC601
at
pin
10
by
Q615
is
not
possible.
Setting
is
now
performed
by
pressing
the
button
"SINGLE".
This
causes
IC601
pin
13
to
be
LOW
for
some
time
via
C640
and
this
will
set
this
flip-flop.
Now
pin
15
becomes
HIGH,
and
the
time-base
is
ready
to
be
triggered
for
only
one
sweep.
For
a
new
sweep,
pushing.again
the
button
"SINGLE"
is
necessary.
The
lamp
"RESET"
is
lit
when
the
time-base
is
ready
for
triggering.
4.2.4 Horizontal switching circuit
The
horizontal
switching
circuit
defines
whether
the
MTB
or
the
external
horizontal
input
signal
is
fed
to
the
horizontal
output
amplifier.
The
circuit
consists
of
IC602.
Input
signals
are
the
MTB
at
pin
13
and
the
external
horizontal
input
signal,
if
any,
at
pin
15.
A
control
signals
is
connected
to
pin
9.
This
pin
is
LOW
if
external
X-input
has
been
selected.
(position
"X
DEFL"
of
the
TIME/DIV
switch).
The
output
signal
of
the
horizontal
switching
circuit
is
available
at
1C602
pin
4
and
from
there
sent
to
the
horizontal
output
amplifier.
4.2 HORIZONTAL DEFLECTION