External Input - Philips PM3055 Service Manual

4-2
The impedance converter serves
as
an inverting buffer circuit for
the
high impedance attenuator.
For the
1
.
f.
-feedback
the output signal
of this stage
is
routed
to the l.f.
summation point NlOOl-2.
The low impedance attenuator reduces
the gain by
xl
,
x2.5
and x5,
depending
on
which relay
is
activated.
RELAY RESISTOR DIVIDER
xl
K1006
x2
.
5
K1007
R1053 vs R1056, R1057
and
R1058
x5
K1008
R1053, R1056 and R1057 vs R1058
The continuous circuit (OQ0203)
,
the
differential input voltages
of
which are
fed to
pins 4 and
5.
This stage comprises the following functions:
- Continuously variable control
(pin
11).
- Gain
xl
(pin
2
and
3)
with offset adjustment R1064
(R1164) and gain
adjustment R1069
(R1169).
- Gain xlO (pin
6
and
7)
with offset adjusting R1072
(R1172) and gain
adjustment R1076
(R1176).
-
xl/xlO control,
(pin
10)
to select the 2,5 and
10
mV/DIV
settings.
The
differential output current from pin
13
and pin 14
is
routed via
a
common— base circuit V1063, V1064
and applied
to
the pre-amplifier
unit.
4.2 EXTERNAL INPUT
The external input can be subdivided into four stages:
Input coupling
,
basically similar
to
the ch.A input coupling.
High impedance attenuator
for the
xl
attenuator
only, where the
l.f.
square-wave can
be adjusted with trimmer C1206. The
l.f. part
is
routed
to
the summation point N1201—
2,
R1217 serves
as an
offset compensation
for N1201, For
1
.
f
.-feedback the output of the
impedance converter
is
also routed
to
this
summation
point.
Note that
the
output of
this
stage
is
also
a
reconstituted version
of
the input signal.
Impedance converter
,
is
basic similar
to
the
ch.A impedance
converter.
The differential amplifier V1211, V1212 converts the voltage from
emitter-follower V1209
into the differential current signals EXT+ and
EXT-. This signal
is
applied
to
the pre-amplifier
unit and serves as
external trigger signal or
as an external deflection signal.
The current for this stage
is
applied from current source V1213.