Philips PM32671 Service Manual page 32

2-16
TV circuit
With the TV CIRCUIT
In the TV mode, the FINAL TRIGGER AMPLIFIER i s inoperative and instead, the TV CIRCUIT triggers the
MTB. The LEVEL control R6 is also inoperative and the
positive and negative video signals.
The input of the TV circuit is the base of transistor V869. For positive video signals V869 functions as an
amplifier with a phase shift of 180 degrees between base and collector. In this mode, collector resistor R983 is
connected to +14 V via transistor V897, which is conducting.
For negative video signals, V896 functions as an emitter-follower. As
base and collector. This collector now functions
situation, transistor V897 is not conducting. The collector of V869 is direct-coupled to the base of emitter-
follower V871. This is followed by a clamping stage formed by C904 and the base-emitter junction of V899,
the synchronisation pulses being available on i t s collector. These pulses have
of 0 V approx. In the TV line trigger mode, the pulses
level
switching diode V894 to the MTB trigger circuit.
In the TV frame trigger mode (MTB TIME/DIV
the TIME/DIV switch. As
suppressed.
Only frame trigger pulses can now reach the MTB trigger circuit.
Transistor V908 functions as
the diode V894 and in transistor V898.
The TV CIRCUIT is switched off by a 0 V applied to the cathode of diode V901.
This circuit stage produces the supply voltage for the MTB LEVEL control R6. The integrated circuit multi-
plexer 0851 contains two 4-way analog switches that select the voltages applied to both ends of the LEVEL
control. These voltages depend on the selected trigger mode.
The four possible modes are:
Switch position depends on the logic levels
In the TV (TTL, ECL) mode, the control input D851-10 is a t logic L and input D851-9 is also L.
Thus, inside the multiplexer points 1+3 are interconnected and also points 12+13.
As a result, the potential from voltage divider R907,R909 i s connected to both ends of R6. The position of R6
i s now irrelevant and the trigger level i s fixed.
Note that circuit differences necessary to adapt the instrument from TV into TTL, ECL triggering are indicated
in the table given in the circuit diagram.
In the AUTO mode, the control input D851-10 is
2+3 and points 15+13 are interconnected. This results in one end of R6 being connected to D852-1 output,
which carries a voltage proportional to the top
output D852-7. This operational amplifier output carries a voltage that is proportional to the bottom level of
the trigger signal. In this mode the MTB stays triggered in all positions of the LEVEL control since the voltage
on R6 is proportional to the signal voltage.
In the TRIG mode, the control input D851-10 is a t logic H, D851-9 is H and internally, points 4+3 and points
The MTB trigger level can now be adjusted over approximately +8 or -8 divisions of the displayed signal.
In the modes described above, transistor V866 conducts and D851-6 is a t logic L; as a result, the multiplexer
input levels are applied to output pins 3 and 13. In the EXT X DEFLECTION mode however, transistor V866
blocks and D851-6 is a t logic H. In this case, the multiplexer input levels are isolated from the outputs, which
now float.
it is possible to trigger the MTB on television line signals (TIME/DIV
=
as
=
result, C917 and C918 are switched into the circuit and line trigger pulses are
a
a 'current mirror' in order to give the correct current ratio between the current in
TV (TTL, ECL) mode
AUTO (peak-peak level mode)
TRIG mode
EXT X DEFL mode
control inputs pins 10 and 9 of multiplexer D851.
a t
level
+/-
slope switch S8 permits selection between
result, there i s no phase shift between
a
an emitter, connected to -7 V via R 101'1
top level of +5 V and
a
routed via diode V901, transistor V898 and
are
0,5 s...50ps/DIV), switching transistor V909 conducts
logic H and D851-9 is L. Internally, multiplexer points
a t
of the trigger signal. The other end of R6 is connected to
V approx. from voltage divider R951, R952.
...
20 ps
=
and
bottom
a
via