Philips PM32671 Service Manual page 20

The MAIN TIME-BASE GENERATOR produces a sawtooth voltage, the repetition time being controlled by
the TIME/DIV switch. To enable the capacitors that determine the repetition rate sufficient time t o discharge,
the HOLD-OFF CIRCUIT is employed. This time is adjustable with the HOLD-OFF control.
After the HOLD-OFF time, the HOLD-OFF CIRCUIT sends a signal to the SWEEP-GATING LOGIC, which in
turn starts the next time-base sweep.
The repetition rate of the MTB sawtooth voltage is continuously variable with the continuous control CAL.
The output sawtooth voltage from the MTB i s fed to the HORIZONTAL CHANNEL SWITCH circuit.
2.1.2.2. Delayed time-base
Channel A and B trigger signals are fed to the DTB TRIGGER SELECTION CIRCUIT
.
STAG E
Trigger selection i s controlled by the DTB trigger source selection pushbuttons A, B.
The symmetrical output current signal from the TRIGGER SELECTION CIRCUIT is converted to an asymmetri-
cal voltage signal in the TRIGGER AMPLIFIER INPUT STAGE. This signal i s then fed via the TRIGGER
COUPLING circuit t o the FINAL TRIGGER AMPLIFIER. Trigger coupling i s selected by thn DC, LF, HF
pushbuttons.
The FINAL TRIGGER AMPLIFIER comprises the LEVEL/SLOPE controls and their associated circuits. The
asymmetrical input voltage signal is converted t o an asymmetrical current signal, which i s fed to the CURRENT-
VOLTAGE CONVE RTE R
The output of the CURRENT-VOLTAGE CONVERTER and the output of the COMPARATOR are fed t o the
SW EEP-GAT I NG LOG IC.
The COMPARATOR circuit compares the amplitude of the MTB sawtooth voltage with a d.c. voltage selected
by the DELAY TIME control. If the amplitude of the MTB sawtooth i s equal to the d.c. voltage, the
COMPARATOR produces a signal that is then fed t o the SWEEP-GATING LOGIC.
If the MTB pushbutton of the delayed time-base trigger source switches is depressed, the SWEEP-GATING
LOGIC starts the DELAYED TIME-BASE GENERATOR immediately after the DELAY TIME selected.
If the A or B pushbutton is depressed, the SWEEP-GATING LOGIC detects the end of the delay time but waits
for a trigger signal ( A or B) from the CURRENT-VOLTAGE CONVERTER, after which the TIME-BASE
GENERATOR starts.
The DELAYED GATE OUT is taken from the SWEEP-GATING LOGIC when this option is available. The
output i s a t logic H during the DTB sweep and L for other conditions.
The DTB sawtooth voltage is produced in the DELAYED TIME-BASE GENERATOR under the control of the
If the UNCAL LED lights up, it indicates that the continuous controls of one or both time-bases
CAL position.
2.1.2.3. Horizontal channel selection and final horizontal amplifier
In the X DELFECTION AMPLIFIER the X DEFL signal derived from the MTB INTERFACE i s amplified and
fed to the HORIZONTAL CHANNEL SWITCH circuit.
The HORIZONTAL CHANNEL SWITCH selects the X DEFL, MTB and/or DTB signals under the control of
the HORIZONTAL CHANNEL SELECTION LOGIC, which in turn i s controlled by the horizontal display
mode switches X DEFL, DTB, ALT TB, MTB.
If the X DEFL pushbutton is selected, the signal chosen by the MTB trigger source selection switches A, B,
EXT, LINE, will determine the horizontal deflection.
Horizontal deflection i s performed by the sawtooth output of the DELAYED TIME-BASE GENERATOR if the
DTB pushbutton is selected.
Similarly, the MTB pushbutton selects the MAIN TIME-BASE GENERATOR sawtooth for horizontal
deflection.
If the A L T TB pushbutton is selected, the HORIZONTAL CHANNEL SWITCH alternates from the MTB saw-
tooth t o the DTB sawtooth voltage a t the end
.
everty time-base sweep.
of
2-5
the TRIGGER INPUT
via
are not in the