Philips PM 5775 Manual page 34

At moment t1, the differential amplifier Z235/Z236 switches over to its initial
state and the same procedure described above is repeated, however in the reverse
direction. The result is now that a negative-going and positive-going pulse appears
at the collector of Z235 and Z236 respectively.
The pulse are fed to differential amplifiers Z240/Z245 and Z241/Z244 via emitter
followers Z238 and Z246 respectively.
When switch SINGLE/DOUBLE, S7, is in position SINGLE, the differential
amplifier Z241/Z244 is inoperative because emitter resistor R259 is connected to
earth. In the quiescent state, transistor Z240 is conducting and Z245 cut off.
When a positive pulse is injected into the base of Z240 and a negative pulse into
the base of Z245, the differential amplifier will remain in its initial state. These
pulses are present at the moment to as can be seen in the diagram of Fig. VII-3
so at the beginning of the pulse delay time.
When, however, at the same time a negative and a positive pulse — which are formed
at the end of the delay time — are simultaneously injected into the bases of Z240
and Z245 respectively, the differential amplifier Z240/Z245 switches over and a
negative pulse arises in the collector circuit of Z245. This pulse is taken from the
collector of Z243 and further fed to the pulse width circuit.
When switch SINGLE/DOUBLE, S7, is in position DOUBLE, the differential
amplifier Z241/Z244 is in operation. In the quiescent state Z241 is cut off and
Z244 is conducting. The differential amplifier switches over when at the same time
a positive and a negative pulse is injected into the bases of Z241 and Z244
respectively. As can be seen in Fig. VII-3, these pulses are present at the start of
' the delay time.
The resulting negative pulse is also taken from the collector of Z243.
Resuming one can say that negative trigger pulses are produced by the differential
amplifiers Z240/Z245 and Z241/Z244.
With switch SINGLE/DOUBLE, S7, in position DOUBLE, two negative pulses are
generated whose time interval between the negative-going edges equals the delay
time which is adjustable with switch DELAY, S2, and fine delay control RV2.
. Pulse width circuit
The operation principle of the pulse width circuit (Z250/Z251, Z255, Z252 and
timing capacitors C229//C22...C28) is exactly the same as that of the pulse delay
circuit.
Transistor Z242 and diodes Z248 and Z249 form the clipping circuit. The pulse
width is adjustable both in steps and continuously by means of switch WIDTH, S3,
and potentiometer RV3 respectively.
The differential amplifier Z256/Z257 is controlled by the negative trigger pulses
coming from the emitter of Z255. In order to enable normal or inverted pulse
operation there should be positive or negative pulses.
By means of switch NORM. INV., S8, positive and negative pulses can be taken
from the collectors of Z256 and Z257 respectively.
The selected pulses are fed to the output circuit.