Telequipment DM64, Manual

Page: 13 / 63

3.3.8 In the AL T mode a negative-going pulse coinciding with
the start of the sweep flyback is fed, via 0606 or 0609, to
the above bistable causing it to switch. When TR614 is
conducting, it passescurrent from the switching stage,TR609
and TR611, allowing the CH1 signal to passto the shunt feed-
back amplifier, TR612 and TR613. At the same time TR615
is off, its collector rises to 16 V approximately taking the
emitters of CH2 switching stage, TR626 and TR627 with it
and so cutting
off the current. Oiodes 0610 and 0612
prevent the base-emitter junctions from breaking down in the
reversecondition.
3.3.9 In the CHOP mode, R664 and R648 are returned to H.T.
via R639 and R649, forming an astable multi vibrator. The
frequency is mainly determined by R664, R648, C613, C622,
R647, R663, R639 and R649.
3.3.10 In the SUM mode, the current supplied via TR616 is
switched off, so both TR614 and TR615 are non-conducting.
Both switching stages, TR609, TR611 and TR626, TR627
are required to be on, so extra current is bled from the 115 V
line via R637 and R638. Current flows through the switching
stages, via R646 and R662 through R673 to earth. Hence
these signals are added at the bases of TR612 and TR613.
CH2 signal can be inverted by switch, S604, to provide
addition or subtraction of the two signals. Also in the SUM
mode, CHl
POSITION becomes a coarse shift control. CH2
POSITION being the very fine shift control.
3.3.11 The table below shows the state of the switched
components for all switch combinations; followed by a
resume on the part of circuit activated.
Condition A denotes R637, R638 connected to + 110 V.
Condition B denotes TR616 conducting.
Condition C denotes R673 connected to junction R646/R662.
AL Ternate CHOP SUM
x-y
I
CHl CH2 A
B C
A
B
C
A B
C A
B C
ON OFF NO YES NO NO YES NO NO YES NO NO NO NO
OFF ON
NO YES NO NO YES NO NO YES NO NO NO NO
ON ON NO YES NO NO YES NO YES NO YES NOiNO,NO
OFF OFF NO YES NO NO YES NO NO YES NO
N°iNOINO
1. CHl On, CH2 Off.
TR614, TR609 and TR611 are conducting, this feeds the
output of TR609 and TR611 to the bases of TR612 and
TR613; TR626 and TR627 being reversed biased by the
potential at TR615 collector.
TR615, TR626 and TR627 are conducting,so only the output
of TR626 and TR627 may passto the basesof TR612 and
TR613; TR609
and TR611 being reversed biased by the
potential at TR614 collector.
TR614 and TR615 are connected to form a bistable circuit.
At the end of each sweep, a negative-going pulse appearsat
the junction
0606/0609 which reverses the state of the
bistable. Hence TR614 and TR615 conduct alternately and
allow the outputs
of CHl and CH2 alternately to reach the
basesof TR612 and TR613.
-1
R648 and R664 are returned to H.T. via R639 and R649 to
form an astable multivibrator, which free runs at 150 kHz
approx. Thus the outputs of CHl and CH2 are successively
switched into
TR612 and TR613 at 150 kHz. At each
transition a pulse is fed from the emitters of TR614 and
TR615 via C642 to the unblanking amplifier Fig.6, which
blanks the CRT beam and thus provides automatic transient
blanking in the chopped mode.
The tail of the multivibrator and R633 are disconnected;
TR614 and TR615 are non-conducting; R673 is connected
to
ground providing a current path for both channels
simultaneously; TR609, TR611, TR626 and TR627 are
conducting; extra current being fed to their collectors, via
R637 and R638 from the + 110 V line, to maintain correct
conditions. CHl and CH2 may be used as a summing or
differential (with INVERT pressed) amplifier. In this mode,
the CHl POSITION control provides a coarseshift, and CH2
POSITION
control provides a very fine shift control
(reference 3.3.10).
TR614 and TR615 are non conducting, preventing outputs
from either CH1 or CH2 from reaching TR612 and TR613.
When the X-Y switch is selected,the circuit is connected for
X-Y operation as follows, regardlessof any vertical amplifier
mode switching. R641 is returned to ground, ensuring CH1
signal is connected to TR612 and TR613; TR626 and TR627
are biased off, TR616 is non-conducting and the junction
R646/R662 is returned to HT.
The basesof trigger input amplifiers TR2 and TR3, reference
Figure 5, are fed with internal or external trigger signalsvia
switch, S4, whi'ch selectsthe source from either the collectors
of TR753 and TR754 in the vertical amplifier or TRl the
external trigger amplifier. S2 selectsthe polarity of the signal
on which the triggering occurs.
3.4.1 When switched in by S3a orS3b, R15,the LEVEL control
varies the base potentials of TR2 and TR3 in antiphase. This
alters the quiescent voltage on the baseof TR4 and O.C. level
of signal required to trip TR4 and TR5.
3.4.2 When S3a and S3b are open in the AUTO position, feed-
back is applied from TR4 collector via R27 and R9 to TR2
base and from TR5 collector via R26 and R23 to TR3 base.
This feedback causesTR2, TR3, TR4 and TR5 to oscillate,in
the absence of a trigger input, at a low frequency primarily
determined by Cll,
R26 and R27. Input signals are A.C.
coupled only and override the above oscillation, causing the
circuit to lock to the input frequency. The trigger sensitivity
is set by R34, this adjusts the hysteresis of TR4 and TR5.
R17 is set to provide symmetrical operation of TR2 and TR3.
3.4.3 When Sla and b are in the NORMAL position, TR4
and TR5 form a Schmitt trigger. The constant amplitude
rectangular-wave output at the collector of TR5 is