Tektronix 067-0521-00, Instruction Manual

Page: 19 / 53

Circuit Description— 0 6 7 -0 5 2 1 -0 0
+ 75 v |
+ 67 .5 v
+ 75 v
■ i X - W -
Q 6 4
+ 6 7 .5 v
Q 7 4
-K H O -
> _ r
+ 67.5 v
+ 67 .2 v
> 3 L
- 0 . 6 v
+ 67.8 v
+ 6 7 .5 v
+ 6 0 v I
Fig. 3 -8 . M ajor current path during rise and top of pulse with TEST FUNCTION switch at + PULSE and AMPLITUDE control fully clockwise.
During the multivibrator cycle, when Q45 is conducting and
Q55 is cut off, the positive voltage at the collector of Q55
causes the current through Q74 to be limited to approxi­
mately 10
fiA
or less through R61. Since the current through
Q74 is limited to a few microamperes the voltage across the
base-emitter junction of Q74 is not enough to forward bias
Q64 and it is held in cutoff. W ith no current through Q64
and a limited amount of current through Q74, the disconnect
diodes are forward biased and are conducting current from
the supply voltages through R62/R64/R66 and R72/R74/R76.
Current through the disconnect diode circuit and thus through
the output resistors (R66/R76) is adjusted by ganged AMPLI­
TUDE controls R62/R72. The output voltages are applied to
pins 1 and 3 o f the interconnecting plug and produce the
baseline voltage on the crt display of the + pulse. W ith the
AMPLITUDE control turned fully clockwise and the disconnect
diodes conducting, this output is approximately +67.2 volts
at the junction of D75 and R76, and approximately +67.8
volts at the junction of D65 and R66.
During the time that Q55 is cut off, C52 and the capac­
itor selected by REPETITION RATE SW50 charge to ground
Fig. 3 -9 . M ajor current path during fall and bottom of pulse with TEST FUNCTION at + PULSE and AMPLITUDE control fully clockwise.
3 -9