Tektronix 067-0532-10 Calibration Fixture Manual page 15

As the secondary voltages build up, Q718 receives base drive and
collector voltage from rectifiers CR726 and CR729. Q716, as it
comes into conduction, starts to share the primary current with
Q707. When the rectifier voltage from CR726 and CR729 reaches about
80% of its final value, Q71l1 receives base drive through R713 and VR713.
The conduction of Q711 causes Q707 to turn off, transferring the total
current load to Q716. The secondary voltages will continue to increase
until the voltage reaches a level that turns on Q725.
Biasing conditions of Q725 require either pin 5 or 7 of the feedback
winding to be driven sufficiently negative with respect to pin 6.
When the current through the network consisting of R722, R723, R724,
R725, CR723 and CR727-CR728 is sufficient to produce a voltage drop of
6.2 volts across R724 and part of R723 (to the CR723 cathode), Q725
will be forced into conduction. The required 6.2-volt drop is estab-
lished by zener diode VR725. CR723 offsets the Q725 base-emitter
voltage and provides thermal compensation for the transistor.
When Q725 turns on, the current forced into R720 reduces the Q716-Q718
base drive. This causes a voltage to be developed across Q716, thus
limiting the voltage across the transformer. The degree of limiting,
and hence control of the voltages across the secondary windings, is
dependent upon the setting of R723. R723, the Reg Set control, provides
an adjustment of the resistive network across the feedback winding,
which changes the current requirements necessary to produce the 6.2-volt