
Circuit Description—Type 324
causing collector current to flow through the T538 N1
winding. The voltage induced into the N2 winding is regen
erative to the base of Q529, turning it on fully. When the
maximum Q529 base and collector current permitted by
Q518 is flowing, current stops changing in the N1 winding.
The N2 voltage decreases, decreasing the drive to Q529.
Q529 collector current then decreases, and the field built
up by N1 starts to collapse.
The voltage induced into the N2 and N3 windings is of
opposite polarity with respect to the previous half cycle. It
now simultaneously back biases D523 and causes Q525 to
conduct. This rapidly cuts off Q529 and induces power into
the secondary windings.
A t the end of turn-off time, the N2 field collapses, Q518
again pulls the D523 anode positive, Q529 starts to con
duct, and the cycle repeats itself.
The oscillator frequency varies indirectly with oscillo
scope power requirements, since the power induced into
the secondary varies directly with Q529 "on" time.
The collapsing magnetic field that occurs when Q529
turns off causes a large positive voltage at the Q 529 collec
tor. This charges C531 through D531. When Q529 again
saturates, C531
attempts to discharge through D533
keeping C533 charged up to approximately —100 V.
Q518 current (Q529 drive current) is controlled by
Q515 collector current, which is established as a function
of reference voltage (from D547 through R513 and R514)
in combination with the - 1 0 0 V (through R535) and cath
ode current sensing feedback (through R 515). As CRT
cathode current increases, the voltage developed across
R572 increases, causing an increase in Q515 and Q518 cur
rent. The resulting increase in Q518 drive keeps Q529 con
ducting longer, stores more energy in T538, and delivers
more power to the secondary of T538.
The longer Q529 "on" time causes the charge on C533
to become more negative. The resulting increase in feed
back current through R535 offsets most of the cathode
current feedback (from R 572), stabilizing the circuit. This
action prevents an appreciable change of high voltage from
occurring as a result of increased CRT current.
D 5 1 6 temperature-compensates Q515 and sets its
emitter at —0.6 volt. D517 bypasses R516 during turn on.
D523 protects the base circuit of Q529 from large negative
spikes which develop in the N2 winding when Q529 turns
off. D525 bypasses Q518 during Q529 turn-off time,
clamping one side of the N2 winding at the value of the
Power Pack voltage.
C521 bypasses R521 to speed up on the Q529 switching
action. C529 and L501 perform the dual function of filter
ing input pulses during AC operation, and minimizing radi
ation out of the power supply line. C587 provides decoup
ling on the reference voltage line.
+100 V Power Supply
When Q529 is conducting, energy is being stored in the
magnetic circuit of T 538. When Q 529 turns off, the voltage
generated at the Q529 collector causes current to flow
through D543, charging C543 to approximately -MOO V.
+175 V Power Supply
The N4 secondary winding of T 5 3 8 has one side refer
enced to the +100 V pickoff point and the other side con
nected to D541. During the time the primary field is collap
sing, voltage is induced into this secondary, adding its value
to that at the +100 V pickoff point. Current flows through
D541 and L541, developing +175 V across C540.
+5 V Power Supply
The +5 V Power Supply is directly powered by voltage
from the Power Pack. However, regulation is dependent
upon the +8.6 V reference provided by Zener diode D547,
and series regulator Q557.
Error sensor Q555 constantly compares the voltage at
the wiper of R552 against the +5 V output at the Q555
emitter. This comparison determines the drive current to
Q558, which then controls the total Q557 emitter current.
For example, if the +5 V supply tends to increase, Q555
decreases conduction, which decreases Q558 current drive.
This decreases the current drive to Q557. Q557 decreases
conduction and holds the +5 V supply within its specified
limits. The +5 V output is filtered by C559 and L559
before being applied to external circuits.
—5 V Power Supply
The output of the +5 V Power Supply is used as the
reference for the —5 V supply. The —5 V supply is derived
from the output of the N7 winding of T 538 and is rectified
by D 560 and D561. This voltage determines the voltage at
the emitters of Q567 and Q569. Current through R565,
R566 and R567 determines the drive to Q562. Q562 pro
vides drive current to Q 567, which then controls Q569. A
comparison between the —5 V output at the collector of
Q569 is made against the +5 V supply, and a voltage near
—0.6 V is applied to the base of Q562. If the —5 V supply
tends to go positive, current through Q562 decreases. This
decreases the Q567 emitter-base current, which decreases
its collector current. The Q567 collector voltage tries to
rise, thereby increasing the Q569 emitter-base current. This
causes an increase in Q569 collector current. The increase
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