Tektronix 323, Инструкция по эксплуатации

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Circuit Description— Type 323
emitter of Q450 follows the base down until D452 conducts
and supplies additional current through Q450 to enable
rapid charging of the capacitance associated with the de­
flection plate.
POWER REGULATOR AND CRT CIRCUIT <9>
General
The Power Regulator converts DC voltage from the Power
Pack into the various operating voltages required by the
Oscilloscope. Employing a blocking oscillator, a flyback-
type transformer, rectifiers and filters, it develops the fol­
lowing voltages: + and —5, + 9 , - f!4 , 4- and —100, +175,
and -1 9 0 0 V DC, and 0.6 V AC. The + 9 and - 1 0 0 V
supplies are used only within the regulator circuitry, and
the 0.6 V AC supplies the CRT filament power.
Block Diagram Description
Refer to the block diagram contained on the Power Regu­
lator and CRT Circuit schematic page. When the POWER
switch is closed, the Blocking Oscillator goes into opera­
tion, alternately causing the Energy Storage Switch to turn
on and off. When the Energy Storage switch conducts, cur­
rent flows through T538 primary, storing energy in the trans­
former. When Q529 stops conducting, the energy stored
in T538 is delivered to the secondary windings, providing
power to the previously mentioned supplies.
A Feedback Circuit, an Error Amplifier (Q515) and a
Blocking Oscillator Control (Q518) circuit combine to de­
termine the frequency at which Q525 operates. Initially,
input power is applied through a Start and Reference Cir­
cuit and a summing network to the Error Amplifier. Cur­
rent flows through Q518, permitting the Q525 circuit to
oscillate. The + 9 V Zener reference (developed in the trans­
former secondary) feeds back to the Start and Reference
Circuit, where it over-rides the input voltage which started
the oscillator. When the —100 V supply builds up, its feed­
back current flows through the summing network to offset
the reference current from the + 9 V Zener reference. A
slight difference between the + 9 V and —100 V feedback
currents provides a drive current to the Error Amplifier,
holding the Blocking Oscillator at its required frequency.
When CRT intensity is at a minimum, practically no cath­
ode current flows, and a minimum amount of power is re­
quired by the high-voltage circuit. As CRT intensity is in­
creased, cathode current increases. The CRT cathode volt­
age starts to diminish, due to the increased drop across the
high-voltage multiplier components. A CRT Cathode Current
Sense circuit is designed to counteract this voltage loss by
sending proportional changes of feedback current to the
summing point. The Error Amplifier and Blocking Oscillator
Control circuits cause the Blocking Oscillator to decrease its
frequency. The Energy Storage switch then stays on for
longer periods of time, delivering more energy to the T538
primary. The additional high voltage power required by
the increased cathode current is thus made available.
Blocking Oscillator Operation
Refer to Fig. 3-8. When power is applied, a positive volt­
age appears at the collectors of Q525 and Q529, at the
emitter of Q518, and at the base of Q515. Q515 conducts
and supplies Q518 with base-emitter current, turning Q518
on. Q518 collector current flows through D523 and turns
Q525 on. The Q525 collector current passes through T525
primary and the resulting regenerative feedback transformer
action puts Q525 and Q529 into saturation. When the Q525
collector current reaches
(3 times the Q518 current, the rate
of change of current through T525 decreases, decreasing
the signal coupled into the Q525 base circuit. Q525 de­
creases conduction. The accompanying decrease of cur­
rent through T525 induces a negative signal into the base
circuits of Q525 and Q529, turning them off. As Q518 cur­
rent increases, Q525 "on" time increases, and repetition rate
decreases.
The collapsing magnetic field that occurs when Q529 turns
off causes a large positive voltage at the Q529 collector.
This charges C531 through D531. When Q529 again sat­
urates, C531 attempts to discharge through D533, keep­
ing C533 charged up to approximately —100 V.
Q518 current is controlled by Q515 collector current,
which is established as a function of + 9 V reference in com­
bination with the —100 V and cathode current sensing feed­
back. As CRT cathode current increases, the voltage devel­
oped across R572 increases, causing an increase in Q515
and Q518 current. The resulting increase in forward bias
keeps Q525 conducting longer, stores more energy in T538
and delivers more power to the secondary of T538.
The longer Q525 "on" time causes the charge on C533
to become more negative. The resulting increase in feed­
back through R534-R535 offsets the cathode current feedback
voltage (from R572) at the base of Q515, stabilizing the
circuit. This action prevents an appreciable change of high
voltage from occurring as a result of increased CRT current.
D516 temperature-compensates Q515 and sets its emitter
at —0.6 V. D523 protects the base circuit of Q525 from
large negative spikes which develop in the secondary of
T525 when Q525 turns off.
A further understanding of circuit operation can be ob­
tained by studying the idealized waveforms (Fig. 3-9) with
respect to the points indicated in Fig. 3-8.
Refer to the Power Regulator and CRT schematic. D521
protects the collector circuit of Q518 by limiting positive
transients to the value of the input supply voltage. D525
protects Q525 by limiting transients to +100 V at its col­
lector. C526 bypasses R526 to speed up the Q529 switch­
ing action. R524 forms part of the Q525 base-biasing cir­
cuit. C512 and R512 are decoupling components. C529 and
L501 perform the dual function of filtering input pulses dur­
ing AC operation, and minimizing radiation out of the power
supply line.
+ 100V Power Supply
When Q529 is on, energy is being stored in the magnetic
circuit of T538. When Q529 turns off, the changing magnetic
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