Tektronix 2465B - Page 83

Theory of Operation—2465B/2467B Service
MCP-BIAS-SUPPLY OSCILLATOR. The MCP-Bias-
Supply Oscillator transforms power obtained from the —15
volt unregulated supply to the voltage necessary to bias
the MCP-CRT element of the crt. The circuit consists of
transformer T4480, transistor Q4460, and associated
components. The low-voltage oscillations in the primary
winding of T4480 are raised by transformer action to a
high-voltage in the secondary winding. This ac secondary
voltage is half-wave rectified by CR4490, filtered by
C4390, and then applied across the MCP.
Oscillation occurs due to the positive feedback from the
primary winding (pin 3 to pin 4) to the smaller base-drive
winding (pin 2 to pin 5) for transistor Q4460. The fre-
quency of oscillation is about 86 kHz, and is determined
primarily by the resonant frequency of transformer T4480.
Initially, when power is applied, the MCP-BIAS-voltage
regulator circuit detects that the MCP voltage is too low
and pulls pin 2 of transformer T4480 negative. The nega-
tive level is applied to transistor Q4460 through the
transformer base-drive winding and forward biases it.
Current begins to flow in the primary winding through the
transistor collector-to-emitter circuit and induces a mag-
netic field around the transformer primary winding. The
increasing magnetic field induces a current in the base-
drive winding that further increases the base drive to the
transistor. This in-phase feedback causes current in
Q4460 to increase until the primary winding current
reaches its maximum value. As the rate of change of the
primary current peaks and then reverses, the induced
magnetic field begins to decay. This decreases the base-
drive current and begins turning Q4460 off.
As Q4460 is starting to turn off, the magnetic field
around the primary winding continues to collapse at the
resonant frequency rate of the transformer. This induces
into the base-drive winding a voltage that completely turns
off the transistor. The collapsing magnetic field goes to
zero, then builds in the opposite direction to a maximum
before collapsing again (resonant flywheel effect). This
sequence of events occurs repetitively as the circuit con-
tinues to oscillate.
The oscillating magnetic field in the primary winding
couples power into the secondary winding of the
transformer. The amplitude of the voltage induced in the
secondary winding is a function of the turns ratio of the
transformer windings.
High-Voltage Oscillator
The High-Voltage Oscillator transforms power obtained
from the —15 volt unregulated supply to the various ac
levels necessary for the operation of the crt circuitry. The
circuit consists of transformer T4340, switching transistor
Q4350, and associated circuitry. The low-voltage oscilla-
tions set up in the primary winding of T4340 are raised by
transformer action to high-voltage levels in the secondary
windings. These ac secondary voltages are applied to the
DC Restorer, the Cathode Supply, and the anode multi-
plier circuits.
Oscillation occurs due to the positive feedback from the
primary winding (pin 4 to pin 5) to the smaller base-drive
winding (pin 2 to pin 3) for transistor Q4350. The fre-
quency of oscillation is about 58 kHz, and is determined
primarily by the resonant frequency of the transformer.
When power is first applied, the High-Voltage Regulator
circuit detects that the negative crt cathode voltage is too
positive and pulls pin 2 of transformer T4340 negative.
The negative level forward biases transistor Q4350 via the
base-drive winding of the transformer. Current begins to
flow in the primary winding through transistor Q4350,
inducing a magnetic field around the transformer primary
winding. The increasing magnetic field induces a current in
the base-drive winding that further increases the base
drive to the transistor. This in-phase feedback causes
current in Q4350 to increase until the primary winding
current reaches its maximum value. As the rate of change
of the primary current peaks and then reverses, the
induced magnetic field begins to decay. This decreases the
base-drive current and begins turning Q4350 off.
As Q4350 is beginning to turn off, the magnetic field
around the primary winding continues to collapse at the
resonant frequency rate of the transformer. This induces
into the base-drive winding a voltage that completely turns
off the transistor. The collapsing magnetic field goes to
zero, then builds in the opposite direction to a maximum
before collapsing again (resonant flywheel effect). This
sequence of events occurs repetitively as the circuit con-
tinues to oscillate.
The oscillating magnetic field in the primary winding
couples power into the secondary windings of the
transformer. The amplitude of the voltages induced in the
secondary windings is a function of the turns ratios of the
transformer windings.
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