Sony CTV-25R1 Manual page 25

A magnetic field is created when current flows through pins 1-2 of PRT
transformer T602. This induces a negative voltage that outputs the trans-
former at T602/pin 3. This negative voltage is applied to the base of
IC601-2, turning it OFF.
At the same time, a positive induced voltage from T602/pin 4 is applied
to the base of IC601-1. This voltage is held there by capacitor C616 and
coupled to the base via C611. The positive voltage drives IC601-1 into
saturation (ON). The voltage at the collector of IC601-1 becomes zero
by transistor action. This zero volts also appears at T603/pin 6 because
the inductance of T602 is small (few windings).
Because C614 initially acts like a momentary short, the full +167 supply
voltage is applied to T603's primary transformer windings (pins 4-6).
T603's rising magnetic field is coupled into the secondary windings.
Top Transistor IC601-2 Turns ON
The conduction of the transistors in the IC601 package alternate when
the magnetic field in T603 collapses. Eventually, C616's charge leaks
off so IC601-1 can no longer be held in conduction. At this time current
stops flowing through IC601-1 and PIT T603's primary winding. The
magnetic field that is built up in the primary winding of T603 now col-
lapses and current through the primary winding of T603 flows in the op-
posite direction. During the collapsing magnetic field, current takes this
path through IC601-2:
Collapsing Magnetic Field Current Path
Component
T603/pin 4
C614
R608 & R609 R608
IC601-2 base
IC601-2 collector
T602 pin 2
T603/pin 6
Both IC601 transistors receive a change in base bias. While current is
flowing through T602/pins 2-1, a positive voltage is induced and output
T6502/pin 3. This is coupled into the base of IC601-2, turning it ON. At
the same time, a negative voltage is induced and output T602/pin 4.
This turns IC601-1 OFF. As a result of IC601-2's conduction, its emitter
rises to 167Vdc.
Input Output
R609
Emitter
Emitter
Pin 1
21
When the collapsing magnetic field in T603 has depleted its energy, the
cycle repeats, starting with the charging of C614. The result is a square
wave at the junction of the two IC601 transistors when they alternately
turn ON and OFF.
The following waveform shows the oscillator's square wave output (chan-
nel 2) at IC601. It is shown with IC601-1 base bias (channel 3) and the
sine wave at the junction of transformer T603 and C614 (channel 1).
Oscillator Stage. TV = ON, 120Vac input.
Channel 1 – T603/pin 4; 50V/div.
Channel 2 – IC601-1 Collector; 50V/div.
Channel 3 – IC601-1 Base; 5V/div.
Time base = 2usec/div.
Oscillator Stage Protection
Protection 1 – VDR602
All of the TV's power comes through C614 and T603. A bright scene
accompanied by a surge in AC line can pass more current through C614
than normal. More current means there would be a greater voltage drop
across C614. If the capacitor's maximum voltage is exceeded, it will
short and damage the transformer. VDR602 is placed across C614 for
its protection. VDR602 is a Voltage Dependant Resistor that only shows
low resistance when there is a high voltage across it. When good, it
measures like a small capacitor.