Theory of Operation—2205 Service
CR986, CR987, CR988, and CR989 produces the
8,6 V and -8.6 V. Filtering of the 8.6 V is done by
C986, C987 and L986; filtering of the -8.6 V is done
by C988. C989 and L988. The 6.2 V supply is proî™–
duced by CR990 and CR991 and filtered by C990,
C991. and L990.
CRT Supply
The 900 V from the high-voltage winding of T902
is doubled by C980, CR975, and CR976 to produce
-1.8 kV for the crt cathode. The -1.8 kV supply is
filtered by a low-pass filter formed by R976, R978.
C976, and C979. Neon lamps DS853 and DS856 proî™–
tect against excessive voltage between the crt
cathode and crt grid by conducting if the difference
exceeds about 180 V.
Focus Circuit
Focus voltage is also developed from the -1.8 kV
supply by a voltage divider formed by R892, R894.
FOCUS potentiometer R893, R888, R889, R890, and
R891.
Line Signal
Transistor Q900 is a floating differential amplifier
with a dc bias network comprising R905, R904 and
R902. Resistors R906 and R903 apply a small line
frequency signal from the secondary of T901 to the
base-emitter junction of Q900. The resultant collecî™–
tor current of Q900 is a line-frequency sine-wave
that is applied to the trigger circuit.
Probe Adjust
The Probe Adjust circuitry, shown on Diagram 4,
is a square-wave generator and diode switching netî™–
work that produces a negative-going square-wave
signal at PROBE ADJUST connector J590. Amplifier
U580a forms a multivibrator that has an oscillation
period set by the time constant of 19687 and C587.
When the output of the multivibrator is at the positive
supply voltage, CR588 is forward biased. This
reverse biases CR589, and the PROBE ADJUST conî™–
nector signal is held at ground potential by R590.
When the multivibrator output switches states and
goes to the negative supply voltage level, CR588 is
reverse biased. Diode CR589 becomes forward
biased, and the circuit output level drops to about
-0.5V.
3-17
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