Sony CTV-25R1, Manual

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When the picture tube warms up and draws current, these three pulses
all appear next to each other at the common cathode IK pin 5 of IC701.
The amplitudes represent the current of each cathode.
Clipper Stage – Q301, Q302.
This IK signal is applied to a signal clipper stage to cut off low and high
level parts of the waveform. The clipper circuit consists of a voltage di-
vider and two transistors. The voltage divider resistors R321 and R322
place the base of both transistors at a threshold voltage. The IK signal is
applied to both transistors’ emitters. When the IK signal rises above this
threshold, both transistors conduct and pass only the signal above this
voltage threshold. Therefore this stage is said to clip the lower voltage
components of the positive IK signal, leaving the just the peaks.
In normal operation, the IK line consists of the three IK pulses and the
video signal from the picture tube cathodes. Q301 passes the signal to
the Y/C Jungle IC301 so the IK pulses can be identified and used for
cathode current balancing. Zener D301 limits the signal input to the Y/C
Jungle IC301/pin 21 to a 5-volt maximum pulse.
Q302 is used to clip the larger amplitude signals. Q302 operates similarly
to Q301, but passes the IK signal to ground, dividing the signal current.
As the signal amplitude becomes larger, more current flows through R329.
The additional voltage dropped by R329 turns Q302 ON harder than Q301.
As Q302 turns ON harder, the level of the video signal (or any other volt-
age spike from the picture tube) is reduced so it does not damage the Y/
C Jungle IC301.
The following waveforms show the IK signal entering the signal clipper
stage (CH 1) and the signal leaving (CH 2). Note that only the top half of
the IK waveform is allowed to pass.
Color bar pattern on TV screen. Digital scope is set for peak waveform
presentation. The 3 IK pulses are difficult to see in this digital picture, but
are present to the right of the open (vertical) area. The vertical lines in the
waveform represent the horizontal scan lines.
Channel 1 – CN301/pin 4; 2v/div.
Channel 2 – D301/Cathode; 1V/div.
Time base = 2msec/div.
The output IK signal is applied to IC301/pin 21. Inside IC301, each one of
the three IK pulses is measured and compared to technician set levels
stored in memory. These levels stored in memory are accessed from the
service mode (see the service manual). The information is adjusted and
used to set the gain of the RGB signals. When the gain is within the
automatic adjustment range, the RGB signal is unblanked and leaves
IC301 (pins 22-24) with the IK pulses.
CH 1
CH 2
T r a n s is t o r V o lt a g e s – C o l o r B a r T e s t P a t t e r n
T r a n s is t o r E m it t e r B a s e C o ll e c t o r
Q 3 9 2 0 1 . 5 V d c 2 . 2 V d c
Q 3 9 3 0 1 . 5 V d c 2 . 1 V d c
Q 3 9 4 0 1 . 6 V d c 2 . 2 V d c
Q 3 0 0 0 0 . 4 6 V d c 3 . 4 V d c
Q 3 0 1 3 . 8 V d c 3 . 7 3 V d c 0 . 7 8 V d c
Q 3 0 2 3 . 9 V d c 3 . 7 3 V d c 0
Spot Elimination Circuit
Since many cable boxes shut off power by removing AC when the TV is
ON, the TV could leave a spot of light at the center of the picture tube.
This is because the TV’s magnetic deflection would collapse first being
the most power hungry, leaving residual beam current to produce a cen-
ter spot.
Q701, D700, D702, and D703 will drive the CRT harder depleting the
beam current when AC is lost. Normally with the TV ON, Q701 is turned
ON with bias from B+ via zener D700. Q700’s collector voltage is LOW.
When AC vanishes, B+ is lost and Q700”s collector voltage rises to apply
Set +9Vdc to IC701/pins 1&2 (via diodes). This drives the blue and
green beams full ON before deflection ceases, depleting the power pro-
viding beam current. The reduced beam current eliminates the possibility
having a spot on the screen.