Panasonic PV-20DF25-K, Service Manual

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2 X-RADIATION
WARNING :
1. The potential source of X-Radiation in TV sets is the High
Voltage section and the picture tube.
2. When using a picture tube test fixture for service, ensure
that the fixture is capable of handling 33.0 kV without
causing X-Radiation.
NOTE:
It is important to use an accurate periodically calibrated high
voltage meter.
1. Reduce the brightness to minimum.
2. Set the SERVICE switch to SERVICE.
3. Measure the High Voltage. The meter reading should
indicate 28.0 kV±1.5 kV (For model with 20 inch CRT), 31.0
kV±2.0 kV (For model with 27 inch CRT).
If the meter indication is out of tolerance, immediate service
and correction is required to prevent the possibility of
premature component failure.
4. To prevent an X-Radiation possibly, it is essential to use the
specified picture tube.
2.1. HORIZONTAL OSCILLATOR
DISABLE CIRCUIT TEST
SERVICE WARNING :
The test must be made as a final check before set is returned
to the customer.
CONFIRMATION OF X-RAY MOVEMENT
1. Turn off TV set.
2. Connect the circuit below between TP554 and TP553 on
the Deflection C.B.A.
Figure 2
3. Turn on SW1 and then turn on the set. Confirm that the
picture is on the screen properly.
4. Confirm that the picture goes out of horizontal sync while
getting down by varying VR.
5. If this does not occur, it means that X-ray protect circuit is
not operating. Further confirmation and repair is required.
2.2. REPAIR PROCEDURES OF
HORIZONTAL OSCILLATOR
DISABLE CIRCUIT
1. Connect a DC voltmeter between capacitor C513 (+) on the
Main circuit board and chassis ground.
2. If approxi21.5 V (For model with 20 inch CRT),
+20.0 V (For model with 27 inch CRT) is not present at that
point when 120 V AC is applied, find the cause. Check
R508, R503, R5504, D5505, D503, C513 and C5507.
3. Carefully check above specified parts and related circuits
and parts. When the circuit is repaired, try the horizontal
oscillator disable circuit test again.
2.3. CIRCUIT EXPLANATION
2.3.1. HORIZONTAL OSCILLATOR
DISABLE CIRCUIT
The positive DC voltage, supplied from the D503 cathode for
monitoring high voltage, is applied to the IC5301 Pin 11 through
R508, R503 and R5504. Under normal conditions, the voltage
at IC5301 Pin 11 is less than approx. 6 V. If the high voltage at
Flyback Transformer Pin 5 exceeds the specified voltage, the
positive DC voltage which is supplied from the D503 cathode
also increases. The increased voltage is applied to IC5301 Pin
11 through R508, R503 and R5504. Due to the increased
voltage at IC5301 Pin 11, the horizontal oscillator frequency
increases, the picture goes out of horizontal sync, the beam
current decreases and the picture become dark in order to
keep X-radiation under specification.
Figure 3
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PV-20DF25 / PV-27DF25 / PV-27DF5 / PV-20DF25-K / PV-27DF25- K