NEC PF-68T31 User manual

PF-68T31
PF-68V21
CONTENTS
zSPECIFICATIONS …………………………………………………………………….. 2
zSAFETY PRECAUTIONS …………………………………………………………….. 3
zSERVICE ADJUSTMENTS …………………………………………………………… 7
zPARTS LIST …………………………………………………………………………… 19
zSTANDARD CIRCUIT DIAGRAM
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COLOUR TELEVISION
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2
SPECIFICATIONS
Item Specifications
Dimensions(W x H x D)
Weight
730 mm x 590 mm x 520 mm
49.5 kg
TV RF System B/G
TV Mode PAL Colour System
Video Mode PAL/ NTSC 3.58/ NTSC 4.43
VHF (VL) 46.25 MHz to 168.25 MHz
VHF (VH) 175.25 MHz to 463.25 MHz
UHF 471.25 MHz to 863.25 MHz
Receiving
Frequency
CATV None
VIF Carrier 38.9 MHz Intermediate
Frequency SIF Carrier 33.4 MHz
Colour Sub Carrier Frequency PAL (4.43 MHz)
Aerial Input Terminal 75 Ohm Unbalanced DIN
Power Input Rated Voltage 240 V AC, 50/60 Hz
Power Consumption 145 W (Max.)
Picture Tube Visible Size: 68 cm measured diagonally
High Voltage 30 kV ±1.5 kV (at Zero beam current)
Speaker 8 cm x 13 cm Oval x 2
Audio Output 8 W + 8 W
Video 1 Vpp, 75 Ohm, RCA x 2
S-Video Input Y, C
Components Input Y, Cb, Cr (PF-68T31)
Input
Audio 500 mVrms (-4dBs), High impedance, RCA x 4
Video 1 Vpp, 75 Ohm, RCA x1
Audio 500mVrms (-4dBs), Low impedance, RCA x 2
Output
Headphone jack Stereo Mini jack (φ3.5mm)
Remote Control Unit RE-002 (PF-68V21), RE-003 (PF-68T31)
(Battery size: AAA/R03/UM-4 x 2)
Design & specifications are subject to change without notice.

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3
SAFETY PRECAUTIONS
1. The design of this product contains special hardware, many
circuits and components specially for safety purposes. For
continued protection, no changes should be made to the
original design unless authorized in writing by the
manufacturer. Replacement parts must be identical to those
used in the original circuits. Service should be performed by
qualified personnel only.
2. Alterations of the design or circuitry of the products should
not be made. Any design alterations or additions will void
the manufacturer’s warranty and will further relieve the
manufacturer of responsibility for personal injury or property
damage resulting therefrom.
3. Many electrical and mechanical parts in the products have
special safety-related characteristics. These characteristics
are often not evident from visual inspection nor can the
protection afforded by them necessarily be obtained by
using replacement components rated for higher voltage,
wattage, etc. Replacement parts which have these special
safety characteristics are identified in the parts list of
Service manual. Electrical components having such
features are identified by shading on the schematics
and by (
 ) on the parts list in Service manual. The use
of a substitute replacement which does not have the same
safety characteristics as the recommended replacement
part shown in the parts list of Service manual may cause
shock, fire, or other hazards
4. Don’t short between the LIVE side ground and
ISOLATED (NEUTRAL) side ground or EARTH side
ground when repairing. Some model’s power circuit is
partly different in the GND. The difference of the GND is
shown by the LIVE: ( ) side GND, ISOLATED (NEUTRAL) :
( ) side GND and EARTH : ( ) side GND. Don’t short
between the LIVE side GND and ISOLATED (NEUTRAL)
side GND or EARTH side GND and never measure with a
measuring apparatus (oscilloscope etc.) the LIVE side GND
and ISOLATED (NEUTRAL) side GND or EARTH side GND
at the same time. If above note will not be kept, a fuse or
any parts will be broken.
5. If any repair has been made to the chassis, it is
recommended that the B1 setting should be checked or
adjusted (See ADJUSTMENT OF B1 POWER SUPPLY).
6. The high voltage applied to the picture tube must conform
with that specified in Service manual. Excessive high
voltage can cause an increase in X-Ray emission, arcing
and possible component damage, therefore operation under
excessive high voltage conditions should be kept to a
minimum, or should be prevented. If severe arcing occurs,
remove the AC power immediately and determine the cause
by visual inspection (incorrect installation, cracked or melted
high voltage harness, poor soldering, etc.). To maintain the
proper minimum level of soft X-Ray emission, components
in the high voltage circuitry including the picture tube must
be the exact replacements or alternatives approved by the
manufacturer of the complete product.
7. Do not check high voltage by drawing an arc. Use a high
voltage meter or a high voltage probe with a VTVM.
Discharge the picture tube before attempting meter
connection, by connecting a clip lead to the ground frame
and connecting the other end of the lead through a 10kΩ
2W resitor to the anode button.
8. When service is required, observe the original lead dress.
Extra precaution should be given to assure correct lead
dress in the high voltage circuit area. Where a short circuit
has occurred, those components that indicate evidence of
overheating should be replaced. Always use the
manufacturer’s replacement components.
9. Isolation Check
(Safety for Electrical Shock Hazard)
After re-assembling the product, always perform an isolation
check on the exposed metal parts of the cabinet (antenna
terminals, video/audio input and output terminals, Control
knobs, metal cabinet, screwheads, earphone jack, control
shafts, etc.) to be sure the product is safe to operate without
danger of electrical shock.
(1) Dielectric Strength Test
The isolation between the AC primary circuit and all metal
parts exposed to the user, particularly any exposed metal
part having a return path to the chassis should withstand a
voltage of 3000V AC (r.m.s.) for a period of one second.
(…Withstand a voltage of 1100V AC (r.m.s.) to an appliance
rated up to 120V, and 3000V AC (r.m.s.) to an appliance
rated 200V or more, for a periode of one second.)
This method of test requires a test equipment not generally
found in the service trade.
(2) Leakage Current Check
Plug the AC line cord directly into the AC outlet (do not use
a line isolation transformer during this check.). Using a
“Leakage Current Tester”, measure the leakage current
from each exposed metal part of the cabinet, particularly
any exposed metal part having a return path to the chassis,
to a known good earth ground (water pipe, etc.). Any
leakage current must not exceed 0.5mA AC (r.m.s.).
However, in tropical area, this must not exceed 0.2mA AC
(r.m.s.).
●Alternate Check Method
Plug the AC line cord directly into the AC outlet ( do not use
a line isolation transformer during this check.). Use an AC
voltmeter having 1000 ohms per volt or more sensitivity in
the following manner. Connect a 1500 Ω10W resistor
paralleled by a 0.15 μF AC-type capacitor between an
exposed metal part and a known good earth ground (water
pipe, etc.). Measure the AC voltage across the resistor with
the AC voltmeter. Move the resistor connection to each
exposed metal part, particularly any exposed metal part
having a return path to the chassis, and measure the AC
voltage across the resistor. Now, reverse the plug in the AC
outlet and repeat each measurement. Any voltage
measured must not exceed 0.75V AC (r.m.s.). This
corresponds to 0.5mA AC (r.m.s.).
However, in tropical area, this must not exceed 0.3V AC
(r.m.s.).
This corresponds to 0.2mA AC (r.m.s.)

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4
EXPLODED VIEW PF-68T31/PF-68V21
FRONT CABINET
SPEAKER &
SPEAKER BOX
REAR TERMINAL
BOARD
CRT PWB
CONTROL PWB
IR/LED PWB
TELETEXT PWB
(PF
-
68T31)
STEREO PWB PF-68T31
MAIN PWB
POWER PWB
REAR CABINET
AC POWER CORD

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5
REPLACEMENT OF MEMORY IC
1. MEMORY IC.
This TV uses memory IC. In the memory IC are memorized data for correctly operating the video
and deflection circuits.
When replacing memory IC, be sure to use IC written with the initial value of data.
2. PROCEDURE FOR REPLACING MEMORY IC
(1) Power off
Switch the power off and unplug the power cord from AC outlet.
(2) Replace IC
Be sure to use memory IC written with the initial data values.
(3) Power On
Plug the power cord into the AC outlet and switch the power On.
(4) Check and set SYSTEM default value:
1) Press “TEST” key on the special service use Remote control unit.
2) The “TEST” will be displayed on the screen.
3) Press “0” to “8“ key, (Mkey) and corresponding on-screen display will be appeared.
4) In this section, except “3”, “4” and “5” key function would be confirmed.
5) Check the setting value of the SYSTEM default value of Table 1. If the value is different,
select items by [CH+]/[CH-] keys and set value by [VOL+]/[VOL-] keys.
6) Press “TEST” key again and return to the normal screen.
SERVICE ADJUSTMENT
B1 POWER SUPPLY
1. Receive normal colour bar signal.
2. Connect DC voltmeter to TP91 and isolated ground.
3. Adjust VR651 to get the voltage as 130V ±1.0V.
FOCUS ADJUSTMENT
1. Receive a crosshatch signal.
2. While watching the screen, adjust the FOCUS VR to make the vertical and horizontal lines as
fine and sharp as possible.
STEREO ADJUSTMENT (PF-68T31)
1. Receive A2 stereo signal.
2. Adjust L3101 to get the voltage at TP1 (N3101 Pin #3) as 2.5V ±0.1V
3. Adjust VR3101 to get same amplitude at TP3 and TP4.
BUS CONTROL ADJUSTMENT
To enter BUS control mode, use special remote control with “TEST” key, and push “TEST” key.
On TV screen “TEST” will be indicated, this means entered bus control mode.