GE 27GT720TX1, Technical Service Data

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SAFETY PRECAUTIONS
INDEX
SET 4776
MODEL 27GT720TX1 (CHASSIS CTC203AX)
GE
GridTrace Location
Main Board ..................................... 4
High Voltage Shutdown Test .................. 1
IC Functions ........................................... 2
Important Parts Information ................... 4
Miscellaneous Adjustments ................... 1
Parts List ................................................. 4
Placement Chart ..................................... 1
Safety Precautions .................................. 1
Schematic Component Location ............ 4
Schematic Notes ..................................... 2
Schematics
Audio ............................................... 3
Gemstar 3 ........................................ 3
Power Supply .................................. 2
System Control ................................ 3
Television ........................................ 2
Tuner ............................................... 3
Test Equipment ....................................... 1
Tuner Circuit Voltage Chart ................... 1
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SEPTEMBER 2003 SET 4776
GE
Model 27GT720TX1 (Chassis CTC203AX)
4776
Technical Service Data
4776
SERVICE WARNING
Only qualified service technicians who are familiar with safety checks
and guidelines should perform service work. Before replacing parts,
disconnect power source to protect electrostatically sensitive parts. Do
not attempt to modify any circuit unless so recommended by the
manufacturer. When servicing the receiver, use an isolation transformer
between the line cord and power receptacle.
SERVICING THE HIGH VOLTAGE AND CRT
Use EXTREME CAUTION when servicing the high voltage circuits. To
discharge static high voltage, connect a 10K ohms resistor in series with a
test lead between the receiver ground and CRT anode lead. DO NOT lift
the CRT by the neck. Always wear shatterproof goggles when handling
the CRT to protect eyes in case of implosion.
X-RAY RADIATION AND HIGH VOLTAGE LIMITS
Be aware of the instructions and procedures covering X-ray radiation. In
solid-state receivers and monitors, the CRT is the only potential source of
X-rays. Keep an accurate high voltage meter available at all times. Check
meter calibration periodically. Whenever servicing a receiver, check the
high voltage at various brightness levels to be sure it is regulating
properly. Keep high voltage at rated value, NO HIGHER. Excessive high
voltage may cause X-ray radiation or failure of associated components.
DO NOT depend on protection circuits to keep voltage at rated value.
When troubleshooting a receiver with excessive high voltage, avoid close
contact with the CRT. DO NOT operate the receiver longer than
necessary. To locate the cause of excessive high voltage, use a variable
AC transformer to regulate voltage. In present receivers, many electrical
and mechanical components have safety related characteristics which are
not detectable by visual inspection. Such components are identified by a
# on both the schematic and the parts list. For SAFETY, use only
equivalent replacement parts when replacing these components.
GENERAL GUIDELINES
Perform a final SAFETY CHECK before returning receiver to customer.
Check repaired area for poorly soldered connections, and check entire
circuit board for solder splashes. Check board wiring for pinched wires or
wires contacting any high wattage resistors. Check that all control knobs,
shields, covers, grounds, and mounting hardware have been replaced. Be
sure to replace all insulators and restore proper lead dress.
SAFETY CHECKS — FIRE AND SHOCK HAZARD
Cold Leakage Checks for Receivers with Isolated Ground
Unplug the AC cord, connect a jumper across the plug prongs, and turn
the power switch on (if applicable). Use an ohmmeter to measure the
resistance between the jumped AC plug and any exposed metal cabinet
parts such as antenna screw heads, control shafts, or handle brackets.
Exposed metal parts with a return path should measure between 1M
ohms and 5.2M ohms. Parts without a return path must measure infinity.
Hot Leakage Current Check
Plug the AC cord directly into an AC outlet. DO NOT use an isolation
transformer. Use a 1500 ohms, 10W resistor in parallel with a .15µF
capacitor to connect between any exposed metal parts on the receiver and
a good earth ground. (See figure below.) Use an AC voltmeter with at
least 5000 ohms per volt sensitivity to measure the voltage across the
resistor. Check all exposed metal parts and measure voltage at each point.
Voltage measurements should not exceed .75VAC, 500µA. Any value
exceeding this limit constitutes a potential shock hazard and must be
corrected. If the AC plug is not polarized, reverse the AC plug and repeat
exposed metal part voltage measurement at each point.
Page 1 SET 4776
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4776
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Schematics
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Component locations
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Parts list
Essential coverage
for servicing a television receiver...
Representative Model
HIGH VOLTAGE SHUTDOWN TEST
Momentarily short BC14901 (see Q14901 base) to ground. The receiver should lose raster and sound. If
receiver does not lose raster and sound, the shutdown circuit should be repaired. To resume normal
operation, remove AC power for approximately 30 seconds and then turn the receiver on.