GE 27gt720tx1 schematic
SAFETY PRECAUTIONS
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.
4776
SET 4776
GridTrace Location
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
Test Equipment....................................... 1
Tuner Circuit Voltage Chart ................... 1
INDEX
Main Board ..................................... 4
Audio ............................................... 3
Gemstar 3 ........................................ 3
Power Supply .................................. 2
System Control ................................ 3
Television ........................................ 2
Tuner ............................................... 3
Technical Service Data
GE
Model 27GT720TX1 (Chassis CTC203AX)
Representative Model
Essential coverage
for servicing a television receiver...
• Schematics
4776
4776
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.
The listing of any available replacement part herein in no case constitutes a recommendation, warranty, or guarantee by
SAMS Technical Publishing, LLC as to the quality and suitability of such replacement part. The numbers of the listed parts
have been compiled from information furnished to SAMS Technical Publishing, LLC by the manufacturers of the specific
type of replacement part listed.
Reproduction or use, without express permission, of editorial or pictorial content, in any manner, is prohibited. No patent
liability is assumed with respect to the use of the information contained herein.
© 2003 SAMS Technical Publishing, LLC
9850 E. 30th St.
Indianapolis IN 46229
www.samswebsite.com
Printed in the United States of America 5 4 3 2 1 03PF02160
Page 1 SET 4776
• Component locations
• Parts list
MODEL 27GT720TX1 (CHASSIS CTC203AX)GE
UPC
HERE
For a Complete List of Manuals,
Visit www.samswebsite.com
SEPTEMBER 2003 SET 4776
Page 1 SET 4776
TUNER CIRCUIT VOLTAGE CHART
Pin VHF Low VHF High UHF
No. Band Band Band
U17301
1 4.9V 4.8V 4.8V
2 3.1 3.1V 3.1V
3 7.6V 7.6V 7.6V
4 3.1 3.1V 3.1V
5 7.6V 7.6V 7.5V
6 0V 0V 0V
7 6.4V 6.3V .2V
8 9.2V 9.2V 9.2V
9 3.4V 3.4V 3.4V
10 3.2V 3.2V 3.0V
11 7.2V 6.8V 9.2V
12 3.2V 3.2V 3.0V
13 0V 0V 0V
14 9.3V 9.2V 6.4V
15 3.5V 3.5V 3.2V
16 3.5V 3.5V 3.2V
Pin VHF Low VHF High UHF
No. Band Band Band
U17401
1 1.8V 1.8V 1.9V
2 1.2V 4.0V 5.3V
3 2.6V 2.6V 2.6V
4 34.0V 34.0V 34.0V
5 2.3V 4.4V 4.8V
6 1.6V 4.8V 4.9V
7 1.8V 4.4V 4.8V
8 2.1V 5.1V 4.3V
9 5.0V 5.0V 5.0V
10 1.5V 1.5V 1.5V
11 1.5V 1.5V 1.5V
12 0V 0V 0V
13 1.2V 1.2V 1.2V
14 11.1V 11.1V .3V
15 0V 0V 3.9V
16 3.9V 3.9V 3.9V
17 11.7V 11.7V 11.7V
18 4.9V 4.9V 4.9V
19 4.9V 4.9V 4.9V
20 0V 0V 0V
Pin VHF Low VHF High UHF
No. Band Band Band
Q17101
G1 0V 0V 1.8V
G2 5.1V 5.1V 5.1V
D 0V 0V 0V
S 0V 0V 0V
Q17102
G1 1.8V 1.8V 0V
G2 5.3V 5.3V 5.3V
D 11.7V 11.6V 0V
S 0V 0V 0V
Q17402
E 12.0V 12.0V 12.0V
B 11.7V 11.0V 11.7V
C -11.3V -11.6V -11.1V
Q17403
E 12.0V 12.0V 12.0V
B .5V .6V 11.4V
C 11.8V 11.6V .1V
TEST EQUIPMENT
Test equipment listed by participating manufacturer illustrates typical or equivalent equipment used by Sams engineers to obtain
measurements. This equipment is compatible with most types used by field service technicians.
Equipment Sencore No.
Oscilloscope SC3100
Generators
RGB CM2125
Multiburst Signal VG91
Color Bar VG91
TV Stereo VG91
Digital VOM SC3100
Frequency Meter SC3100
Hi-Voltage Probe HP200
Accessory Probes TP212
Equipment Sencore No.
Isolation Transformer PR570
Capacitance Analyzer LC102
CRT Analyzer CR7000
AC Leakage Tester PR570
Inductance Analyzer LC102
Flyback Yoke Tester TVA92
Field Strength Meter SL753
Transistor Tester TF46
Horizontal Analyzer HA-2500
Video Analyzer VG91, TVA92
NOTE: VHF Low Band voltages taken on channel 2.
VHF High Band voltages taken on channel 7.
UHF Band voltages taken on channel 14.
Q17404
E 12.0V 12.0V 12.0V
B 11.8V 11.7V 0V
C .5V .6V 11.4V
MISCELLANEOUS ADJUSTMENTS
SET 4776 Page 1
NOTE: All procedures require an antenna connected and power applied to the set.
HIGH VOLTAGE CHECK
Tune in a picture. Set brightness, contrast, and color to minimum. Connect a high
voltage probe to the CRT anode. High voltage should measure 26kV to 28kV.
COLOR TEMPERATURE
NOTE: See Service Adjustment Parameters to change drive and bias values.
Press menu button for collapsed raster service line. Disconnect the antenna. Preset
the red, green, and blue drive values to 32. Adjust screen control for a service line
that is just visible. Adjust red, green, and blue drives to obtain a white raster. Check
the low light to high light gray scale tracking. Repeat the procedure, if necessary, to
obtain the best performance
SERVICE MENU
The following adjustment procedures are accessed thru a service menu. To access the service menu, turn the receiver on, press the menu button and hold it down while
pressing the power button. While holding down the menu button, release the power button and press the volume + button. The screen will display a one line menu, on the
left the parameter P0, and on the right the value of that parameter V0. Release buttons. Adjustments are made by selecting the proper parameter and changing the value of
that parameter. To change the parameter number use channel up and down buttons. To adjust the current value of that parameter use volume + and - buttons. To access and
change any of the adjustments, the proper parameter pass number must be entered. This information is listed at the beginning of the alignment. When these parameters are
modified, the T-Chip and the corresponding EEPROM are updated. All service adjustments are bus controlled, except focus and screen.
NOTE: In order to adjust the RF AGC, audio or video levels, tuner, PIP, or stereo circuits, the ChipperCheck hardware and software must be used. This can be purchased
from Thomson Electronics. Before making any changes to any of the values, record the On Set values.
SERVICE ADJUSTMENT PARAMETERS
No. Parameter Name Value Range Comment
0 Pass number for service Must set to 76 - May not advance until value is set to 76.
1 Error Code 1 0 0 - 255 Displays the first error detected. Set to 0 before exiting.
2 Error Code 2 0 0 - 255 Displays the second error detected. Set to 0 before exiting.
3 Error Code 3 0 0 - 255 Displays the last error detected. Set to 0 before exiting.
4 Horizontal Phase 10 0 - 15 Tune in a crosshatch pattern, adjust to center the pattern on the screen.
5 EW DC (Width) 16 0 - 31 Tune in a crosshatch pattern, adjust for slight horizontal overscan.
6 EW Amplitude 8 0 - 15 Set value to 8.
7 EW Tilt 8 0 - 15 Set value to 8.
8 Top Corner Pin Correction 2 0 - 7 Set value to 2.
9 Bottom Corner Pin Correction 2 0 - 7 Set value to 2.
10 Vertical DC 33 0 - 63 Tune in a crosshatch pattern, adjust to center vertically.
11 Vertical Size 84 0 - 127 Tune in a crosshatch pattern, adjust for slight vertical overscan.
12 Vertical Countdown Mode 0 0 - 3 Set value to 0. (0 = Standard, 1 = Non-Standard, 2 = 50Hz, 3 = 48Hz)
13 Red Bias 30 0 - 127 Press menu button on the TV set for setup line.
14 Green Bias 15 0 - 127 Press menu button on the TV set for setup line.
15 Blue Bias 34 0 - 127 Press menu button on the TV set for setup line.
16 Red Drive 41 0 - 63 17 Green Drive 33 0 - 63 18 Blue Drive 32 0 - 63 19 Gemstar Horizontal OSD Position 166 0 - 255 Set value to 166.
20 Gemstar Vertical OSD Position 68 0 - 255 Set value to 68.
21 Gemstar PIP Horizontal Position 40 0 - 255 Set value to 40.
22 Gemstar PIP Vertical Position 43 0 - 255 Set value to 43.
23 Gemstar PIP Window Vertical Size 3 0 - 13 Set value to 3.
Parameter On Set Value
adjustment parameters.
See Error Codes Chart.
See Error Codes Chart.
See Error Codes Chart.
ERROR CODES CHART
ERROR CODE
DEC HEX Error Location Condition Indicated
0 00 No error code 1 01 16.0V fault 16.0V STBY source is failing.
3 03 12.0V run fault 12.0V source is failing.
4 04 T4 Chip Run supply failed.
8 08 T4 Chip X-ray protection caused high voltage shutdown.
9 09 T4 Chip (POR) Power supply problem at (POR) power on reset.
10 0A F2 PIP module error (POR) Power supply problem at (POR) power on reset/PIP.
11 0B Stereo decoder (POR) Power supply problem at reset/Stereo decoder.
16 10 Run IIC Bus held low Run IIC clock or data held low.
18 12 Standby IIC Bus held low Standby IIC clock or data held low.
23 17 Gemstar 3 Board Guide fatal error on set using Gemstar 3 Board.
24 18 Gemstar 3 Board Task monitor error on set using Gemstar 3 Board.
25 19 Gemstar 3 Board Watchdog error on set using Gemstar 3 Board.
26 1A Gemstar 3 Board Task monitor error on set using Gemstar 3 Board.
32 20 Gemstar 3 Board CPU error on set using Gemstar 3 Board.
34 22 Gemstar 3 Board Gemstar 3 fails to acknowledge.
44 2C F2 PIP module error F2PIP fails to acknowledge.
102 66 Octal DAC Octal DAC fails to acknowledge.
128 80 Stereo decoder Stereo decoder fails to acknowledge.
160 A0 Main or PIP tuner EEPROM Main or PIP tuner EEPROM fails to acknowledge.
186 BA T4 Chip T4 Chip fails to acknowledge.
196 C4 Main tuner PLL/DAC Main tuner PLL IC fails to acknowledge.
198 C6 Main tuner PLL/DAC Main tuner DAC IC fails to acknowledge.
ERROR CODES
If certain failures occur, the matching error codes will be stored in the
EEPROM. These error codes will be displayed in parameters 1, 2, and 3.
The first failure error code will be stored at parameter 1 and the second
failure error code will be stored at parameter 2. Parameter 3 will be
updated to display the most recent failure occurred in the chassis. If a
failure of a bus IC occurred, the normal acknowledgment checking of that
bus will be disabled in the service mode and the address of that IC which
failed will be stored in one of the error code parameters. After every repair
is done to the chassis it is recommended to check the error code
parameters, and reset them back to value 0.
GE MODEL 27GT720TX1 (CHASSIS CTC203AX)
SET 4776 Page 1
Page 2 SET 4776
Loading...