How to enter service mode – Akira 14THS3, 14 inch color TV, Troubleshooting, circuit diagram, working principle
The high voltage in the TV reaches to 22KV when the TV is in operation. Be more careful during opening the back cover. The high voltage existing in the TV is very dangerous. Refer servicing to qualified personnel only.
SIGNAL PROCESS
The TV signal enters into tuner (A201) from cable or antenna. Pin 10 and pin 11 of N301 are combined to select the band. Pin 4 of N301 outputs PWM tuning signal. The 38.9MHz IF signal is coupled to V308 (pre-amplify) and then to SAWF (Z301). After processed in the SAWF, the 38.9MHz signal gets to pin 23 and pin 24 of OM8370. The IF circuit in OM8370 includes such unit as the AGC amplifying circuit, 38.9MHz oscillator, PLL video demodulator, video amplifier, IF identify circuit and AFT circuit. The demodulated signal (CVBS) comes from the pin 38 of OM8370, the sound signal comes from the pin 44.
The internal CVBS signal needs norm identification then outputs from pin 38 of OM8370, via the trap circuit (composed of the V351, Z351, Z354, V352 and so on) feeds back to the pin 40 of OM8370. The RGB signal comes from pin51, Pin52, Pin53 of OM8370, and outputs to the CRT board. The internal sound signal comes from pin 44 of OM8370 and then input to pin 1 and pin 12 of HEF4052BP. The sound signals from RF and AV are selected in it. The selected signal is output from Pin 3 and pin13, then input AN7522N (sound amplifier) or output from AV terminals.
HORIZONTAL
AND VERTICAL CIRCUIT
Through Synchronous separating circuit, the video signal is divided into
horizontal-Synchronizing signal and Vertical-Synchronizing signal. The
horizontal-Sync pulse coming from pin 33 is transferred to the horizontal-drive
transistor. The cathode driving voltage
180V vertical drive voltage +15V and –15V are all generated from
FBT. The vertical sawtooth waveform is
generated on the pin 21 and 22, and then enters the vertical amplifier
STV9302A. STV9302A is a 7 pins vertical deflection circuit (2 Amperes) for
DC-coupled 90° or 110° deflection systems with frame frequencies from 50 up to
120 Hz. one supply voltage for the scanning and second voltage supply for the
flyback are needed. The vertical drive
currents of OM8370 pins 21 and 22 are connected to input pins 1 and 7 of STV9302A.
The vertical scanning output is coming from pin 5 and feed back to pin 1 by
R405. The supply voltage is ±15V at pin 2 and pin 4. C406 is the pump
capacitor.
POWER
SUPPLY
The IC of KA5Q0765RT is adapted in this chassis; it is the product of Fairchild.
It supplies four DC voltages, one is the +B＝110V, another is Hcc＝26V,
the third is 13V (the sound drive voltage), the fourth is +16V. The +16V can
generate the +8V, +5V and +3.3V by the special generators.
FACTORY
MENU / Service mode
Main power +B setting
Receive standard color pattern RF signal. Set picture to “Standard Mode”.
Adjust VR501, to get +B (VD542 -) voltage =110V.
I2C bus control adjustment method
How to enter and exit factory mode (with customer’s remote controller)
1. Press [menu] key > display picture menu
2. Press digital key “6” “4” “8” “3” - display “M”
3. Push [standby] key one time to exit factory mode and return to normal.
How to select menus in factory mode
In factory mode {“M” mode} there are ten menus totally. Menu 1 to Menu 4 can be
selected directly but Menu 5 to Menu 0 must be select by these steps:
1. Press “CALL” key, to call “M”.
2. Press “LOCK” key, then press any one numeric key from 5 to 0.
For example, to select Menu 7, you should press “call” key and LOCK key in
turns, then press the numeric key 7
Troubleshooting
No
OSD (On Screen Display)
This failure is usually cause by the circuit of character generated and
located. Most of reasons are that the horizontal and vertical flyback pulse
signals do not come to UOC. We can judge
this failure by measuring the wave of the character in an oscilloscope. The detail checking and repairing steps are
as follow:
Click on the pictures to zoom in
UOC
does not work
In television, remote-control system is similar with the computer system. In
theory , it can work if it holds two conditions as follow:
1) The power supply: In general, it is 5V, the error is not above 10% and the
disturbance pulse is as small as possible.
2) The clock pulse: In TDA93XX circuit, the clock pulse is generated by pin58 /
pin59 of N301 and 12M crystal oscillator.
Television’ s remote-control system also needs reset circuit that can preset
the values in internal register. The circuit around pin57 of N204 is called
auto-reset circuit. If UOC detects errors in resetting, it will come to the
state of program protected.
Schematic
No
raster, no picture, no sound
This failure is mainly caused by big-power circuit such as power supply,
horizontal scanning, vertical scanning. The
detail checking and repairing steps are as follow.
No picture, no sound
The failure shows that the set does not display the picture but it has noise wave or blue background or OSD on the screen. This means that the circuits of power supply, horizontal scanning, vertical scanning and video amplification are normal and they are not considered in the repairing. The failures are mainly in the small signal processing circuits.
Before checking these circuits, a kind of practical test method is introduced. It is called “Signal-input way ”. The detail is described as follow: We can use the resistance function of an analog multimeter, connect the red pole (negative in ohm scope) on the circuit board ground, then touch softly the test point with another pole (black pole) in ohm scope meanwhile observe the reactivity on the output device.
Note : In the TV test, we mainly observe the noise wave on the CRT and listen to the noise voice liking as “Ka…..Ka” from the loudspeakers.
No
sound
In this kind of failure, first of all we should observe if there is the picture
on the CRT. It proves the small signal circuit to work correctly with the
picture on the CRT and we only check the sound signal processing and sound
amplification circuit. The repairing method(B1) may be referred without
picture. The detail checking and repairing steps are as follow.
Note: Before repairing, assure that the volume is on and the state of set is in “TV”
Only
horizontal line in the middle of the screen
If vertical deflection circuit does not work, this kind of failure will happen.
In deflection yoke, there only has horizontal sweep, the electron beam in
the CRT only moves in the horizontal orientation , so form this failure. (While checking horizontal and vertical
deflection circuit’ s failure , we have better to use an oscilloscope.)
Only
vertical line in the middle of the screen
This is a dangerous failure. It probable causes flashover and smoking inside
the set. Don’ t let your TV work for a long time as this failure appears.
Because the electron beam can not move in the horizontal orientation, the
failure should be in the horizontal deflection circuit. We mainly check the
open-circuit fault in horizontal deflection circuit. The detail checking and
repairing steps are as follow
OM8370/TDA9340 pin functions
