The following TVs are produced on the basis of the MS-994A chassis: CA-14/20/21 F89W, CA-14/20/21 F89X, CF-2O / 21 D79, CF-2O / 21 F39, CF-14/20/21 F69X, CF-14/20/21 F89, CF-14/20/21 F89W, CF-14/20/21 F89X. The main technical characteristics of these models are presented in table. 1.
Design and features of the chassis MS-994A
Structurally, the chassis consists of a main board, a kinescope board, a board "EYE" (see table. 1) and the teletext module. The last two nodes are installed optionally. The main feature of the new chassis is the use of the TOSHIBA multifunction IC IC01 type TB1238AN, containing an OCH, a video detector, a sound demodulator, a video processor, a clock processor and an I 2 C interface circuit. Its use greatly simplified the chassis circuitry, which, in turn, led to an increase reliability of TVs.
Table 1. The main technical characteristics of LG TVs based on the chassis of the MS-994A
The chassis control system is built on the IC01 microcontroller (MK) type MC37221 of MITSUBISHI firm, working in tandem with the IC02 type 24C04 non-volatile memory chip. For the exchange of data between microcircuits and the transfer of commands to the IC501 chip, the MK uses the I 2 C digital interface.
A feature of the new chassis is the presence of an interface for a video camera that allows you to use a TV, in particular, as a monitor for a video surveillance system.
In the table. Figure 2 shows the parameters of interchangeable elements, depending on the diagonal of the kinescope screen.
Table 2. Parameters of replaceable elements depending on the diagonal of the kinescope screen
Consider the operation of the main components of the chassis and the paths of the passage of audio and video signals, as well as the main elements of their processing.
The power supply unit (PSU) generates stabilized voltages 115 (V), 20 (S-VCC), 14, 12, 9 and 5 V (ST-5V) to power the chassis nodes in operating and standby modes. It is built according to the scheme of a quasi-resonant reverse pass converter on the IC803 chip type STR-F6707 from SANKEN. The composition of the IC803 includes: a master oscillator, start-up circuits, protection against overload, overvoltage and overheating, as well as an output stage on a powerful bipolar transistor. The microcircuit turns on when the voltage on the pin. 4 is about 8.5 V, but turns off at a voltage of 5 V and consumes a current in operating mode of 30 mA, and in standby mode. 200 μA. The R809 R810 circuit generates a starting voltage, and obm. 1-2 T802 and a rectifier on the elements D806, C801. supply voltage in operating mode. The output voltages are stabilized by the feedback circuit IC804 IC801, the input of which (pin 11C804) is connected to bus B, and the output is connected to the input of the error signal amplifier of the controller IC803 (pin 1). To control the current limit, the voltage drop is removed through the power switch of the transmitter (R805) and fed to the pin. 11C803 (protection operation level is about.0.9.1.2 V). Using transistor switches Q805-Q807 and optocouplers IC802 MK signal with pin. 5 switches the power supply to standby mode. In this case, the converter operates at the minimum operating frequency.
The circuit on the elements R807, C831, Q831, Q832 generates an ABNORMAL alarm on the MK (pin 6) in case of malfunctions in the output circuits of bus B or in the horizontal scanning circuit. Secondary voltages of 5 and 9 V are formed by integrated stabilizers IC805 and IC844, the latter being controlled. The IC844 chip is controlled by the MK ON / OFF signal (pin 5).
The television radio signal from the antenna input is fed to the input of the TU101 tuner, which is controlled by the MK (pin 31, 33 IC01) via the I2C interface (pin 4, 5 TU101). The tuner is powered by a voltage of 5 V (pin 7). The output signal of the tuner (vyv. 11) from the IF equal to 38 MHz, through the band-pass filter Z101, forming the frequency response of the IF path, is fed to the input of the IF-output. 6 and 7 of the IC501 chip. Here are its main functions:
- generating a full color video signal (CVBS) from an IFI signal;
- formation of an audio signal from a PCP signal;
- AGC voltage generation for the tuner;
- automatic detection of the color system and decoding of PAL and NTSC systems;
- control of an external SECAM decoder (IC502);
- luminance signal extraction from CVBS;
- formation of color difference signals: luminance signal and primary colors (RGB);
- switching of RGB signals and on-screen menu (OSD), their amplification to the level necessary to control the output video amplifiers on transistors Q901-Q903;
- the selection of clock pulses from CVBS and the formation of pulses launch line scan and sawtooth voltage to control the frame scan;
- receiving and processing control commands from MK via I 2 C. interface
The purpose of the findings of the TB1238AN chip is presented in table. 3.
Table 3. Terminal assignment for TB1238AN
The signal processor OSD-R, G, B, teletext TXT-R / G / B or external signals SCART-R / G / B can be input to the video processor switch (pin 14-16 IC501). The required signals are selected by the IC751 switch, controlled by the FB-ID (pin 39 IC01), TXT-FB (pin 8 P701V) or SCART-FB (pin 16 PJ201) signals. Output video signals of primary colors with pin. 18.19, 20 IC501 via contact 2, 4 and b of the P901B connector are fed to transistors of the output video amplifiers Q901-Q903, which are powered by a voltage of 180 V from the horizontal scanning circuit. In addition, through cont. 1 P901V, a bias voltage of 12 V is applied to the video amplifiers, which determines the operating points of the transistors. The adjusting elements of the video amplifiers are absent in the circuit because all the adjustments are performed by the IC501 video processor in the service mode using MK on the I 2 C interface.
The main part of the sound path is located in the IC501 chip. To isolate sound signals of various standards, use the IC151 switch with filters F151-F154, controlled by MK signals: SO, S1, and M4.5 (vyv. 38, 39, 14). The PCP signal from the output of the video detector (pin 47 IC501) is supplied through the Q507 buffer to the inputs of the F151-F154 filters connected to the IC151 switch. PCHZ output signal with pin. 3 IC151 goes to the input of the demodulator. pin. 53 IC501. The audio output from the demodulator is amplified and sent to the INT / EXT switch (inside IC501) to select the appropriate signal. External sound signal on the pin. 55 IC501 comes from SCART connectors or "tulip". The sound source selected by the IC01 microcontroller via the I 2 C interface is removed from the pin. 2 IC501 and is fed to the input of an audio power amplifier (UMZCH). pin. 5 IC601 type TDA7253 microcircuit, which is a single-channel push-pull class AB amplifier with short-circuit protection and a MUTE sound blocking input (pin 3). From its output (vyv. 8) the signal through the isolation capacitor C605 and connector P601 is fed to the dynamic head. The UMZCH is powered by a 20 V power supply (S-VCC).
A teletext module can be installed on the MS-994A chassis, which is connected via the R701V, R702V connectors. The module is based on the IC701 type SAA5281 chip, which has 8 Kx8 RAM for 8 teletext pages. It is designed to work with the 625-line standard WST (World System Teletext). In addition, the chip decodes VPT (VCR programming) signals. It is controlled by MK via the I2C interface (vyv. 24, 25). For the work of IC701 on its pin. 9 with IC501 (pin 35), the TXT-CVBS video signal is input. At the outputs of the microcircuit, the teletext R, G, B signals (pin 16, 17,18) and the TXT-FB blank signal (pin 20) are generated, which are fed to the IC751 switch, and from it to the IC501 video processor.
To power the teletext module on cont. 3 P701V 9 V voltage is supplied from the power supply. The IC701 chip is powered by a voltage of 5 V from the stabilizer IC702.
Horizontal and vertical scan nodes
Line scan is built according to the usual two-stage scheme (transistors Q401, Q402) with serial power supply to the output stage. Transistor Q401 is powered by a voltage of 14 V, while Q402 is powered by a voltage of 115 V (V) from a power supply. The output transistor has an internal damping diode. The line transformer T402 generates the supply voltage of the kinescope, vertical scanning (24 V) and output video amplifiers (180 V). All T402 secondary circuits are protected against overload by breaking resistors FR301, FR401 and FR501.
A voltage is formed on the capacitor C418, inversely proportional to the current of the kinescope rays. The signal ABL (OFF) from the output of the shaper enters pin. 21IC501 for controlling dimming and beam current limiting circuits.
The output stage of the frame scan is implemented on the IC301 chip type LA7833 from SANYO. Sawtooth pulses of frame scan are fed to the input of the microcircuit (pin 4) with pin. 24 IC501. The output coils of the microcircuit (pin 2) are connected to the frame coils of the OS V-DY. To control and stabilize the vertical size, the V-NFB feedback signal is removed from the amplifier output and fed to the pin. 23 IC501.
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As already noted, the IC301 chip is powered by a voltage of 24 V (pin 6) from the horizontal scanning circuit.
To synchronize the OSD circuit, the OX pulses of the horizontal (pin 10 T402) and personnel (pin 7 IC301) scans through the inverters Q01, Q02 are fed to the MK (pin 1 and 2).
MK IC01 performs the function of controlling all nodes of the chassis. The operation of the MK is ensured by a quartz resonator X01 (vyv. 19, 20), a reset circuit IC03 and a non-volatile memory IC02. The purpose of the conclusions of the chip are presented in table. 4.
Table 4. The purpose of the conclusions of the IC01
As in any modern television receiver, the necessary after repair or replacement of the RF controls, video processor and other components on the MS-994A chassis are performed in service mode. To work in this mode, you must have a remote control with teletext control buttons. Before making adjustments, turn on the TV, apply a signal to its antenna input "Test table" and warmed for 15. 20 minutes
To enter the service mode, simultaneously press the buttons "OK" on the remote control and on the front of the TV, holding them until a list of adjustable parameters appears on the screen (Fig. 1). Last line "LINE SVC 0" shows the menu number, and there are five in total (LINE SVC 0-4).
Fig. 1. List of adjustable parameters in service mode
The required parameter is selected with the joystick buttons "up down", and adjust its value with the buttons "right left". To save the new parameter value, press the button "OK". To exit the service mode, put the TV into standby mode using the remote control button "Power". Consider the sequence of adjustments to the main parameters on the MS-994A chassis.
RF AGC adjustment
This adjustment is required after replacing the tuner, as well as when significant noise (interference) appears on the image.
- Connect a voltmeter to the pin. 1 tuner TU101.
- A signal is fed to the antenna input of the TV from the television signal generator "Colored polo
- sy" with a level of 65 dB, turn on and configure the TV to receive this signal, and then switch it to service mode.
- Select a parameter from the menu "AGC" and its adjustment, a voltmeter reading of 2.3 V is obtained for a tuner of type 6700VPV002A or 3.0 V for a tuner of type 6700VPV002B. Button "OK" remember the new parameter value "AGC".
Acceleration Voltage Adjustment
Accelerating voltage, as a rule, is regulated after replacing the kinescope or after repairing the horizontal scanning circuit.
- They send a signal to the antenna input of the TV "Colored stripes" from a television signal generator.
- In service mode, select the menu "LINE SVC 3", and in it is a parameter "Cutoff".
- The Screen controller on the T402 transformer ensures that the light horizontal line is barely visible.
White balance adjustment
This operation must be performed after adjusting the accelerating voltage.
- A signal is sent to the antenna input of the TV "White field" and set the contrast adjustment to maximum, and the brightness to 90% of the maximum position.
- In service mode, select the menu "LINE SVC 0".
- Adjustable parameters "Gg" and "BG" achieve white balance in "bright".
- Set the brightness and contrast settings so that the screen hardly glows, and adjusting the parameters "RC", "Gc" and "Sun" achieve white balance in "dark".
- If necessary, repeat the adjustment several times, achieving optimal balance
Factory values of the parameters are given in table. 5.
Table 5. Factory-set white balance adjustment options
This operation is performed in the same cases as the previous one, as well as in case of poor focus. Turn on the TV, apply a signal to its antenna input "Grid" or "Test table" and warmed for 15. 20 minutes Then, using the Focus knob on the horizontal transformer, you achieve the best image focus.
Adjust image geometry
This adjustment is performed as needed.
- They feed the same signal to the antenna input of the TV as in the previous case.
- Before adjusting the button "ARC" on the remote control select the image format "STANDARD".
- Enter the service mode, and in it. select the menu "UNESVC2".
- Sequentially select parameters "Vl" (vertical linearity) "VS" (vertical alignment) "VA" (vertical size) "Hs" (horizontal alignment) "SC" (S-correction) and adjust the geometry of the image.
Adjustment of options is necessary in order to configure a specific TV model. Setting options is done in the menu "OPTION 1" and "OPTION 2".
Options and their possible values are given in table. 6 and 7.
Common problems and solutions
TV does not turn on, indicator "Power" off, mains fuse F801 blows
They disconnect the TV from the network and use an ohmmeter to check for a short circuit the elements of the demagnetization circuit, line filter, rectifier: ТН801, ТН802, С806-С810, VD801, Т801, RT801, RT802, DB801. If there is no short circuit in these circuits, then the power transistor is checked with an ohmmeter for a short circuit (pin 2 and 3 of IC803). If these elements are operational, the T802 transformer is soldered and checked according to one of the known methods.
TV does not turn on, indicator "Power" off, mains fuse F801 good
Turn on the TV with a network switch SW801 and measure the voltage 300 8 on the pin. 3 IC803. If the voltage is zero, then the elements of the following circuit are checked for open circuit: F801, SW801, T801, R811, DB801, pin. 8-5 T802, FB803, conc. 3 IC803. If the voltage is 300 V on the pin. If there is 3, but the converter does not work (there are no pulses with a span of about 500 V on pin 3 of IC803), then we check the external elements of the microcircuit that provide its power in the startup and operating modes (see description).
If the converter is working (there are pulses on pin 3 of IC803), check the 5 V stabilizer (IC805). If it is working, then check the MK and its external elements (see description and table. 4).
Indicator "Power" lights up, the TV is in standby mode and does not switch to working
First of all, they check the ON / OFF signal (pin 5 IC01). If the signal is high (i.e., the TV is in standby mode), it may be that the protection of the stream overload of channel B of the power supply has worked.
In this case, the signal ABNORMAL pin. 5 IC01 will be active (low). They turn off the TV and determine the cause of channel B overload. If the protection signal is passive, the MK itself may be faulty or IC02 memory failure. The chip is rewritten and, if the TV still does not turn on, replace the MK. If the ON / OFF signal (pin 5 IC01) is low, the Q807 key must be closed and the 9 V stabilizer (IC844) turned on.
No raster and sound, power supply is working
Perhaps one of the voltages on the kinescope board and the kinescope itself is missing: HV, USCREEN, UHEATER. 180 V. Check for the presence of the indicated voltages, determine the absent and eliminate the cause. If there is no high voltage (there is no characteristic crack during turning the TV on and off), then most likely the reason is the horizontal scanning scheme. Check for start pulses on pin. 32 IC501, their arrival at the preliminary stage on the transistor Q401 and the operation of the output stage on the transistor Q402 (the collector should have pulses of positive polarity with a range of about 1000 V). If the output stage does not work, disconnect the TV from the network and check all its external elements. If there are pulses, but there is no high voltage, the reason is in the T402 horizontal transformer.
There is a raster, no sound or image
First check the IF path and the video processor (IC501 chip). Control the power of the microcircuit (see table. 3). If there is no 5 V supply voltage (pin 46), the stabilizer IC505 is checked. If the power is normal, the test signal with a frequency of 38 MHz with a level of 65 dB is fed to the input of the IF path (pin 11 of the TU101 tuner) and the signal is transmitted along the path (see description and oscill. 1, 3-5). Defective elements are identified and replaced. If there is no test generator, you can use a VCR or video camera connected to the corresponding inputs as a signal source, but in this case only the video processor is checked.
No color image in SECAM color system
Most likely, the IC502 chip or its external elements is faulty. Set the saturation adjustment to the maximum level position. Check the power of the microcircuit (5 V on pin 9 and 18. If there is no voltage. check the stabilizer on the elements ZD504, R531), the presence of a video signal on pin. 13 and 15 IC502, gating pulses on the pin. 17, all external capacitors. If the elements are working, they replace the microcircuit.
The TV only works through the bass input
Check the power of the A101 tuner (33 and 5 V). Then a test signal is supplied to the antenna input of the tuner from the generator, the auto-tuning mode is turned on, and the corresponding control signals are checked at the terminals of the tuner (see description). If there are signals, but there is no output IF signal (range 0.25. 0.5 V), replace the tuner.
Check the power of the IC601 chip (20 V on pin 9) and the absence of a blocking signal (high potential on pin 3), the health of the dynamic head and the presence of a contact in the P601 connector. Then they touch a metal object (for example, tweezers) pin. 5 IC601. If the background appears in the dynamic head, then the UMZCH is working. Otherwise, replace the chip.
There is no sound in TV reception mode
Check the presence and passage of an audio signal along the following chain: pin. 47 IC501, Q507, Q151, F151-F154, conc. 1, 2, 4, 5 IC151, conc. 3 IC151, conc. 53 IC501, conc. 2 IC501. Identify and replace the faulty path element.
No sound or picture when working through the bass input
Check the appropriate paths.
Video path: PJ201, C251, vyv. 7, 8 IC251, conc. 41IC501.
Sound path: PJ201, C227, Q221, vyv. 10.11 IC251, C257, conc. 55 IC501.
TV does not respond to remote control commands
Faulty remote control. Initially, known-good batteries are installed in the remote control. To check, use the infrared photodiode, for example, FD-8K, connect its findings to the input of the oscilloscope, send the remote control to the photodiode and press one of the remote control buttons. On the oscilloscope screen there should be bursts of pulses with an amplitude of about 0.5 V. If they are not there, check the serviceability of the elements of the remote control circuitry: microcircuit, resonator, output transistor and LED.
If the remote control works, press one of the buttons on the remote control and check for a signal with an amplitude of 4. 4.5 V pin. 1 RA01. If there is no signal, then replace the photodetector. If there is a signal, the microcontroller IC01 is faulty.
One of the colors prevails in the image, the black-and-white image has a color of the same color
As a rule, this is due to changes in the parameters of the radioelements and picture tube due to their aging. To eliminate, adjust the white balance in service mode (see "Service mode")
A thin horizontal bar is visible on the TV screen.
Check the sawtooth pulses (Os. 9) on the pin. 24 IC301. If they are not there, check the capacitor C313 (Os. 6) and all the elements in the feedback circuit: C308, R314, R313, R306, R407, C301.
If the sawtooth pulses on the pin. 4 IC301 is, and the output signal on the pin. 2 is absent (signal span is about 45. 50 V), the microcircuit’s power is checked (24 V on pin 6) and the following elements: R303, R304, C311, R310, C310, V-COIL If they are working, replace the IC301 chip.
The vertical image size is small and not adjustable in the service mode.
Check the elements of the boost circuit D302 and C307.