Repair of TVs with defects in memory chips. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Repair of TVs with defects in memory chips. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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In televisions with a microcontroller control system, malfunctions are very often associated with the appearance of defects in the program stored in memory chips. How to identify and fix this is described in the published article.

Most of the problems in the repair of modern TVs arise due to a violation (failure) of the boot information (BIOS), which is necessary for the normal operation of the control microcontroller and for setting up TV nodes controlled via a digital bus (tuner, video processor, teletext module, etc.). As a rule, this information is contained (recorded) in a memory unit, made in the form of a separate microcircuit, located next to the control microcontroller.

Currently, electrically reprogrammable serial read-only memory (EEPROM) chips are most widely used. Moreover, for the exchange of information between these microcircuits, as a rule, the HC (12C) bus is used, consisting of two bidirectional lines: for information data - SDA (serial data) and for synchronization signals - SCL (serial clock). Most often, televisions use microcircuits from ATMEL, MICROCHIP, PHILIPS, containing at the end of the designation the alphanumeric combinations 24С01, 24С02, 24С04, 24С08, 24С16 or PCF8582 (domestic analogues 1568РР1, 1566РР1,1609, 1ХПXNUMX, etc.)

Sometimes TVs use a three-wire 3-Wire (Microwire) bus of a serial type, in which synchronization signals are exchanged - SK (shift clock), input - DI (data input) or DO (data output) data output and crystal selection - CS (chip select). An example of a memory device with such a bus is microcircuits, the designations of which end with the combination 93C46.

The protocols and addressing methods for working on these buses and microcircuits are described in detail in various popular literature, for example, in reference books on microcircuits from the DODEKA publishing house. However, they are not discussed here. Let's just focus on practical things.

As a rule, all memory chips are replaceable by others with more memory. For example, a 24C01 or 24C02 chip can be replaced with any of the 24C04, 24C08, 24C16. The 24C01 chip has a memory capacity of 1K (128x8 bits), and the 24C16 chip has 16K. It is also possible to replace the PCF8582 chip with 24C02, but not vice versa, since an external programming clock generator is required when writing to the PCF8582 memory.

The manifestations of a malfunction due to a defect in information in the memory chips can be very diverse: the TV does not turn on, the on-screen menu does not appear or the control through it is performed incorrectly, the raster geometry is changed, the selected program is not stored, channels switch spontaneously, the sound is turned off, etc. In addition , after turning on the TV, all initial settings of volume, brightness, contrast and saturation can be changed to maximum or minimum values. In this case, checking the presence of synchronization signals and information data with an oscilloscope in most cases turns out to be useless, since the exchange occurs both with a "clean" microcircuit and when incorrect data is recorded in it. Even one defective ("dangling") cell (that is, which cannot be reprogrammed) can be the cause of a malfunction.

In the most general case, when the TV is turned on, the control microcontroller is reset by the RESET signal. Usually, a specialized reset chip is used for this purpose. Then the program written in the microcontroller starts from some point (not necessarily from address zero). As a rule, after fetching two or three commands (port initialization, ADC, etc.), the microcontroller accesses the memory chip and reads the initial settings into its own RAM memory block. Usually these are the initial values of brightness, contrast, saturation, image and sound volume, information about the geometry of the raster, and others, depending on the software and functionality of the TV.

Unfortunately, there is no standardization in the development of programs for microcontrollers: each developer does it at its own discretion. Therefore, the following options for the operation of a microcontroller-memory chip pair are possible: either the processor reads all the information and, after external influence, for example, commands from the control panel, accesses to it, or the processor constantly interrogates the memory chip via the digital bus and responds to external influences in the intervals between polls. As a result, it may be difficult to determine the faulty link (processor or memory chip).

To unambiguously check the contents of the memory chip, it is best to use a programmer. Its use represents, of course, a simplified approach in such a situation, but in most cases it solves the problems associated with the malfunction of these microcircuits.

Let's consider ways to eliminate a defect in the operation of memory nodes using the example of the most common imported TV models FUNAI-TV-2000MK7 and SONY-KV-T14M1.

In FUNAI TVs, the memory chip fails due to faults in horizontal scanning, the memory unit, lightning strikes the antenna. In this model, a 24C01 chip is installed. After replacing the chip with a "clean" one, the values of all settings (sound volume, brightness, saturation and contrast of the image) take on the maximum values.

To resume information in the memory chip in this case, when the TV is turned off, output 13 of the control microcontroller (one of the TMP47C4 series) must be disconnected from the common wire and connected through a 1 kΩ resistor to the point where the supply voltage should be +5 V. As a result, after turning on the TV, all the necessary boot factory information will be rewritten from the microcontroller to the memory chip. Then you need to turn off the TV and to restore the normal operating mode, output 13 of the microcontroller is again connected to the common wire.

After such a "firmware" of the memory chip by the microcontroller, the information presented in Table 1 is recorded in it. XNUMX (left and top addresses of rows and columns of memory cells, respectively, all numbers in the table are in hexadecimal form).

Repair of TVs with defects in memory chips

Let's decipher some boot information located in the cells that are circled. So at address 0-6 (row-column) a sound volume level of 14 H is recorded, where the letter H marks the value in hexadecimal form, and at addresses 0-7 - 0-A - the level of 20H of brightness, saturation contrast and image clarity.

If you are not satisfied with some factory settings, you can adjust them up or down with the programmer. The minimum values in the indicated cells are 00 N, the maximum ones are 3H N. It is not recommended to change the information in the cell at address 0-E, equal to 06 N, since this is information about the color system and you can turn off the color by manipulations. Cell 0-F with a value of 01H stores the channel number (in our case, the first channel). The value of 22 N corresponds to the 22nd channel. Cells from address 2-0 are used to store stations. In the cell at address 7-C (AC H value), the information does not change, it is reserved by the microcontroller.

After checking the chip for writing-reading and writing factory settings to it, you can be sure that the memory chip is working.

In SONY TVs, in addition to the occurrence of malfunctions due to the reasons listed above, a change in the contents of the memory chip was observed for an unknown reason (especially in early models). The above model uses a 24C02 chip. It contains more information than 24C01. After replacing the chip with a new one ("clean"), a common problem is a violation of the geometry of the raster. In addition, problems also arise when converting this TV from PAL-NTSC standards to SECAM-PAL-NTSC standards.

To set the correct geometry of the raster, the "firmware" of the memory chip is shown in Table. 2.

Repair of TVs with defects in memory chips

There are two options for conversion. In models (European) on the Motorola MC44006 video processor, the color is turned off by software. To enable it in the cell at address E-A, you need to write the value 20 N. In TVs on the MC44007 video processor, in addition to writing the value 20 N to the above cell, you need to replace the video processor itself with the MC44002 or MC44006. No other modifications are required.

A little about the content of "vital" cells in the memory chip for this model. Address C-6 with value 31 H - brightness level. From address C-7 to CF - factory presets. From address D-0 to DA - all settings for the geometry of the raster. Address DB with a value of 6A H - blocking the video signal, DD (value 10H) - color reproduction. Address E-A (value) 20 H - color system. To turn off the color in this cell, the number 7D H must be written. At the same time, it must be remembered that the choice of the system, the amplitude of the three-level synchronization signal (Sandcastle) depends on the video processor used.

Of course, all the changes listed above can be made when entering the service mode. For SONY TVs, as a rule, the following combination of pressing buttons on the remote control in the standby mode of the device is required: Display, "5", Volume "+", PWR ON. To record the changes made, click on the Mute write button. In this case, the entire memory is reprogrammed. However, it should be remembered that another combination of buttons is also possible to enter the service mode for specific TV models.

From the above, it can be seen that in the case of using a programmer, you can check both the performance of the memory chip and its contents, and also, if necessary, change some settings (even just one bit). The service mode is convenient when making some adjustments (for example, to the raster geometry) without disassembling the TV, but you cannot be sure that the memory chip is working and that all contents correspond to the factory settings. Unfortunately, branded service instructions are expensive and almost inaccessible for many models. Therefore, the purchase of a programmer or its independent production is economically beneficial when repairing TVs. In this case, the main task is to collect a library of "firmware" from known good TVs. In any case, it is preferable to use the factory settings, which will completely avoid any errors.

Author: A.Goncharenko, Odessa, Ukraine

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