TCL - M28 - SUPER ONE CHIP CHASSIS-CRT TV_REPAIR HINTS_ Common Faults to CRT based TVs

The indicator is always light. Cannot turn it on by remote handset or local key.

  Can make a conclusion that the MCU section is not working. So the remote / local key cannot wake up the machine. Perhaps the reset circuit of MCU or the crystal oscillator causes the failure. Having checked the circuit, find that all components of the reset section of MCU are OK. So suspected that the crystal oscillator is failure. Replaced it with a new crystal oscillator, so the machine can work normally.

No sound, no picture blue screen in TV status

  The blue screen indicates that the video output and scanning circuit and MPU are all OK. The failure may exit in the section from the tuner to pin26 of TMPA8803CSN. Used an oscilloscope to observe the waveform of pin30, and the waveform is OK, but no signal input from the pin26. Checked the circuit carefully and found the emitter of Q209 is cold joint. So the TV signal cannot pass the Q209.  That was the failure.

Power is switched on, and makes noises. No sound, no picture.

    First, disconnected the power supply circuit in the position of +B and switch the machine on. Used a multi meter to measure of +B terminal. The voltage of +B terminal is OK. While connect the +B power supply, measured the +B again, and found the voltage is 0V. There must be something wrong with the horizontal power supply circuit, which is supplied by the +B voltage. Having checked the horizontal output triode, found that there is a short between the collector and the emitter of Q402.  The defective horizontal output transistor is replaced and the machine is OK.

The horizontal output transistor is too hot.

     The low or heavy drive current can cause this phenomenon. The resolution of right area of the raster is low. So  suspected that the low drive current for horizontal output transistor make the current linearity worse.  Checkd the horizontal drive section. The power supply for it is OK. Two 1.5W resistors in parallel compose R402. One of the two's is cold soldering, and the resistance of R402 is changed from 2.4K to 4.8K. The voltage that is added to T401 drop down, and therefore the driver current is weak. The work temperature of horizontal output triode is rising. Having re-soldering R402, the temperature of horizontal output triode is OK.

Sound Ok, no picture (Black Screen)

     The standby and working status can be changed freely. But the screen is black. Menu or picture is invisible. The heater is lighting, so the horizontal scanning circuit is OK. Retrace lines are visible if we adjust the screen voltage.

   All these above indicate that RGB level is cut off, and this is a kind of protection method of Toshiba chassis for the CRT. If there is something wrong, which make TMPA8803 not receive the FBP impulse through pin12, with the scanning section. TMPA8803 will change the RGB output to black level via the I2C bus automatically. 

    At the same time, TMPA8803CSN will make the beam current smallest to prevent the screen of the CRT not be hurt by the electron beam.  Checked the circuit according to the clew above and find that D404 is breakdown, so the FBP impulses disappear and screen is “black”.

Screen is white with visible retrace lines:

     Generally this kind of failure may occur at the section of the end video amplifying.  The cathode current cannot be cut off during the trace back time, because the three cathode current is too large, and the three cathode voltages are low.  Only the failure of bios circuit can make the three cathodes not be cut off at the same time because the fail probably of the three video amplifying circuits is very small.  Further checking revealed  that there is a short at the collector of Q510.  

     This matter makes the emitters of Q501, 502,503 on the lower power voltage.  The retrace lines appear on the screen for the lower cathode voltage. Replaced Q510 with good one and tested OK.

One vertical line on the screen:

     Means that there is something wrong with the horizontal deflecting circuit and the horizontal scanning has stopped. For the trace back impulses from the primary coil of the FBT still exist, the heater voltage and EHT voltage affect the display of vertical line in the screen.  Checked the horizontal deflecting section, and find that R411 has been breakdown, and the terminal pad of L412 has been damaged by high temperature The L412 is heavier than the other small components, and it is supported by two legs.  So the legs are easy to loose to cause the cold soldering. And the deflecting current all pass R411, and R411 is blew.

No AV signal input:

     First, checked the AV input circuit but not find anything abnormal. So exchange TV input channel and AV input channel. The AV signal can be displayed normally. So there is something wrong with the inner circuit of Pin24 of IC201. Having replaced TMPA8803CSN, all is OK.

Fuse blew once switch the set on:

   If this failure happens, first  check the rectifier circuit and the switch power circuit to find whether there are some shorted components. Second, check whether there are shorts at the horizontal output section.  While checking,  found a short at the load section of +B power supply. Checked the horizontal output transistor. In addition a short happened between the collector and the emitter, there was an open inside the retrace cap C406. Replaced the components, tested OK.

Some channels cannot be searched out:

    This is a defect of tuning function not the failure of MCU or the tuner.  Perhaps the input channel signal cannot reach the MCU.  Besides the AFA/AFB circuits integrated inside the TMPA8803, M117 includes a horizontal sync separation circuit, which is composed by Q202 and Q203. 

     This circuit can separate the horizontal sync signal from the composite signal, which is output from pin45 of TMPA8803. And the H sync signal input to MCU from pin62. This is a mark for MCU to judge whether the set has received a channel.

No picture, no sound, no raster, remote and local key unavailable:

     First  check the power section. As this example, the switch power supply circuit can work normally, and can output a main power of 112V. Checked the second voltage regulation section, and found that the H-Vcc ( 9V ) was about 4.5V only. As the 18V power supply was OK, suppose that there should be a short in the load section. 

     Found a short at pin17 of IC201. Q207 and Q206 were not damaged.  C030 was broken down.  Once we had replaced C030 and IC201, H-Vcc was OK. Because C030 as short, the base voltage of Q206 became higher. And the voltage of H-Vcc became higher also. So IC201 was damaged.

Above stated faults can occur to any model, brand televisions, which uses picture tube.  Can take these as a reference.