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.
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:
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:
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.
