Teac CTM6845, Service Manual

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② Main points in color cast and retrace fault repair in 8838/8842 series color TV
The traditional method to repair a color cast fault in color TV is to adjust the saturation,
then observe if the picture turns to black and white after cutting off the saturation. If so, the
fault is in the video amplification circuit or there is color cast in the CRT. With a color TV set
with automatic white balance adjustment function, if the color cast remains after turning off
the saturation, the check should be focused on the elements of video amplification circuit and
the associated elements of 8838 main chips.
In case of a retrace fault, first analyze the cause of the fault according to the color of the
retrace horizontal appearing. For example, if it is too blue, the video amplification circuit
should be checked to see if the video amplification output voltage is too low, or any element
such as the video amplification transistor is damaged. The video amplification circuit element
for the color with problem should be checked. In most cases, it is damaged video
amplification circuit element that lead to retrace and color cast, only in extreme few cases the
CRT is damaged. In case of retrace fault without color cast, it is usually due to abnormality in
the supply system for video amplification circuit and CRT acceleration pole voltage. Often
open R215 or D490 and damaged D407 will result in retrace fault. Of course, sometimes the
transistors in the video amplification circuit will damage at the same time.
2 . Circuit analysis and maintenance for “three no’s” fault
The fault of “no picture, no sound and no power supply indication” (hereinafter referred
to as “three no’s” fault) is a fault with high probability of occurrence in TDA8842/OM8838
series chassis. The following faults can be repaired using the methods described below:
①
Indicator is not ON. The fault is mainly in the power supply circuit section.
In this circuit, there are the following main voltage power supply systems:
●
The power transformer T801 outputs the following voltage:
a. +B (main power supply)
b. +24V (power supply to horizontal drive and pillow distortion equalizing)
7805 +5V (supply to CPU, indicator and remote control
head)
c. +15V V809 +12V (12V supply to tuner)
7805 (5V supply to tuner)
7808 (supply to N501, N001)
d.
+16V (sound power supply)
● Horizontal output transformer T402 outputs the following voltage:
a. +16V (vertical power supply)
b. +45V (vertical power supply)
c. +180V (video amplification voltage)
②
The indicator is ON and the power supply output voltage is normal. The fault is mainly in
the horizontal section, vertical section and video amplification section as follows:
In the fault automatic protection circuit in an ordinary color TV set, normally some
external sampling circuits triggers a testing port of the thyristor or microprocessor, and after
logic operation and determination by the microprocessor, a cut-off (waiting) signal level is
output from a specific pin position of the microprocessor, to cut off the switching stabilized
power B voltage or horizontal scan triggering signal power supply output, to realize
automatic protection cut-off for the main circuit of the color TV set.