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I think is matter of color killer signal that is normally activated when the input signal is too low. If your input signal is good, you can check the circuits coming to/from an IC called chroma processor (the actual part number depends from the TV model) where the color killer is generate. Of course check the IC as well
Many things can cause the loss of colour, it could be the big IC but there are more likely reasons. The trouble is you need test equipment to track the fault, in particular you need an oscilloscope to check the signal paths and ensure the ident signal is being correctly generated. Do you have a schematic and oscilloscope?
OK, your TV is a single chip receiver based on LA76931. I don't know that, however I could suggest you to check all the components connected to pin 53 (chroma APC filter). However I agree with betwixt: you need schematic and oscilloscope.
Thanks to all experts !!!!!
My Dear Experts, I solved the problem by changing the crystal near by the BIG IC, then the colour was came back...... Thanks for your valuable help in this topic........Thanks once again......!!!!!!!!!