Repair of SONY CRT TV KV-B21L1 for no sound problem, the picture is OK

I want to repair my Sony KV-B21L1 color CRT TV. Its picture is OK but sound vanishes few minutes after power on. I have changed the Tuner at Antenna input but no result. I also need schematic circuit diagram to study the sound circuit also.If any body can give tips or solution please suggest.

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