Recently i repaired a NEC TV 21 CRT and this is the story that how i have fixed this issue.
The sound of this tv was not stable. Sometime good and other time not. After i opened the tv first i checked the speaker (tv is off) the label of speaker indicated that the speaker must be 8 ohm. My ohmmeter confirmed this, when the sound of tv got problem and speaker is ok the most problematic part could be the sound ic amplifier. Since i did not find any diagram for this i asked which ic is sound ic?
By following the speaker wires back ward from pins of speaker to main pcb and following the circuit on the pcb lines i directed to IC TDA 7245… which must be checked?
By searching Google, i found that this is sound amplifier ic. And by diagram and data
from google i found out that the pin 3 of ic is vcc and pin 2 is sound output. Supply voltage is between 14-30 volt dc. I turn on the tv and by measuring the voltage
of pin 3 my volt meter confirmed that the voltage is good enough. And then my scope confirmed that the ic does not have any output . By changing the ic with a good one the sound return to normal.