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A couple of thoughts regarding sound buzz on white characters:
1) fine tuning - tune towards lower frequency (softer picture) to move the video carrier up the IF slope. If this helps, it's an indication that the IF frequencies in the lettering are too strong compared to the video carrier, causing overmodulation. However, this may cause a different sound problem by moving the audio IF carrier down the other slope of the IF response.
2) If (1) indicates improvement is possible, an IF alignment adjustment to move the video carrier up the slope of the IF response curve may help, while keeping the audio IF carrier at the correct frequency and therefore at the correct place on the other side IF slope.
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