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Hum may be regarded as an unwanted
signal and may be analyzed
and located by signal-tracing methods.
In the ordinary receiver operating on a.c., the hum frequencies may be either 60 or 120 cycles. The oscilloscope is an important aid in determining the frequency. Line frequency (60 cycles) may be introduced through direct pickup by a grid circuit, through leakage in a tube, or by reason of emission from a heater. Ripple frequency (120 cycles) occurs in the "B"-supply. It is strong at W (Fig. 4), negligible at V, and practically zero at the output side of any filter beyond this point. Screen-grid circuits should be exceptionally well filtered since a hum on this grid will be amplified by the tube. By means of signal tracing, hum levels can be compared and the source of excessive hum may be quickly located. |
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