4-16
4.7
HUM MEASUREMENT
Hum is undesirable modulation of the television video carrier by
power line frequencies and harmonics (e.g., 50, 60, 100, 120 Hz), or
other low frequency disturbances (FCC limit: < 3%). When using
Hum Measurement Mode, the icons at the left of the display screen
show the hum test frequencies. When you press the related hum
frequency soft icon, the left side of the icon darkens. The dB/% icon
toggles the displayed value between dB and percentage.
When there is an undetectable level of hum the following message
appears on the screen:
ERROR
INSUFFICIENT SIGNAL Level TO
PERFORM THE MEASUREMENT
Figure 4-7: HUM Measurement
To measure Hum, simply press the HUM Mode key when tuned to
any non-scrambled channel.
Soft keys allow you to select 50, 60, 100, 120 Hz, or <1,000 Hz filters
for this measurement. A 60 Hz modulation component suggests a
possible corroded connector, a 120 Hz component tends to indicate
a possible failure related to the DC supply in the amplifier--possibly a
capacitor failing and thus causing ripple to increase.