Chapter 8 431
IF Section
Log Amplifier Assembly (P/O A4)
Video MUX
The video MUX switches the video output between linear, log and 4.8
kHz IF (for digital RBWs). The demod video is an unused feature. The
easiest way to troubleshoot this circuit is to look for blown FETs. Bad
FETs are characterized by having significant gate current. Only one of
the signal lines LIN_VIDEO, IF_VIDEO or LOG_VIDEO should be
high (+15 V) at any given time. The others should be low (−15 V). Also
look for a voltage drop of several volts across the gate resistors R601,
R605, R609, or R613 when in either the off or on state. This indicates
gate current and thus a bad FET.
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