Theory of Operation
Fan Control
There are multiple temperature sensors in different locations in the instrument,
including some mounted on the Option boards. The control processor reads the
temperature
sensors and sets a target speed for the fan. The fan circuit holds the
fan speed at the target by measuring the fan tachometer output, allowing reliable
operation at low speed. If the fan is not turning, the circuit senses the stall and
turns on a red LED (DS861 on the Main board). The fan is tested at power up. If
it fails, a message is shown on screen and also entered into the diagnostic log.
Power Sup
ply and Distribution
The power supply has a universal AC input, enabling it to accommodate 100 to
264 VAC without any user range switching. The output is 5 VDC. Circuits convert
this out
put to any other supply voltage that they require at the point of usage.
Fuses on the Primary supply 5 V output protect the main board.
The secondary supplies and their tolerances are specified in the troubleshooting
section. The location of the supply test points is shown in the Maintenance section.
2–10 WFM6120, WFM7020, and WFM7120 Waveform Monitors Service Manual
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