Fluke PM6690 Service Manual page 79

Fan Control
The fan is connected to +15 VU over a speed control circuit. It
is only ON if a control signal from the processor is present.
The first 8 minutes after power-up the fan will run at a fixed
speed, fed with +8.3 V. After that the fan is temperature con-
trolled. The processor reads the temperature from U39 via the
2
I C bus. Depending on the temperature, the fan is fed with a
DC voltage between 8 and 13.5 V. The processor uses a PWM
signal that is filtered to control the fan.
Figure 6-58
Important power supply locations #2.
Figure 6-57
Important power supply locations #1.
Troubleshooting 6-35