KX-MB2110RUW, KX-MB2130RUW, KX-MB2170RUW, KX-MB2117RUB, KX-MB2137RUB, KX-MB2177RUB - Panasonic Copying Equipment Service Manual (repair manual). Page 48

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KX-MB2110RUW/ KX-MB2130RUW/ KX-MB2170RUW/ KX-MB2117RUB/ KX-MB2137RUB/ KX-MB2177RUB

This unit is equipped with two FAN motors to prevent the developing devices, Low Voltage Power Supply board (SMPS Board) and
other devices from overheating during printing.
The FAN rotates at high speed (Approx. 3000rpm) while printing.
After printing is finished, FAN rotates at low speed (Approx.2200rpm) while predetermined period.

FAN motors are controlled by following circuit.

For the control of FAN speed, comparator IC (IC504) is used. 
This IC compares (+) side input level and (-) side input level.
If (+) side input level is bigger than (-) side input level, output of this IC is "OPEN".
If (+) side input level is less than (-) side input level, it outputs "L" level.

In half speed mode, IC300_pin K25 outputs pulse (frequency is about 30KHz, duty is about 37.5%).
This pulse is integrated by R644 and C618 then Pin2_IC504 becomes approx. DC1.24V. Input level of IC504_pin2 is determined by
the voltage of between R624.
If voltage between R624 is less than 1.24V, output of pin1_IC504 becomes "L".
Then Q603 is turned on.  So voltage between R624 (=the voltage of pin2_IC504) rises gradually.
When the voltage between R624 exceeds 1.24V, output of pin1_IC504 becomes "H".
Then Q603 is turned off.  So voltage between R624 falls gradually.
By repeating these sequences, voltage between R624 is controlled approx. 1.24V.
On the other hand, if FAN voltage is represented Vo and voltage between R624 is represented VL, VL is determined as below for-

mula.

VL=Vo*R624/(R623+R624) 

 Vo=VL*(R623+R624)/R624

Since each value is as follows, Vo is determined approx. 9.7(V).

VL=1.24(V), R623=150(Kohm), R624=22(Kohm)
Vo=1.24*(150K+22K)/22K=9.7(V)

Therefore by reducing the voltage of FAN power supply, FAN rotates with half speed.