Ricoh VT3500, Service Manual

Page: 51 / 330

[Lamp Heater]
The thermistor [C] mounted in the lamp heater maintains the lamp
temperature at about 40°C.
If the lamp temperature drops too low, the voltage at [B] goes High. This is
because the resistance of thermistor [C] increases, causing the voltage at
IC-pin 7 (operational amplifier) to go High. Q602 then turns on and the lamp
heater [D] is energized. If the lamp temperature rises, the voltage at [B]
becomes less than that at IC-pin 6. This causes IC-pin 7 to go Low, which
turns off the lamp heater.
Thermistor TH601 in the A/D conversion PCB monitors the temperature
inside the machine. If the temperature is low, the increased resistance of
TH601 drops the voltage at [A] and the control temperature of the heater
thermistor (heater ON/OFF temperature) is raised slightly. If the temperature
is high, the control temperature is lowered slightly.
If Q605 turns on, the voltage at [B] becomes 0 volt and the lamp heater turns
off. Q605 is turned on when the original scanning signal (active low) is sent
from the main PCB. Consequently, the heater is always off during the original
scanning process.
CN601
-A13
24K
110K
+5V
4.3K 10K
TH601
+
_
18K
5
6
7
Q605
Q602
+24V
CN608
-1
-2
-3
-4
IC
Fluorescent
Lamp
A/D Conversion PCB
Main PCB
Original
Scanning
Signal
Lamp
Heater
[A]
[B]
[C]
[D]
1 December 1993
2-15