Ricoh FT7770 Service Manual page 59

3. Drum Heater Circuit
The main board monitors the temperature of the drum surface through TH-1.
When the main board determines that the drum heater should be on, CN301-
B15 goes LOW. This action energizes SSR10 on the ac drive board and 100
volts ac is applied to the drum heater.
As shown by the illustration, the drum heater turns on when the drum
temperature drops below 32°C and turns off when it goes over 33°C.
A thermostat inside the heater turns the heater off if the internal heater
temperature goes over 82°C.
the TH-1 overheat protection circuit (SC 4) fails to work properly.
To limit power consumption, the drum heater is held off during the copy
cycle.
This guards against extreme overheat in case
2-3
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April, '89