Oki ML390 Service Manual page 48

Overheat Condition
The printhead contains a built-in thermistor (rated at 130 degrees Celsius). The thermistor monitors the
printhead temperature.
Printhead temperature rises when the unit is printing. When the temperature of the printhead rises, the
resistance of the thermistor decreases and the potential falls at the negative input of comparator Q7. Q7 is
located on the main controller board.
When printhead temperature reaches approximately 90 degrees Celsius, this comparator output inverts,
causing the HEAD TEMP signal to be sent to the motor control LSI. The unit prints unidirectionally.
When printhead temperature reaches approximately 130 degrees Celsius, the thermistor sends a signal to
the main control board, generating the HEAD OVERHEAT alarm. Printing stops when this alarm is
detected (A BUSY signal is sent to the host computer. Data remains in the print buffer as long as power is
supplied to the printer). This allows the printhead temperature to fall. When the printhead temperature falls
below the alarm detection level, printing is resumed (the BUSY signal to the host computer becomes
inactive).
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