Kyocera KM-1820 Service Manual page 180

2GM
The AC power for the heater is applied in one of the five variations of the zero cross switchings as shown in Figure 2-3-7.
Each variation is constituted with the unit of ten positive and negative envelops in five cycles, as obtained by varying the
duration during which TRC1 turns on. The heater lamp is energized while TRC1 is kept on; the heater lamp is turned off
while TRC1 is kept off. For example, the duty cycle (the period of a cycle during which the heater lamp is turned on) is
maximum for variation No.1 as the heater lamp is energized for the whole envelops. The duty cycle is 60 % for variation No.3
as the heater lamp is energized for the 6 positive and negative envelops out of 10. The duty cycle is 0 since the heater lamp
is kept turned off for the whole envelops.
CPU (U1) selectively switches among those variations for applying voltages to the heater lamp according to the THERMA
signal which appears at pin #98 as feedback.
A fraction of THERMA is applied to pin #2 of comparator U6-1. The comparator maintains comparison of the potential at pin
#2 and pin #3 which gives a reference for the possible anomaly in the heater temperature (bred by resistors R52 and R57).
Should the voltage at pin #2 exceed that at pin #98, the level at pin #1 becomes low. Since pin #1 is wired to the output line
for the HEAT signal, the HEAT signal is enforced to be low regardless the behavior of CPU (U1), thus preventing possible
heat overrun.
2-3-11