(5) Triac drive circuitry. The triacs require a pulse at the zero crossing of
each half cycle of the power line. The triacs are opto-coupled to the logic, as described
in the power board section. U9 generates the opto drive pulses. These are open
collector NOR gates. Because the optos, and their associated indicator leds, DS1 and
DS2 (for the developer and dryer), are capacitive coupled, the circuit will not function if
the zero crossing pulses are missing or defective.
(a) In order for the optos to be triggered, all inputs to U12A or B must
be low. Thus, both zero crossing pulses must be low, which will occur at the true zero
(b) The respective comparator output must be low and the thermistor
must be connected.
(c) At each zero crossing, U12A or B generates a pulse. This is
inverted by U13A or B, which is then inverted by U9B or A. In addition, the solution
level signal must be satisfied, and be low at U9-2.
(d) When power is called for, DS1 or DS2 will be lit. If the opto-isolator
is removed or its led is open, the indicators will not go on. These indicators are key
(6) Blower control circuit. A circuit is included to prevent the dryer blower
from operating during the developer warm-up period.
(a) A latching circuit is made from the heavy positive feedback around
U13. Initially the output of U13 is low.
(b) This, in turn, pulls the base of the PNP transistor Q-2 low, causing
Q-2 to conduct.
(c) The fan relay, R-3 on the power board, is normally closed.
Energizing the coil turns off the fan.
(d) When the developer reaches temperature, U8B-7 goes high,
causing the latch formed by U13D to go high, and turn off relay R-3. The fans will then
(7) Dryer standby. When the unit goes into standby, the dryer temperature
is reduced to the extent possible to insure recovery when a film is fed. Relay K-1 is
opened, reducing the reference current to the thermistor circuit by removing R30 from
its parallel combination with R29.