Blodgett MT3255 Series Service And Repair Manual page 34

nal of a SPDT thermal switch (9). The switch
toggles if the temperature passing its face ex-
ceeds the rating on the back of the switch.
NOTE: The switch is located in the front con-
trol compartment. It protects the other
components from hi ambient heat.
If this switch is cold, it should be made be-
tween common and N.C. terminals sending
power to the single solenoid gas valve (10).
4. When power is applied to the coils of both mo-
tor contactors (4) the contacts close sending
power to the four convection fans (5) located
in the back of the oven. Power is also applied
to the coil of the SPST relay (3).
NOTE: The SPST relay acts as a hood inter-
lock and is sometimes used as a
means of starting the hood.
5. The oven has six cooling fans (19). Two on the
front control panel keep the panel below 140_F
(60_C). The fan's airflow is from left to right for
flow through ventilation. The other four, in the
rear of the oven, keep the convection fans from
overheating. The cooling fans start when the
motor contactor powers up and closes be-
tween terminals #13 and #14. Power goes to
the N.C. terminal of a SPDT thermal switch
(18). The switch toggles if the temperature
passing its face exceeds the rating on the back
of the switch and may start the fans even if the
oven is off. If this switch is cold, it should be
made between common and N.C. terminals
sending power to the cooling fans.
NOTE: The switch is located on the ceiling of
the convection fan compartment.
6. The conveyor is driven by an open loop D.C.
control system consisting of a conveyor switch
(23), time display (26), 10kΩ potentiometer
(24), D.C. speed control board (28), 130 VDC
motor (27) and #10 Hall effect pickup (25). Re-
fer to page 5---1 for pickup troubleshooting. Af-
ter the conveyor switch is turned on, the time
display illuminates. The D.C. control board
powers up. The output voltage measured on
terminals A1 and A2 of the board to the motor
varies from 20 to130 VDC based on the posi-
tion of the potentiometer. The speed of the mo-
tor should also vary. The time display varies de-
pending on the speed of the Hall effect pickup.
The pickup sends an R.P .M value to the dis-
play. The display converts this value to min-
utes:seconds.
NOTE: This type of system does not sense the
weight of the product and will slow
down slightly if the belt is fully loaded.
7. If the thermal switch (9) in the control panel
toggles due to high heat, the single solenoid
valve (10) closes. Power runs through a mo-
mentary switch (20) to the coil of a 115 volt
relay (21). When this relay closes, a buzzer (22)
sounds and an indicator lamp (29) lights, indi-
cating a control compartment high tempera-
ture. Pressing the momentary switch disen-
gages the relay, silencing the buzzer. The
indicator lamp remains lit until the temperature
drops 20_F across the face of the thermal
switch, allowing the burner to refire.
3---8
OPERATION