Repair manual
MT3255 and MT3270
3-7
SEQUENCE OF OPERATION
NOTE: The following instructions represent the most common configurations. For questions regarding othĆ
er options call the Blodgett Service Department at (800)331Ć5842.
MTĆ70ĆPH DOMESTIC - M2468 REV E
NOTE: The following is also applicable to the
MT3255 with 3 blower motors.
COMPONENT REFERENCE
NOTE: Refer to FIGURE 6 page 3-21 for compoĆ
nent locations.
1. BLOWER SWITCH (M0153)
2. TEMPERATURE CONTROLLER (M3149)
3. SPST RELAY (16988)
4. MOTOR CONTACTORS A & B (M0708)
5. CONVECTION FANS (Clockwise - M4224,
CounterĆclockwise - M4225)
6. THERMOCOUPLES (Dual lead - M3151,
Single - M3152)
7. HI/LO LIMIT BOARD (M3150)
8. HEAT SWITCH (M0152)
9. SPDT THERMAL SWITCH (M2453)
10. SINGLE SOLENOID GAS VALVE (20287)
11. PRESSURE SWITCH (M0595)
12. 220/24 VAC TRANSFORMER (M2381)
13. PURGE RELAY (M2385)
14. COMBUSTION MOTOR (M0767)
15. CENTRIFUGAL SWITCH
16. IGNITION CONTROL (M1054)
17. PILOT VALVE (LP - 22190, Natural - M5495)
18. MAIN VALVE (LP - 22190, Natural - M5495)
19. SPDT THERMAL SWITCH (M2453)
20. COOLING FANS (4Ć1/2" - M2469,
3Ć1/5" - 21134)
21. MOMENTARY SWITCH (M2497)
22. 120 VOLT RELAY (16241)
23. BUZZER
24. CONVEYOR SWITCH (M0152)
25. 10kW POTENTIOMETER (M3145)
26. #10 PICKUP (M3147)
27. TIME DISPLAY (M3146)
28. D.C. MOTOR (M2378)
29. D.C. SPEED CONTROL BOARD (M2379)
30. INDICATOR LIGHT (M0791)
OPERATION
1. Turn the blower switch (1) to ON. The N.O. conĆ
tacts close, the N.C. contacts open. 115 VAC
runs to L1 of the temperature controller (2),
both coils of the motor contactors (4) and terĆ
minal #7 of the hi/lo limit board (7). Terminal
#7 is an output. It remains powered after the
oven is shut down to keep the convection fans
(5) operating until the unit reaches 135-170F
(57-77C) as sensed by the thermocouples
(6).
NOTE: Two thermocouples are located beĆ
tween the middle convection fans in
the rear of the oven. One thermocouple
provides DC millivolts to the Hi/Lo limit
board. The other provides DC milivolts
to the temperature controller. Check
thermocouples with a millivolt meter.
2. Turn the heat switch (8) to ON. Power goes to
the common terminal of the temperature conĆ
troller (2) and terminal #5 of the Hi/Lo limit
board (7). A switch is made between terminals
#5 and #6 of the Hi/Lo limit board. This switch
opens if the oven cavity temperature exceeds
600F (316C). Terminal #6 of the Hi/Lo board
is an output and sends power to a pressure
switch (11). The switch reacts from a vacuum
created by the convection fans. If the switch is
closed, power runs to the primary side of a
115/24 VAC transformer (12) and a contact on
the purge relay (13).
NOTE: These components are located in a
box mounted on top of the combustion
motor.
When 24 VAC is applied to this relay, the conĆ
tacts close sending 115 VAC to start the comĆ
bustion motor (14). When the combustion moĆ
tor reaches full speed, a centrifugal switch (15)
closes sending 24 VAC to the ignition module
(16). After the module's self diagnostics are
complete, the pilot valve (17) opens. When a
proof of flame is established, the main burner
valve (18) opens.
3. Output from the temperature controller (2)
goes from N.O. terminal to the common termiĆ