Repair manual
MT3255 and MT3270
3-9
MTĆ70ĆPH GENERAL EXPORT - M2501 REV D
NOTE: The following is also applicable to the
MT3255 with 3 blower motors.
COMPONENT REFERENCE
NOTE: Refer to FIGURE 7 page 3-22 for compoĆ
nent locations.
1. BLOWER SWITCH (M0153)
2. TEMPERATURE CONTROLLER (M3155)
3. SPST RELAY (16988)
4. MOTOR CONTACTORS A & B (M0708)
5. CONVECTION FANS (Clockwise - M4597,
CounterĆclockwise - M4598)
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 (M2245)
11. PRESSURE SWITCH (M0595)
12. 220/24 VAC TRANSFORMER (M2384)
13. PURGE RELAY (M2385)
14. COMBUSTION MOTOR (M2276)
15. IGNITION CONTROL (M1054)
16. PILOT VALVE (LP - 22190, Natural - M5495)
17. MAIN VALVE (LP - 22190, Natural - M5495)
18. SPDT THERMAL SWITCH (M2453)
19. COOLING FANS (4Ć1/2" - 23034,
3Ć1/5" - 21430)
20. MOMENTARY SWITCH (M2497)
21. 240 VOLT RELAY (90250)
22. BUZZER
23. CONVEYOR SWITCH (M0152)
24. 10kW POTENTIOMETER (M3145)
25. #10 PICKUP (M3147)
26. TIME DISPLAY (M3154)
27. D.C. MOTOR (M3128)
28. D.C. SPEED CONTROL BOARD (M3153)
29. INDICATOR LIGHT (16037)
OPERATION
1. Turn the blower switch (1) to ON. The N.O. conĆ
tacts close, the N.C. contacts open. 220 or 240
VAC runs to terminal #3 of the temperature
controller (2), both coils of the motor contacĆ
tors (4) and terminal #7 of the hi/lo limit board
(7). Terminal #7 is an output. It remains powĆ
ered 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
terminal #6 of the temperature controller (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 220/24 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 220 VAC to start the comĆ
bustion motor (14). The relay also sends powĆ
er to terminal #2 of the ignition control system
(15). After the module's self diagnostics are
complete, the pilot valve (16) opens. When a
proof of flame is established, the main burner
valve (17) opens.
3. Output from the temperature controller (2)
goes from terminal #4 to the common terminal
of a SPDT thermal switch (9). The switch
toggles if the temperature passing its face exĆ
ceeds the rating on the back of the switch.