Repair manual
MT3255 and MT3270
3-11
MT3270 WITH STANDARD CONTROLS - 21575 REV E
NOTE: The following is also applicable to the
MT3255 with 3 blower motors.
COMPONENT REFERENCE
NOTE: Refer to FIGURE 8 page 3-23 for compoĆ
nent locations.
1. BLOWER SWITCH (M0153)
2. TEMPERATURE CONTROLLER (M3149)
3. MOTOR CONTACTOR A & B (M0708)
4. THERMOCOUPLES (Dual lead - M3151,
Single - M3152)
5. HI/LO LIMIT BOARD (M3150)
6. CONVECTION FANS (Clockwise - M4224,
CounterĆclockwise - M4225)
7. HEAT SWITCH (M0152)
8. COMBUSTION MOTOR (22132)
9. PRESSURE SWITCH (M0595)
10. 110/24 VAC TRANSFORMER (M2381)
11. PURGE RELAY (M2382)
12. CENTRIFUGAL SWITCH
13. IGNITION MODULE (M1054)
14. PILOT VALVE (LP - 22190, Natural - M5495)
15. MAIN VALVE (LP - 22190, Natural - M5495)
16. SPST THERMAL SWITCH (M1362)
17. SINGLE SOLENOID GAS VALVE (20287)
18. COOLING FANS (4Ć1/2" - M2469,
3Ć1/5" - 21134)
19. SPST THERMAL SWITCH (M0635)
20. CONVEYOR SWITCH (M0152)
21. TIME DISPLAY (M3146)
22. 10KW POTENTIOMETER (M3145)
23. D.C. SPEED CONTROL BOARD (M2379)
24. 130 VDC MOTOR (M2378)
25. #10 PICK UP (M3147)
OPERATION
1. Turn the blower switch (1) to ON. The N.O. conĆ
tacts close, the N.C. contacts open. 115 VAC
runs to terminal #1 of the temperature controlĆ
ler (2), both coils of the motor contactors (3)
and terminal #7 of the hi/lo limit board (5). TerĆ
minal #7 is an output. It remains powered after
the oven is shut down to keep the convection
fans (6) operating until the unit reaches
135-170F (57-77C) as sensed by the therĆ
mocouples (4).
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 (7) to ON. Power goes to
terminal #6 of the temperature controller (2)
and terminal #5 of the Hi/Lo limit board (5). A
circuit is made between terminals #5 and #6
of the Hi/Lo limit board. This switch opens if the
oven cavity temperature exceeds 600F
(316C) as sensed by the thermocouples. TerĆ
minal #6 of the Hi/Lo board is an output and
sends power to a pressure switch (9). 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 transĆ
former (10) and a contact on the purge relay
(11).
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 (8). When the combustion moĆ
tor reaches full speed, a centrifugal switch (12)
closes sending 24 VAC to the ignition module
(13). After the module's self diagnostics are
complete, the pilot valve (14) opens. When a
proof of flame is established, the main burner
valve (15) opens.
3. Output from the temperature controller (2)
goes from terminal #4 to a terminal of a SPDT
thermal switch (16). The switch toggles if the
temperature passing its face exceeds the ratĆ
ing on the back of the switch.