MT3255 and MT3270
3 --- 9
MT-70- PH 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 compo-
nent locations.
1. BLOWER SWITCH (M0153)
2. TEMPERATURE CONTROLLER (M3155)
3. SPST RELAY (16988)
4. MOTOR CONTACTORS A & B (M0708)
5. CONVECTION FANS (Clockwise --- M 4597,
Counter-clockwise --- M4598)
6. THERMOCOUPLES (Dual lead --- M3151,
Single --- M3152)
7. HI/LO LIMIT BOARD (M 3150)
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 THERM AL SWITCH (M2453)
19. COOLING FANS (4-1/2” --- 23034,
3-1/5” --- 21430)
20. MOMENTARY SWITCH (M2497)
21. 240 VOLT RELAY (90250)
22. BUZZER
23. CONVEYOR SWITCH (M0152)
24. 10kΩ 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. con-
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 contac-
tors (4) a nd terminal #7 of the hi/lo limit board
(7). Terminal #7 is an output. It remains pow-
ered after the oven is shut down to keep the
convection fans (5) operating until the unit
reaches 135---170_F (57---77_C) as sensed by
the thermocouples (6).
NOTE: Two thermocouples are located be-
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 600_F
(316_C). 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 sw itch is closed,
power runs to t he primary side of a 220/24 VAC
transformer (12) and a contact on t he purge
relay (13).
NOTE: These components are located in a
box mounted on top of the combustion
motor.
When24VACisappliedtothisrelay,thecon-
tacts close sending 220 VAC to start the com -
bustion motor (14). The relay also s ends pow-
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
ofaSPDTthermalswitch(9).Theswitch
toggles if the temperature passing its face ex-
ceeds the rating on the back of the switch.
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