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MT3255 and MT3270
MT3270 CE --- M8296 REV A
NOTE: The following is also applicable to the
MT3255 with 3 blower motors.
COMPONENT REFERENCE
NOTE: Refer to FIGURE 12 page 3 ---27 for compo-
nent locations.
1. BLOWER SWITCH (M0153)
2. TEMPERATURE CONTROLLER (M3155)
3. MOTOR CONTACTOR (M2247)
4. SPST RELAY/HOOD INTERLOCK(16998)
5. HI/LO LIMIT BOARD (M3150)
6. CONVECTION FANS (Clockwise --- M4597,
Counter-clockwise --- M4598)
7. THERMOCOUPLES (Dual lead --- M3151,
Single --- M3152)
8. HEAT SWITCH (M0152)
9. CONVECTION PRESSURE SWITCH (M0595)
10. COMBUSTION MOTOR (M2386)
11. BURNER PRESSURE SWITCH (M3330)
12. TPDT RELAY/LATCHING RELAY (M6519)
13. 10 SECOND PURGE TIMER (M3173)
14. SPST RELAY (16775)
15. SPDT THERMAL SWITCH (M2453)
16. IGNITION CONTROL MODULE/
LANDIS & GYR (M3168)
17. 2 SECOND PURGE TIMER (M3172)
18. PILOT VALVE (LP --- M6001, Natural --- M6000)
19. MAIN VALVE (LP --- M6001, Natural --- M6000)
20. IGNITION ALARM LIGHT (16037)
21. COOLING FANS (4-1/2" --- 23034,
3-1/2" --- 21430)
22. SPDT THERMAL SWITCH (M2453)
23. CONVEYOR SWITCH (M0152)
24. TIME DISPLAY (M3154)
25. 10KΩ POTENTIOMETER (M3145)
26. D.C. SPEED CONTROL BOARD (M3153)
27. 180 VDC MOTOR (M3128)
28. #10 HALL EFFECT PICKUP (M3147)
29. MOMENTARY SWITCH (M2497)
30. BUZZER
31. INDICATOR LAMP (16037)
32. TPDT RELAY (90250)
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 (3) and terminal #7 of the hi/lo limit board
(5). Terminal #7 is an output. It remains pow-
ered after the oven is shut down to keep the
convection fans (6) operating until the unit
reaches 135---170_F (57---77_C) as sensed by
the thermocouples (7).
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 (5). A
switch is made between terminals #5 and #6
of the Hi/Lo 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 convection pres-
sure switch (9). The switch reacts from a vacu-
um created by the convection fans. If the
switch is closed, power runs to the combustion
motor (10), the common terminal of the burner
pressure switch (11), and terminal #7 of a
TPDT relay (12).
NOTE: This relay acts as a latching relay and
remains powered up even after the
burner pressure switch changes state.
The burner pressure switch should be made
between common and N.C., sending power to
terminal #4 and the coil of the latching relay.
3. When the combustion motor (10) reaches full
speed, the burner pressure switch (11) toggles
between common and N.C. to common and
N.O. Power goes to terminal #9 of the latching
relay (12). This relay is latched due to voltage
passing from terminal #7 through a set of
closed contacts to terminal #4 to its coil. A set
of contacts are also closed between terminals
#9 and #6 of the same relay, sending power
to a 10 second purge timer (13). When the tim-
3---19
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