Midco J121-DS - Manual (Page 10)

Cleaning of the blower wheel is usually the only service required. Need for
cleaning is required if the inlet screen or blower wheel shows an accumulation of dust
and lint. The motor air cooling vents should also be cleaned at this time.
If the motor must be replaced, disconnect the motor wires from the burner terminal
strip. Remove the blower inlet screen and blower wheel and remove the motor case
bolt nuts and lock washers.
When remounting the blower wheel, the distance from the wheel and the outside
of the blower housing air inlet side plate should be 5/16".
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The Thermal Switch is a heat sensitive single pole double throw snap switch that
energizes the blower motor to provide cooling air if, because of a heat back up
through the burner when the motor is off, the temperature of the blower housing
reaches 160° F. The thermal switch is not wired into the gas valve circuit and does
not turn valve off when activated by heat back up.
The switch mechanism is a bi-metallic disc that snaps over when it is heated to the
set temperature. The switch is mounted so that the disc bears against the side plate
of the blower housing.
Testing the switch to prove that it is functional can be done by removing the ignitor
assembly and blowing hot air through the opening toward the blower inlet. If hot air
is not available, remove the switch and check for continuity. With the switch cool,
there should be continuity between terminals 1 (black wire) and 3 (red/black wire). To
simulate the “hot” condition, place the switch on a firm surface and apply finger
pressure to the switch disc. When the disc “snaps”, there should be continuity
between terminals 1 and 2 (red wire).
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The Electronic Control is a 24 volt AC, solid state electronic device that
automatically ignites and monitors the flame. It has an integral high voltage
transformer and, upon flame startup, applies high voltage to the spark electrode and
24V to the Main Automatic Gas Valve. When the flame is proven, the spark is
terminated and the burner run continues.
If the flame is not proven within 6 seconds, or if the proof of flame is lost during the
burner run and not re-established within 6 seconds, the Control will shut off the Main
Automatic Gas Valve and LOCK OUT. To reset the Control for restart, turn off the
Burner Switch or Timer for at least 30 seconds and then turn back on.
10
Trouble Chart
Figure 4 General Burner Assembly
Part 2
Service
Continued
XI Blower Assembly
Continued
XII Thermal Switch
XIII Electronic Control System