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12
Refrigerator Service Manual
NX6 / NX8 / NXA Models
Theory of Operation - cont’d.
Theory of Operation - Manual DC Mode
(3-way models only)
The power board measures the thermistor value and reports
it to the optical display board. The thermistor, a temperature
device whose internal resistance goes down as the
temperature goes up, is connected to the power board via
terminals P2-1 and P2-6. The optical display board compares
the actual thermistor value to preset cut-in and cut-out
temperature values for each of the particular temperature
settings. Should the thermistor value fall below a particular cut-
in value the optical display will request the DC board to turn
ON the DC heater relay (K1) on the DC board.
Should the thermistor value go above a particular cut-out value
it will request the DC board to turn OFF the DC heater relay.
Any time relay K1 of the DC board is turned ON, DC voltage
passes through the 30A, automotive style, SAE J1284 fuse (F1
on the DC board), through the now closed K1-contacts, and
on out to the DC heater via DC board terminals DC_HTR and
HTR_GND. Any time DC voltage is applied to the DC heater,
a DC current is produced. This DC current is measured by
the DC board and its value is passed on to the optical display
board. The optical display board determines if the DC current
is or is not within the speci
fi
ed limitations. Should the current
fall below the speci
fi
ed limits the optical display board will
display the appropriate DC current related fault.
•
NX64.3 / NX84.3 = "dc HE" with audible alarm/
beeper
•
NXA64.3 / NXA84.3 = "dc HE" with audible alarm/
beeper
Auto Mode
To operate in the Auto mode the optical display must be
placed to operate in this mode. Once done so, 2-way model
refrigerators will cool via the Auto AC mode or Auto LP Gas
mode and (3-way models) via the Auto AC mode, Auto LP
Gas mode, or the Auto DC mode. While in the Auto mode the
refrigerators electronic controls will function as follows:
The power board is responsible for:
•
Measuring the AC input voltage
•
Turning ON/OFF the AC heater output
•
Measuring the AC heater current
•
Turning ON/OFF the gas valve output
•
Turning ON/OFF the igniter output
•
Determining the presence of a
fl
ame
•
Communicating with the Optical Display Board
•
Communicating with the DC Board (3-way models
only)
The optical display board is responsible for:
•
Determining if the AC input voltage is available or
not available
•
Determining if the AC heater current is acceptable
or not acceptable
•
Determining if the thermistor value is above or
below preset temperature range cut-in/cut-out
values
•
Determining when the power board is to turn ON/
OFF the AC heater output
•
Determining when to turn ON and OFF the gas
valve
•
Determining when to turn ON/OFF the DC heater
output
•
Communicating with the Power Board
•
Communicating with the DC Board (3-way models
only)
The DC board (3-way models only) is responsible for:
•
Turning ON/OFF the DC heater output
•
Measuring the DC heater current
•
Communicating with the Optical Display Board
•
Communicating with the Power Board
Theory of Operation -
Auto Modes
AC voltage may or may not be applied to the power board
via the AC power cord at terminals L1 (Hot) and L2 (Neutral).
This AC voltage will pass through the AGC Series, 8A, Fast
Acting, Glass Tube Fuse (F2) where it is then measured. This
measurement is communicated to the optical display board
which determines if the applied voltage is below or above
85VAC. If the voltage is above 85VAC the determination is
that AC voltage is available and the optical display board will
request the power board to operate via the Auto AC Mode.
If the voltage is below 85VAC the determination is that AC
voltage is NOT available and the optical display board will
automatically default to the Auto LP Gas Mode and request
the power board to operate via the Auto LP Gas Mode (see
below). If the control is not able to establish a
fl
ame while
operating in the Auto LP Gas Mode the optical display board
will:
2-WAY MODELS:
Automatically default back to the Auto AC mode and wait for
AC voltage to return, displaying the following fault code
•
NX61 / NX81 Models: Solid RED power indicator
light
•
NX64 / NX84 Models: "no AC" "no FL" with audible
alarm/beeper
•
NXA64 / NXA84 Models: "no AC" "no FL" with
audible alarm/beeper
3-WAY MODELS:
Automatically default to the Auto DC mode and request the DC
board to operate via the Auto DC mode.
