Enter Calibration Mode; Calibration Procedure - Welch Allyn 4802D Service Manual

1. Unplug the scale from the AC line.
2. Unplug the cradle from the scale.
3. Detach the top cover of the scale's enclosure by removing the eight (8) #10 button head screws.
4. The scale's instrument board should now be in full view. Note the location of the trimmer
potentiometer, "P1 Span".

ENTER CALIBRATION MODE

Enter the "calibration mode" by following exactly the procedure outlined below:
1. Be sure scale is off. The scale should be placed on a flat, level surface and the cradle must be removed.
2. Press and hold the "REWEIGH" pushbutton.
3. While pressing the "REWEIGH" pushbutton press and release the "ON / OFF / ZERO" pushbutton.
4. The " Send Data" pushbutton is a special hidden programming and test pushbutton located under the
SCALE-TRONIX® logo "RECALL PRIOR WEIGHT" pushbutton. After the scale displays the test pattern of
"888888" release the "REWEIGH" button and press the "Send Data" pushbutton five (5) times. This will
cause the readout to enter the calibration mode. The display will indicate "CAL".
5. Press the ("Send Data") pushbutton once more; the display will indicate "A-d". This indicates the start of
the "raw" analog-to-digital converter data being inputted to the microprocessor.
6. Press the " Send Data" pushbutton one more time. The number displayed is now the "raw" analog to
digital data. Each "count" of the display is equivalent to 1 gram.
7. The automatic turn-off timer has also been programmed for an extended "on" period to give you time to
calibrate the scale. This time period is three minutes. The scale may be turned off before this time period
by simply pressing and holding the "ON / (OFF) / ZERO" pushbutton. Hold it in for several seconds until
the power shuts off. If additional time is needed to complete the calibration procedure, press the "ON /
(OFF) / ZERO" pushbutton briefly. This will reset the timer for an additional three minutes. (This does not
apply if the scale is being powered by an external DC power supply, since it will remain on continuously.)
The readout is now displaying a number, which represents the zero offset value of the platform and load
cell transducers, in kilograms (0.001 kilogram = 1 (gram).

CALIBRATION PROCEDURE

1. Momentarily press the "ON / (OFF) / ZERO" pushbutton to reset the internal self-test timer. Reattach
the scale's cradle. Note the currently displayed weight reading on the front display. Record this zero
offset reading.
2. Lay the two (2) 10-kilogram test weights side by-side in the center of the cradle. Note the new
number displayed. Subtract the original zero offset value from this new number to obtain the scale's
displayed value of the calibration weight.
Example: The zero offset value is "-1.230 KILOS" (representing -1.230 Kilograms). Adding the specified
two 10-kilogram test weights (equivalent to 20.000 kilograms) to the cradle produces a reading of
"18.763 KILOS". The difference is 18.763 - (-1.230) = 19.993 (equivalent to 19.993 kilograms). This
would indicate the calibration is 20.000 - 19.993 = 0.007 kilograms "low". Using the specified two 10-
kilogram test weights a difference of 20.000 kilograms +/-0.003 kilograms (3 grams) should be
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