E
KERN & Sohn GmbH
D-72322 Balingen-Frommern
Postbox 4052
E-Mail: info@kern-sohn.de
Service Manual
Electronic Precision Balances
Tel: 0049-[0]7433- 9933-0
Fax: 0049-[0]7433-9933-149
Internet: www.kern-sohn.de
Page 2
KERN 440-33 /-45/-47
1 PCB
Version 2.0
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E
KERN 440-33 /-45 /-47
Version 2.0 / 1 PCB
Service Manual
Electronic Precision Balances
1 INTRODUCTION.....................................................................................................3
1.1 Features ...........................................................................................................3
1.2 Digital auto calibration procedure .................................................................3
1.3 Internal calibration procedure........................................................................4
1.4 Display segment test ......................................................................................4
2 FUNCTIONAL BLOCK DIAGRAM / DESCRIPTION.............................................. 5
2.1 Functional block diagram............................................................................... 5
2.2 Function description....................................................................................... 6
3 TROUBLESHOOTING............................................................................................ 7
4 REPLACEMENT OF PCBA .................................................................................... 8
5 REPLACEMENT OF LOADCELL ASSEMBLY......................................................9
6 SCHEMATICS.......................................................................................................10
7 COMPONENTS LAYOUT..................................................................................... 11
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1. Introduction
1.1 Features :
Full Tare
•
Memory function with indicator
•
Stable indicator
•
Negative value indication
•
Two types of Digital Auto Calibration
•
Solder pads to prevent end-user internal calibration
•
Low battery indicator
•
Auto off function (battery mode only)
•
AC adaptable
•
Display segment test function
•
Overload protection
•
1.2 Digital Auto Calibration Procedure :
Turn balance on and allow the unit to acclimatize and stabilize for two minutes.
1.
Press and hold the [CAL/MODE] key until the display shows a flashing weight
2.
reading. The flashing weight reading indicates the correct weight that must be
placed on the tray.
With the display still flashing, press the off key. The display will now show [CAL 0].
3.
Then gently place the correct calibration weight on the center on the tray.
The display will now show [Cal F] and then return to normal operating mode. The
4.
calibration is complete when the display correctly shows the weight that is placed on
the tray and the display has stabilized.
Remove the calibration weight and press the [TARE] key to reset the zero point.
5.
In case [CAL E] is displayed instead of [CAL F], this indicates a calibration
procedure error. Turn the scale off and then on and repeat the procedure
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1.3 Internal Calibration Procedure :
Remove the top housing of the scale.
1.
Connect J1 by soldering the pads together.
2.
With tray top installed, turn balance on. Display shall show zero with the unit
3.
indicator on.
Press [CAL/MODE] key once. Display shall show the internal count. The reading
4.
shall fall in the range from 9000 to 12000. In case out of this range, connect J10 left
side or right side to reduce or increase the reading. Connecting or disconnecting
pads on J7, J8, or J9 can fine tune the reading.
Press [OFF] key once. Display will show [CAL 0] and then the required calibration
5.
weight. Place the corresponding calibration weight on the center of the tray.
Display will now show [CAL L] and then the next calibration weight. Place the
6.
corresponding calibration weight on the tray. Repeat this step until display shows
[CAL F].
Disconnect J1. Reset the zero point and check the accuracy at different weight.
7.
Install the top housing of scale.
8.
(refer to drawing : component layout)
In case [CAL E] is display instead of [CAL F], this indicates a calibration procedure error,
or wrong weight applied for calibration. Turn the scale off and then on and repeat the
procedure.
1.4 Display Segment Test :
Whenever the scale is power on, all segments of the LCD will be turn on for 2 seconds.
Check for any missing segment.
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2.
Functional Block Diagram / Description
2.1 Functional Block Diagram :
Keyboard
L C D
9V Battery
or
Adaptor
Power control /
Regulator &
Low battery
detector
Microprocessor
digital signal
analog
signal
Load cell
(Sensor)
Amplifier
A/D Converter
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2.2 Function Description :
1. Load cell
This is the heart of the whole system. The load cell itself is arranged as a bridge.
The resistance change of the bridge elements is proportional to the load applied on
the load cell. Therefore, the output of the load cell is an analog signal, which is
proportional to the load applied on the scale.
Amplifier
2.
The analog signal from the load cell is very small, of the order of micro-volt. Hence,
a linear and stable amplifier is applied to amplify the analog signal to an appropriate
level.
A/D Converter
3.
In order for the analog signal can be input to the microprocessor, this part converts
the analog signal to its digital equivalent. The operation of the analog to digital
converter is using a SIGMA DELTA technique and under the control of the
microprocessor.
Microprocessor
4.
The microprocessor control all the functions of the scale, such as auto zero, A/D
conversion, timings, weight calculation and display, parts counting, overload
indication, low battery indication, tare function, etc….
5. Display
This is the part where the weight is shown out on the LCD display in digital form.
The whole display is driven by the microprocessor.
6. Power Regulator and Low Battery Detector
This part contains the ON/OFF power control. In order for the external power can be
used by other parts of the scale, a regulator is used to regulate the supply. A low
battery detector is employed to make sure that the power supply is strong enough for
normal operation of the scale.
7. Keyboard
The keyboard provides on user interface. [ON/MEMORY], [CAL/MODE], [TARE]
and [OFF] keys are employed to operate the scale.
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3. Trouble Shooting :
Power on
↓
Full Segments?
If no display, check battery /adaptor; connection between keyboard—main
board, battery/adaptor—main board.
If missing segments, check fixing of LCD frame, zebra connector under LCD.
↓
Display Zero?
If display [LO], check battery>7.5v, adaptor>9v.
If display [ E] , check internal count.
↓
Proper readout?
If unstable reading, check tray top, overload stopper, loadcell
↓
Correct reading?
If not accurate, perform internal calibration.
If cannot reach full capacity, check tray top, overload stopper, loadcell, and
internal zero point.
If always zero, check internal zero point. Internal calibration if necessary.
↓
Normal operation.
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4. To Replace PCBA
Disassemble top housing of scale.
1.
Disconnect PL1, PL2 and JP2 from the PCBA. Disassemble the ground wire screw.
2.
Repleace a new PCBA. Connect PL1 PL2 and JP2 again. Assemble the ground wire
with screw.
Assemble the top housing.
3.
Perform internal calibration as described in section 1.
4.
5. Check accuracy of scale at different weight.
440-33 440-45 440-47
weight(g) tolorance weight(g) tolorance weight(g) tolorance
50.00 0.02g 200.0 0.2g 500.0 0.2g
100.00 0.02g 400.0 0.2g 1000.0 0.2g
150.00 0.02g 600.0 0.2g 1500.0 0.2g
200.00 0.02g 800.0 0.2g 2000.0 0.2g
Check other functions, such as TARE, MEMORY, and Auto-Off.
6.
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5. To Replace Loadcell Assembly
Disassemble top housing of scale.
1.
Disconnect PL2 from main PCBA. Remove the four screws fixing the bottom plate of
2.
the loadcell assembly. Replace the loadcell with a new one. Connect PL2 and fix
the four screws.
Put 110% full capacity loading onto weight stand of scale. Adjust the overload
3.
screws on the weight stand until they just not touch the bottom plate.
Assemble the top housing.
4.
Perform internal calibration as described in section 1.
5.
Check accuracy of scale at different weight.
6.
440-33 440-45 440-47
weight(g) tolorance weight(g) tolorance weight(g) tolorance
50.00 0.02g 200.0 0.2g 500.0 0.2g
100.00 0.02g 400.0 0.2g 1000.0 0.2g
150.00 0.02g 600.0 0.2g 1500.0 0.2g
200.00 0.02g 800.0 0.2g 2000.0 0.2g
7. Check other functions, such as TARE, MEMORY, and Auto-Off.
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6. Schematics
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Components Layout
7.
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