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Seca 717/727/728/737/748/757
Troubleshooting
First and foremost make a visual inspection,
checking the soldering points and for corrosion
and soiling. Also check that all parts are fitted
correctly.
1. Fault description: Rechargeable battery not
correctly charged.
Connect the scale to the plug-in power supply
unit.
For the correct battery charging current see list
30-34-00-561.
Check the battery charging current.
Remove the rechargeable battery and connect
an ammeter to the rechargeable battery
connections.
If the charging current is not in the specified
range, check the polarity of diode 106 and
correct if necessary (anode at input voltage, LED
lit when charging current flows).
If the polarity is correct, calibrate the charging
current using resistor 103.
If no charging current flows, check transistor 104
and replace if required.
2. Fault description: The scale cannot be
started.
When the scale is operated only on
rechargeable batteries, make the following
check:
Measure the rechargeable battery voltage and
recharge the battery if necessary.
When the scale is operated on a plug-in power
supply unit make the following check:
The voltage at Z-diode 102 must correspond to
the power supply unit voltage.
If the voltage collapses, there is a short-circuit
(e.g. in diode 102) or the power supply unit is
defective.
Check the 5 V supply voltage:
The voltage at capacitor 518 must be in the
range 4.7 V - 5.3 V.
Measure the voltage at pin 6 of IC 131. If it is
lower than 6V, replace diode 110 (scale
operated on rechargeable batteries) or diode
111 (scale operated with power supply unit).
Measure the voltage at pin 8 of IC 131. If, after
pressing the start button, the voltage is lower
than 6V, check pin 8 of IC 126 with an
oscilloscope. When the start button is pressed, a
23.04.97 - Jen
short positive pulse must be registered. If there
is no pulse, check the start button and diodes
115 and 118, otherwise replace IC 126.
Measure the voltage at pin 1 of IC 131. It should
be approx. 5V. If this is not the case, check
diode 132 and replace if required, otherwise
replace IC131.
Check the reset circuit:
Check pin 13 of IC 126 with an oscilloscope.
When the start button is pressed, a short
positive pulse must be registered. If there is no
pulse, replace IC 126.
Check pin 9 of IC 520 with an oscilloscope. If no
short positive pulse is measured when the start
button is pressed, replace transistor 128,
otherwise replace microprocessor IC 520.
Check the microprocessor oscillator:
The frequency measured at pins 18 and 19 of
the microprocessor, IC 520, must be 12 MHz.
If this is not the case, replace the quartz.
If the scale still does not work, replace the
microprocessor.
Blatt 1
Description of faults
30-34-00-437 Index G
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