Seca 717/727/728/737/748/757 Description of faults
23.04.97 - Jen Blatt 2 30-34-00-437 Index G
3. Fault description: Scale displays “bAtt”
after starting”
"bAtt" is triggered if the input voltage falls below
approx. 6 V.
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.
When the scale is operated only on
rechargeable batteries, i.e. without a power
supply unit, make the following check:
On seca 748, the voltage at capacitor 120 must
be approx. 9 V, on seca 727 and seca 757 it
must be approx. 7.2 V.
If the voltage is different, recharge or replace the
rechargeable battery.
4. Fault description: Digit defective
Check the driver transistor (201-205):
At the base of the associated transistor there
should be a square-wave signal with an
amplitude of approx. 0.7 V and an offset of
approx.+4 V.
At the collector of the associated transistor there
should be a square-wave signal with an
amplitude of approx. 5 V.
If this is not the case, replace the transistor.
5. Fault description: Segment defective
Check the actuation of the segments with an
oscilloscope.
Check the output of IC 419:
To do so, use an oscilloscope connected to the
relevant pin to measure the segment actuation
signal.
Output of IC 419
If the output does not switch, replace IC 419.
Check the output of microprocessor IC 520 or
the input of IC 419:
To do so, use an oscilloscope connected to the
relevant pin to measure the segment actuation
signal.
Input of IC 419
If the output does not switch, replace the
microprocessor.
6. Fault description: Mains operation monitor
not working correctly.
If the scale is not loaded, a scale operated on a
power supply unit should switch off after 10
minutes.
If operated on rechargeable batteries, the
following applies:
seca 717, 727, 728, 737, 757: 1 minute
seca 748: 2 minutes
Check transistor 109:
If the scale is operated on a power supply unit, a
voltage of approx. 0.2V must be present at the
collector.
If this is not the case, replace transistor 109.
If, when the scale is operated on rechargeable
batteries, a voltage of approx. 0.2 V is present at
the collector, the wrong transistor was fitted, a
PNP instead of an NPN transistor.
If the scale is operated on a power supply unit, a
voltage of approx. 0.65 V should be present at
the base of transistor 109.
If this is not the case, check whether the correct
components have been fitted for LED 106 and
resistors 107, 108. Fault description:
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