KROHNE BM 90 EN - Manual (Page 33)

33
5. Fault Finding
5.1. Fault finding - Hardware
A) The display is blank or frozen, the l.e.d’s are unlit
and the neon does not fire:
1. Ensure that power is being supplied to the board, and
that it is correctly wired. Refer to figure 6, on Page 9.
2. Check fuses.
3. Check that the supply voltage is within specified levels
(See Section 2 Page 10). A large voltage drop can cause
the unit to lock to show last distance or level
reading.
B) The fuse blows continuously:
* In this case the system is drawing excessive current.
1. Power down and fit a new fuse - refer to Page 10
Disconnect all cabling from the unit except for the power
lead. If the fuse does not blow on power up, there is a
fault in the external wiring.
2. Check that the power supply is within specified limits -
refer to Page 10.
3. Check the enclosure for metal debris which may be
under the lower PCB.
C) The system powers up, but displays ’8888’:
* In this case a connection in the PCB is giving a continu-
ous or intermittent fault.
1. Check; with power off, that an Eprom is fitted at U7
and that the chip has not vibrated free from its socket.
D) The display shows ’LOSt’:
* In this case the instrument is not reciving a good signal
grom the transducer.
1.Check the transducer wiring and connections to the
instrument. Note that different connections are used if a
temperature compensated transducer is connected. See
Figure 6, Page 9.
2. Check whether the neon light adjacent to terminal 22
is flashing. If it is proceed to number 3, if it is not then :
a: Disconnect the transducer: If the neon now
lights then there is a short circuit in the cabling.
b: If the neon does not light the transmission fuse
may have blown. Check F1 and F2 T80mA fuses on the
bottom PCB.
c: If the unit still shows ’LOSt’ check that you can
hear the transducer "clicking" when close to the ear.
d: If the transducer cable has been extended, dis-
connect and remove the transducer and connect it direct
to the Liquiflex. If the unit now operates, recheck the
extension cable connections and routing, avoiding power
cables. Re-instal the transducer checking that its aim is
perpendicular to the target surface.
e: If the transducer does not click proceed to 5.
3. Is there a target within the empty distance specified in
Pr.3?
This is particularly important if temperature variations are
experienced and no compensation is applied.
4. Is the vessel empty with a conical, parabolic, sloping
or spherical bottom?
This commonly causes loss of echo if the transducer
cannot be mounted over the centre of the vessel. When
the vessel becomes empty the pulse from the transducer
hits the sloping sides of the bottom section and the signal
is not reflected back to the transducer. Under this condi-
tion the display will indicate ’LOSt’ but the failsafe desig-
nation will operate until product returns and the system
will automatically recover and track level. If the transduc-
er cannot be mounted centrally, the problem may be
overcome by the installation of a target plate.
5. Connect a known good transducer to the instrument
and check the operation.
If the known transducer gives a good signal check the
instruments gain by pressing the ’TEST’ key. The number
displayed ranges from 1 - 100 and the lower the number
the better the signal strength.
If the gain figure is 50 - 100 check the surface level for
foam or other materials which may float in and out of the
beam and cause poor echoes.
6. Check that the ST6 eprom is seated correctly at U6 on
the bottom PCB
E) The keypad fails to respond:
1. Check for correct alignment of connection from keypad
to main board.
2. Check that key press sequence is valid; refer to
Programming Section.
3. Power down unit and wait 5 seconds. Power up and
immediately press ’MODE’. This should result in ’PROG’
being displayed. It is now advisable to reset to factory
parameters; refer to Programming Section 3.
F) Analogue Output is Unstable:
1. Connect a test meter in series with your external
wiring.
Can the fault be seen on the test meter? If YES, then
use Pr.34 to enter a stable value into the current loop.
Suitable values range from 4 to 20.
If the output is still unstable disconnect external wiring