4. VERIFICATION OF NORMAL OPERATION
It is the purpose of this section to aid the user in determining, in the event of a
malfunction to which the Model 5M78 is suspected of contributing, whether the
instrument is functioning normally or whether it is the source of the observed
trouble. In the event the unit requires repair, the user may also contact the
factory Service Department or the local Daytronic Representative for assistance.
Daytronic service information is located on the last page of this document.
If the instrument is suspected of faulty operation, follow the steps below.
(a)
If the unit is totally inoperational (front-panel power indicator does not
light), check the primary power input terminals for proper connection.
Input power can be from 11 to 28 Vdc and will draw less than 2 watts.
If properly connected, the front panel Green LED will be illuminated. Before
reapplying power, visually inspect the power supply and the input power
connections for any discrepancy which could have caused the overload.
(b)
If the transducer has some preloading, the BALANCE controls may not
allow successful zeroing of the instrument output. This condition can be
remedied by connecting a resistor (50 k- 200 k range, metal-film type)
from the +Signal terminal of the transducer to the + or –
Excitation Sense
terminals. The
Excitation
terminal to which the connection is made is
determined by the direction of the loading or off-zero reading.
(c)
The inability to balance correctly, where the instrument output reads
totally off scale and the BALANCE controls have no authority, can very
likely be the result of a damaged or defective transducer or cable. This
possibility can be confirmed (or eliminated) by substituting a transducer
and cable known to be in good condition or by simulating a balanced
transducer, using either a commercially available transducer simulator
or the simple
star bridge
arrangement shown in Figure 7. The
star bridge
simulates a conventional four-arm bridge in an exact condition of bal-
ance. To construct a
star bridge,
connect four 10% carbon resistors as
shown in Figure 5. Use 180-ohm resistors to simulate a 350-ohm bridge
and use 56-ohm resistors to simulate a 120-ohm bridge. Neither the
resistor values nor temperature characteristics are critical since the
balance condition of a star bridge is not determined by the resistance
Figure 7. Star Bridge Construction
20
values. Solder two resistors together, then solder the remaining two
resistors together. Next, connect the two junctions together using a
separate wire as shown. There is a good reason for this method of
construction, and it should be followed. Connect the substitute or simu-
lated transducer to the instrument I/O connector using a short 4-wire
cable configuration as shown in Figure 4. Attempt to balance the substitute
simulated transducer. If conditions now appear to be normal, the transducer
or cable is at fault. If the previous difficulties persist, the 5M78 is faulty.
