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M800-00
T807/808 Fault Finding
B6.3
Copyright TEL
31/08/96
6.1
Visual Checks
Disconnect the power supply from the mains and wait 5 minutes before removing both
covers.
Inspect the PCB for damaged or broken components, paying particular attention to the
surface mounted devices (SMD's).
Check for defective solder joints. If repair or replacement is considered necessary, refer
to Section 3 of Part A.
Check the fuse. If it is blown, check that the correct rating and type was fitted and fit a
new fuse of the correct rating.
Note:
If the fuse was of a lower rating and there are no signs of component dam-
age, it may be worthwhile fitting a new fuse, replacing the covers and
switching the supply on. If the new fuse blows, proceed with fault finding
as described in the following sections.
6.2
Component Checks
6.2.1
General
If a transistor is suspected of faulty operation, an indication of its performance can be
assessed by measuring the forward and reverse resistance of the junctions. First make
sure that the transistor is not shunted by some circuit resistance (unless the device is
completely desoldered). A 20k ohm/V or better multimeter should be used for taking
the measurements, using only the medium or low resistance ranges.
The collector current drawn by multijunction transistors is a further guide to their per-
formance.
If an IC is suspect, the most reliable check is to measure the DC operating voltages. Due
to the catastrophic nature of most IC failures, the pin voltages will usually be markedly
different from the recommended values in the presence of a fault. The recommended
values can be obtained from either the circuit diagram or the component data catalogue.
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