Hardware Reference
In-Depth Information
Figure 13.25
Layout of the controls and adjustments of a typical bench
oscilloscope
4
Is the fault intermittent or is it present at all times?
5
If the fault is intermittent, in what circumstances does it arise? Is it possible
to predict when the fault will occur?
6
To facilitate testing and diagnosis, can conditions be reproduced so that the
fault manifests itself permanently?
7
What parts of the equipment are known to be functioning correctly? Is it
possible to isolate the fault be isolated to a particular part?
8
Is the fault a known 'stock fault'? Has the fault been documented elsewhere?
Having answered the foregoing questions, and assuming that one is con-
fronted with a system which is totally unresponsive, the first step is that of
checking the power-supply rails using a multi-range meter. Where any one of
the supply rails is low (or missing altogether) the power supply should be dis-
connected from the backplane and the measurement should be repeated in order
to establish whether the absence of power is due to failure of the power supply
or whether the fault can be attributed to excessive loading. This, in turn, can
either be due to a short-circuit component failure within an expansion card or
a similar fault within the system motherboard.
The system power supply employs switched mode techniques and it should be
borne in mind that such units generally require that a nominal load be present on
at least one of their output rails before satisfactory regulation can be achieved.
Failure to observe this precaution can lead the unsuspecting test engineer to
conclude that a unit is not regulating correctly when it has been disconnected
from a system. In any event, it is advisable to consult the manufacturer's data
before making measurements on individual supplies.
Having ascertained that the system is receiving its correct power-supply
voltages, the next stage is that of activating the system reset switch and not-
ing whether any changes are produced. After each of the initial diagnostic
