Tektronix ECO422D Instruction Manual page 105

Maintenance
NOTE. A source transition can occur if the channel output amplitude is reduced.
This can be caused by over-termination, cable damage, or an accidental short to
ground. A channel's selected input and its output are directly connected through
an internal relay, therefore the ECO422D detects an output reduction as an input
reduction. This does not apply to the three Option ELSW channels, which are
buffered.
To avoid this problem, work on an output only when the ECO422D is not in
operation. Disconnect the output cable from the ECO422D before working on
the cable, and verify proper termination before reconnecting the cable to the
ECO422D.
Determine and evaluate all trouble symptoms. This is accomplished by isolating
the problem to a general area, such as an assembly. The block diagram and the
Theory of Operation, are valuable aids in signal tracing and circuit isolation.
CAUTION. Use extreme care when probing with meter leads or probes. The
components are very dense and there is only limited access within the instru-
ment. The inadvertent movement of leads or a probe could cause a short circuit
or transient voltages capable of destroying components.
Determine the nature of the problem. Attempt to make the determination of
whether the instrument is out of calibration or if there has been a component
failure. Once the type of failure has been determined, proceed on to identify the
functional area most likely at fault.
Visually inspect the suspect assembly for obvious defects. Most commonly these
will be broken or loose components, improperly seated components, overheated
or burned components, chafed insulation, etc. Repair or replace all obvious
defects. In the case of overheated components, determine the cause of overheat-
ing and correct the cause before reapplying power.
Use successive electrical checks to locate the source of the problem. The primary
tool for the problem isolation is the oscilloscope. Use the Performance Verifica-
tion procedure to determine if a circuit is operating within specifications. At
times, it may be necessary to change a calibration adjustment to determine if a
circuit is operational, but since this can destroy instrument calibration, care
should be exercised. Before changing an adjustment, note its position so that it
can be returned to its original setting.
7- 5
ECO422D SD/HD Changeover Unit Instruction Manual