Zeiss humphrey HFA II-i series, Field Service Manual

Page: 352 / 521

GEN-004B Page 3
5) Resolve the Obvious
Obvious mechanical or physical defects should be resolved when discovered. Often, resolving
the obvious will lead you to the real problem.
6) Substitute the Failing Assembly
Once you have deduced which assembly is at fault, or is suspected to be at fault, it should be
substituted with a known good assembly.
7) Confirm the Repair
After an assembly is replaced, confirmation should be made by reinstalling the failing assembly
to witness the original symptom. You are then assured of the effectiveness of the repair, and of
the need to utilize your spare assembly. Also, the number of "No Problem Found" PCB's
returned to the Carl Zeiss Meditec Repair Center is greatly reduced.
Confirming the repair to the customer often helps to bolster confidence in your abilities. To
avoid repeat service calls it is especially important to confirm intermittent problems. Time spent
confirming a repair is time well spent.
If the problem still exists, reinstall the original assembly that you removed , then continue
troubleshooting.
Solving Solid Failures
It is usually straightforward deductive logic or electrical checks that will lead you to replace the
faulty assembly. After replacement of the assembly in question resolves the symptom, it is
imperative that you
CONFIRM THE REPAIR by reinstalling the original assembly and witnessing
the original symptoms again. For many assemblies this is the only means of positively verifying
that the replacement circuitry has resolved the problem.
Solving Intermittent Duplicable Failures
These failures are resolved in the same manner as solid failures but always require more time to
resolve and confirm.
The frequency of the problem helps you determine how long it will take to verify a repair. A
good rule of thumb when troubleshooting intermittent problems is: 2 times the frequency of the
symptom gives an 80% confidence level of the repair.
EXAMPLE:
Problem
The problem appears only about once an hour. The sequence of events
needed to reproduce the problem leads you to believe a circuit board is the
likely cause.
Solution
The suspected circuit board is replaced. To provide an 80% confidence level
that the circuit board resolved the problem, the instrument will need to be
tested for 2 hours without failing.