Specifications

January – March 2003 ExtroNews 14.1 15
equipment for this type of problem and
should be reserved for the worst case
scenario. In some extreme cases, when the
application has justified the expense, the
Extron RGB 202 Rxi has been used to resolve
this type of problem.
Scenario 4 – Cable Problems
A “Cable Problem”, refers to a defective,
damaged, or poorly terminated cable
exhibiting either a discontinuity or a short
circuit between one end of a cable and the
other. Cable problems may be the most
common problem encountered in A/V
systems, whether sync related or otherwise.
Cable problems might not seem appropriate
material to cover in an article on the technical
tricks of troubleshooting; after all, once a
cable problem is identified, it’s usually time to
either pull another cable or cut off the old
connector(s) and re-terminate. There’s not
much to it except repeating some good old
fashioned hard work. However, identifying
cable problems can be a bit more technically
challenging than fixing them, and thus, the
“Cable Problem” sneaks its way into this
publication.
Solution:
A bad cable for composite
sync, H sync, or sync on green will typically
result in a scrambled image or no image at
all. A bad cable for V sync will result in an
image rolling vertically or no image at all.
Once these kinds of problems are seen,
checking the integrity of cables is a good
next step.
Testing the integrity of a single coax cable
in an RGBHV snake can usually be done by
swapping it with a known existing cable. For
instance, if an image rolls when connected to
RGBHV, vertical sync may not be passed. Try
swapping the blue cable for the V-Sync cable.
If the image passes, but no blue elements of
the image appear, the V-Sync cable is bad.
An oscilloscope can also be used to test
cable integrity of sync cables. If the scope
can see the sync signal at the far end of the
cable run, the cable is good, and if not, it’s
bad. It’s important to note that the signal at
the input end of the cable must be verified
to draw the latter conclusion.
Another means to verify the integrity of a
cable is to use a Digital Multimeter (DMM),
to test for a short circuit or an open circuit
in the cable. While testing for a short circuit
at one end of a cable is easy, the leads of a
DMM don’t typically reach from one end of
a cable to the other to test an open circuit
condition. It is necessary to short or jumper
two conductors together at the far end of a
cable in order to use a DMM to test
continuity at the near end.
The DMM is also quite handy in
identifying if a problem is present in a
connector or somewhere in a cable itself.
Once a cable is determined to be bad, the
terminating connectors become the first
point of failure to be looked at. If, however,
TROUBLESHOOTING TIPS
re-terminating does not resolve the
problem, and/or inspection shows the
terminations to be sound, a DMM may be
the only method to test the cables
themselves for shorts or opens.
For those who carry a Time Domain
Reflectometer (TDR) wherever they go, it
can be used to pinpoint the exact location
of a problem in a cable.
In any case, once the problem cable is
identified, and the problem specific to that
cable is established, the cable must be
re-terminated or replaced. While repairing
an open or shorted cable within an
installation is possible, it is rarely practical or
cost-effective.
Conclusion
And that concludes this guide to
troubleshooting some of the most common
causes of the bad sync phenomenon. We
hope this article leaves you with a few
more shells to shoot your way clear of
A/V trouble.
A Digital Multimeter (DMM) and an oscilloscope can be used to test the integrity of sync cables.