Orban OPTIMOD-FM 8700, Operating Manual

Page: 291 / 390

OPTIMOD-FM DIGITAL TROUBLESHOOTING
5-1
Section 5
Troubleshooting
Problems and Potential Solutions
Always verify that the problem is not the source material being fed to the 8700i, or
in other parts of the system.
RFI, Hum, Clicks, or Buzzes
A grounding problem is likely. Review the information on grounding on page 2-12.
The 8700i has been designed with very substantial RFI suppression on its analog and
digital input and output ports, and on the AC line input. It will usually operate adja-
cent to high-powered transmitters without difficulty. In the most unusual circum-
stances, it may be necessary to reposition the unit to reduce RF interference and/or
to reposition its input and output cables to reduce RF pickup on their shields.
Particularly if you are using a long run of coaxial cable between the 8700i and the
exciter, a ground loop may inject noise into the exciter’s composite input—especially
if the exciter’s input is unbalanced. A Jensen JT-123-BMCF transformer can almost
always cure this problem—see page 1-15.
The AES3 inputs and output are transformer-coupled and have very good resistance
to RFI. If you have RFI problems and are using analog connections on either the in-
put or output, using digital connections will almost certainly eliminate the RFI.
Unexpectedly Quiet On-Air Levels
The ITU412 multiplex power controller may have been turned on accidentally. See
step 12 on page 2-24.
If you are using the ITU412 multiplex power controller and have edited a factory
preset (including by use of
L
ESS
-M
ORE
)
, you may have to readjust the
M
ULTIPLEX
P
OWER
O
FFSET
control in your edited preset so that the average indication on the
M
ULTIPLEX
P
OWER
meter is 2 to 3 dB of gain reduction. (See the notes on the
M
ULTIPLEX
P
OWER
O
FFSET
control on page 3-48.)
Poor Peak Modulation Control / Low On-Air Loudness
The 8700i ordinarily controls peak modulation to an accuracy of ±2%. This accuracy
will be destroyed if the signal path following the 8700i has poor transient response.
Almost any link can cause problems. Even the FM exciter can have insufficient flat-
ness of response and phase-linearity (particularly at low frequencies) to disturb peak
levels. Section 1 of this manual contains a complete discussion of the various things
that can go wrong.