Orban 8300J, Operating Manual

Page: 205 / 280

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 8300, or
in other parts of the system.
RFI, Hum, Clicks, or Buzzes
A grounding problem is likely. Review the information on grounding on page 2-10.
The 8300 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 8300 and the
exciter, a ground loop may inject noise into the exciter’s composite input — espe-
cially if the exciter’s input is unbalanced. The Orban CIT25 Composite Isolation Trans-
former can almost always cure this problem.
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 20 on page 2-22.
Poor Peak Modulation Control
The 8300 ordinarily controls peak modulation to an accuracy of ±2%. This accuracy
will be destroyed if the signal path following the 8300 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.
Digital STLs using lossy compression algorithms (including MPEG1 Layer 2, MPEG1
Layer 3, Dolby AC2, and APT-X) will overshoot severely (up to 3 dB) on some pro-
gram material. The amount of overshoot will depend on data rate — the higher the
rate, the lower the overshoot.