Olson Technology OTR-3550 - Manual (Page 9)

025-000156 REV F Page 9
DEMODULATOR TROUBLESHOOTING
This unit has a restricted AFC range. This is done to eliminate a problem inherent in many low cost modulators
incorporating AFC. That problem occurs when the desired off-air channel goes off the air and the AFC has a wide
enough range to pull in an adjacent channel. In most cases the AFC holds the undesired channel when the desired
channel returns to the air.
However, many independent UHF broadcast stations and translators operate out of specification with regard to F.C.C.
rules. We have found some in excess of 1MHz from their specified channel allocations. Fortunately, the OTR-3550
is capable of selecting the L.O. frequency in -.25MHz increments. This will allow you to select an L.O. frequency to
compensate for your UHF station error.
TROUBLESHOOTING:
1) Make sure that the 12-position DIP switches for input channel select are in the correct position (according to
the input channel select chart). Also, make sure that the 6-position DIP switches are in the proper position.
Set the AFC switch in the “OFF” mode (UP position).
2) If the audio signal is present (you can hear it) but there is no video signal, then the input level to the UHF input
is either too low (-20dBmV or below) which triggers an internal squelch circuit, or the UHF transmitted signal
may be off frequency. An off frequency input will cause the unit to act like a low level input signal because it will
run the video carrier down the rejection slope of the I.F. SAW filter.
3) If a field strength meter is available, measure the UHF input level for the channel of interest. If the signal is
below -10dBmV, a preamplifier is recommended. If possible, view the UHF signal on a TV set and ensure
that you have an adequate picture. If properly set up, the OTR-3550 should produce a picture of comparable
video quality.
4) View the video output on a suitable composite video monitor. If it is of comparable quality to the off-air signal,
the unit and the TV station are both operating properly.
If the picture is of inferior quality or there is no picture, begin changing the L.O. frequency as follows:
1) The first two DIP switches as viewed from left to right are always in the “DOWN” position (Figure #7), and the
third switch is always in the “UP” position for Standard frequency allocations.
2) Moving the first switch to the “UP” position will increase the L.O. frequency by 0.25MHz. If the picture
improves significantly, move the second DIP switch to the “UP” position, and the first switch back to the
“DOWN” position. This increases the L.O. by 0.50MHz. If the picture quality degrades (not as good as
with 0.25MHz offset), then return to the 0.25MHz offset position (1st switch “UP”) and activate the AFC
switch (5th switch in the “DOWN” position).
3) If the picture quality degrades considerably in step (2) by increasing the L.O. frequency, then move the 1st
and 2nd DIP switches to the “UP” position and move the 3rd DIP switch to the “DOWN” position. This
procedure will move the L.O. frequency down by 0.25MHz. If the picture quality improves, try putting the
1st DIP switch in the “DOWN” position. leaving the 2nd DIP switch in the “UP” position and the 3rd DIP
switch in the “DOWN” position.