4
EXTERNAL LINEAR AMPLIFIER SET UP. There are
specific model amplifier application notes on our web site
and why some work on ATV and others do not. Basically,
they all setup in the same way.
1. With no video source plugged in, turn the RF power pot
on the VM-70XS module to its minimum power position
(CCW). Make sure the amp is rated to be linear class A, AB
or B and is connected to a good 50 Ohm dummy load or low
vswr antenna (less than 10% reflected).
2. Turn on the amp and transmitter. Slowly increase the
TX70-5s output to the amplifiers rated 1 dB compression
power output level. In most Amateur amplifiers, this is about
1/2 the rated maximum CW, FM or SSB peak envelope power.
For instance, the Mirage D100-ATVN will take less than 1W
to give 50 W pep in the ATV mode.
3. Plug the camera back in and have a distant station on two
meters talk back to check your picture to make sure there is
stable sync - no rolling. When amplifiers gain compress from
over drive, the sync tips get compressed and the picture
becomes unstable. You may have to back the RF Power pot
down until it becomes stable.
OPERATING NOTES: ATV practices are somewhat different
from the other bands and modes. It is easy for another ham
near you to look for your video transmission by connecting a
roof top 70cm antenna to their TV set to cable channel 58,
59 or 60 and talking back to you on 2 meters. See our ATV
Application Notes web page for info on making a 70cm ground
plane or simple beams if they do not have one. Many ATV
contacts are initiated by calling or listening on an area 2 meter
FM simplex ATV coordination frequency (146.43 in 434.0
areas, and 144.34 in 439.25 transmit video areas due to the
3rd harmonic relationship). Since we must use directional
antennas to make up for the 26 dB higher noise floor
difference compared to NBFM due to bandwidth (15 kHz vs.
3 MHz), the probability of someone pointing their beam at
you while at the same time you at them and calling CQ is
very low.
Two meters, even for FM, has about 9 dB less path loss
than 70CM so that all possible ATVers can be received on 2
meter FM using just an omni antenna. You will find with
experience the correlation between 2 meter simplex and
70CM ATV. It is much easier for all local ATVers to monitor a
squelched 2 meter FM simplex channel than to try tuning
and swinging the 70CM beam looking for sync bars. Once
another ATVer comes up on 2 meters, you can roughly swing
the beams on each other before turning on the ATV
transmitter. Then, if the picture is better than 20% snow, the
video transmitting station can talk on the sound subcarrier,
and all those receiving him can talk back at the same time
on 2 meters (full duplex) to comment on picture content, etc.
Others listening to the 2 meter channel are often hooked
into ATV this way. You can also run full duplex audio and
video with another station on 900 or 1200 MHz bands.
It is more fun as time goes on to have many hams put
their families, other hobbies, and varied interests on the
screen. Let others know your 2 meter ATV freq. by publishing
in local radio clubs newsletters, contact your local ARRL SCM,
or pick a night and time to start an ATV net. The TX70-5s is
portable enough to give a little demo at your local radio club
or hamfest.
IF YOU BELIEVE THE TX70-5s ISN’T WORKING, check
all cables and connections, internal fuse and 16V protection
zener, VSWR, power supply and DC voltages at the boards.
Then call or email us and describe the problem or ask any
questions you might have. It will save us both time and money
if we suggest some things to try that may have been over-
looked, or for us to better evaluate the problem. The TX70-
5s can be repaired by us for $80 plus parts cost in a few
days if we believe the problem is customer caused or nothing
wrong. If we determine that it was due to our workmanship
and materials within a reasonable time and given
circumstances then your cost is only the shipping to us.
However the repair and service policy stated in our latest
catalog will supercede the general policy listed here. Include
with the unit a filled out
Return Authorization Form - download
from our web site. Normal turn around is 2 days after we
receive it. There is no other warranty expressed or implied.
We believe this policy is more realistic than the usual 90 day
warranty other amateur manufacturers have since various
parts have different expected lifetimes.
INTERNAL:
Also reference the wiring diagram on page 6.
RF POWER OUT Control. A pot is located on the VM-70XS
module between the frequency select and Vin solder pads
to set power from 0-5W. Do not adjust unless you have a
70CM RF power meter, 50 Ohm dummy load or antenna
connected with a VSWR of less than 1.5:1. Never exceed
5.0W with no video connected. Permanent damage to the
Videolynx VM-70XS module can result from higher power,
VSWR or ambient temperature which causes the module to
exceed 149 degrees Fahrenheit.
The variable capacitor on the TR-1b board (rear wall of
the chassisis) is NOT to be used to vary the power output.
This cap is to fine tune any reactance in the antenna jack
and relay line and must always remain peaked for maximum
power output. Detuning will present a higher VSWR to the
VM-70XS module and could damage it.
AUDIO DEVIATION. Peak deviation is controlled by a pot
on the FMA5-G board for no more than 40kHz or 25 kHz
average. If you don’t have a communication monitor tuned
to 4.5 MHz above the video carrier, you can come close by
comparing the sound level with a cable broadcast channel.
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