MIC jack accepts any low Z dynamic mic in the range of 100
- 600 Ohms with a mini plug. Mic audio is active at all times
and mixes with the camera or VCR line audio input to give
more direct pickup, commenting while running video tapes,
etc. Mikes must have a shielded cable to prevent RF pickup
hum and buzz. Unidirectional mics are suggested for full
duplex to minimize speaker feedback or to reduce pickup of
unwanted sounds and noise from the sides. Electret and
amplified mics are very susceptible to RF pickup - buzz and
should not be used.
MIC GAIN control varies the level of the low Z dynamic mic.
It is independent of the line audio level. Speak directly into
the microphone at the normal operating distance. Increase
the level to the point where the red XMIT light winks off, and
then back down the gain slightly. There is a volume
compressor that will keep the audio at the standard 25 kHz
deviation and 40 kHz peaks. Audio usually drops out about
the same time as color does in a snowy picture - P3 -
depending on the TV sets audio IF gain and limiting.
3
XMIT/REC switch. It is in parallel with the EXT KEY jack.
The red lamp above this switch will light whenever you are in
the transmit mode and the audio inputs are making <40 kHz
peak deviation. There is a delay of 2 to 3 seconds for this
LED to light and the RF output to come up. In receive, the
a voltage appears on the downconverter power jack
to power a TVC-4s 70CM ATV downconverter.
CHANNEL SWITCH.
This model has 4 synthesized
channels and are the same as our TVC-4s downconv.:
1 - Open - *can be special programmed for 421.25 MHz
2 - 426.25 MHz
3 - 427.25 MHz
4 - 434.00 MHz
5 - 439.25 MHz
*This frequency can only be used with a VSB filter in the
antenna line to keep sideband energy from falling outside
the band edge at 420 MHz as is done at a repeater site.
439.25 MHz only can be used in Canada and those above
the A line - approx 100 miles south of Canada - in the USA
since the band edge there is 430 MHz.
POWER ON switch turns on the a12 to 14 Vdc to the
TX70-5s. If the green light does not come on, check the
fuse, polarity and determine why it blew before replacement.
LINE AUDIO GAIN control varies the high level audio applied
to the subcarrier from the front panel audio input RCA phono
jack. Increase the level to the point where the red XMIT light
winks off, and then back down the gain slightly. This audio is
independent and mixed with the mic audio. This makes
varying the level of a video tape audio verses mic for voice
over comments easy. Peak deviation is set by an internal
pot on the FMA5-G sound subcarrier board.
LINE AUDIO INPUT. High level line audio usually from the
same source as plugged into the companion Video input is
plugged into this jack using another RCA phono plug shielded
cable. Minimum level is .1 v pk-pk into a 10K load.
75 OHMS TO MONITOR. This output provides the composite
video from the front panel Video jack during receive to enable
you to aim the camera and to best adjust the focus and
lighting, etc. before transmitting. In transmit, there is no
output. Use a RCA plug 75 Ohm shielded cable to connect
to your video monitor or VCR video in.
VIDEO INPUT. This input accepts any standard NTSC 1Vp-
p composite video into 75
Ω
from cameras, VCRs, computers,
SSTV or RTTY converters, home satellite converters, etc.
Use RCA phono plug and shielded cable (Radio Shack 15-
1535) up to 12' or RG59 or RG-6 for longer runs. When
unplugging, only twist clockwise to keep the jack from working
lose over time.
FRONT PANEL:
50 OHMS TO ATV DOWNCONVERTER. This BNC output
jack is connected to the antenna input of your 70 CM 420-
450 MHz ATV downconverter or cable channel TV.
Downconverters for other bands are not connected to the
TX70-5s, but rather to their own antenna and left on when
transmitting on 70cm for full duplex or crossband repeat. If
a TVC-4s downconverter is used, a short 50 Ohm cable with
a male BNC on one end and type N adaptor on the other is
supplied. If a TV set to cable channels 57 - 60 is used instead
of a downconverter, you will need to make a BNC to F 50 or
75 Ohm cable or use adaptors. The TX70-5s contains a T/R
relay to switch the antenna input as well as DC power
between the downconverter and the transmitter.
If you have a RF power meter, you should read 4 to 5
Watts forward power typically with no video plugged in - peak
sync level. With video plugged in and using an average
reading meter, it will read less, and down to half with an all
white picture, but sync tip power will still be the same as
measured with no video plugged in. This is the nature of
cable analog NTSC or AM video transmission, similar to SSB
voice peak and average RF power measurements with
complex analog modulation.
On initial turn on, do not transmit more than 15 seconds
if the reflected power is more than 10% or 2:1 VSWR. You
could damage the final power FET. Also, VSWR or being
too near your antenna can cause RF interference in your
camera or buzz in the audio.
Use a good resonant broad bandwidth 70 CM antenna
like the DSFO25-ATV, OAL 5L-70cm, circularly polarized OAL
7CP-70cm or homebrew antennas shown on our app note
web page. Do not be tempted to just try it out with a rubber
duckie, 2 meter antenna, or other antenna not specifically
designed for the video carrier frequency. Place the antenna
as high as practical, at least above the trees or roof tops.
See the section on dx vs. power vs. gain on page 5.
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