REAR PANEL:
POWER INPUT JACK. A 4 pin plug and 3 ft long #18 cable
is provided for connection to your source of 12 to 14 Vdc.
Pin 1 is ground (black) and pin 2 (red) is +. The TC70-20Sa
works best connected directly to the terminals of a well
regulated power supply with leads no longer than necessary.
The transmitter is set up by us with a regulated 13.8 Vdc
supply, if the voltage is <13.3 Vdc see amplifier / pedestal
set up on page 5. Do not exceed 15 Vdc input. There is a
16 v zener which should blow the fuse if this voltage is
exceeded or the supply leads get cross connected, but
semiconductors have been known to protect fuses. Any ripple
or noise on the DC line may be seen in the transmitted video.
You will need a 13.8 Vdc regulated power supply capable of
4 Amps continuous. There is a 5 Amp fuse inside the unit.
Downconverter Output to TV is at this jack during receive
for connection to your TV receiver antenna input. If you want
video output instead, connect to a VCR tuner or channel 3
receiver like the MCM Electronics 333-2060. Run a 75 Ohm
RG6 coax with F connectors. Radio Shack has cables made
up in various lengths (15-1530). Tune the TV to channel 3
or 4 depending on which ever is weakest or not on the air in
your area. With the TC70-20S still off, fine tune the TV set,
with the AFC off, for all snow and no adjacent channel
feedthru. Now turn on the TC70-20S and slowly tune the
REC TUNE knob for the best picture from a known close by
ATV station. Once you have a picture, rotate your antenna
for least snow or strongest picture. Then ask the ATV
transmitting station to swing his antenna for the strongest
picture. It’s best to coordinate the tuning and antenna rotation
on 2 meter FM. The most popular ATV coordination
frequencies are 144.34 and 146.430 MHz simplex. Select
the one that does not have a 3rd harmonic within the video
passband on 70 cm. The video transmitting station then talks
to you on the sound subcarrier, and the receiving stations
talk back at the same time (full duplex) on 2 meters.
TRANSMIT VIDEO MONITOR OUTPUT. This provides
composite video of your demodulated RF directly from the
antenna output during transmission periods to enable you to
best adjust the video gain, set focus and lighting, etc., rather
than a distant station describing these back to you on 2
meters. In receive it outputs the phono jack video input to
enable your setting up the picture on the monitor as you
receive another station on the TV. Use a RCA plug shielded
cable to connect to your video monitor or VCR video in. If
your TV receiver does not have a video input, the Radio
Shack 15-1273 RF Modulator can take the composite video
and modulate it up to channel 3 or 4 to make another TV set
into a monitor. Attempting to see your own video over the air
with a TV set at the same QTH most often gives false
indications due to overload and reflections. Even receiving
the 2nd harmonic 40 or more dB down around channel 80,
or on cable channels between 57 and 60 can give an
erroneous indication of transmitted picture quality. Only the
monitor output will be accurate, except when reset to drive
an external power amplifier.
50 OHM 70 CM ANTENNA. A UG21 type N plug is provided
to attatch to low loss .5" size 50
Ω
coax. Losses at 70 cm are
very high in transmission lines. We suggest using the foam
filled types (3.5 dB/100') such as Belden 8214, or semi rigid
(2.5 dB/100') Belden 9913. Take great care to put the N
plug together properly - see last page. The type N connector
has good moisture resistance and low loss at UHF but use
two layers of vinyl tape or coax seal on all outside connections
to prevent moisture contamination. The antenna and feed
line are the most important part of your ATV system, and
therefore the last item to just try and get by with.
Check out your antenna system with a RF power meter.
On initial turn on, do not transmit more than 10 seconds if
the reflected power is more than 10% (2.0 W max) or 2:1
VSWR. You could blow the M57716 power module. Also,
VSWR or being too near your antenna can cause RF pickup
interference in your camera or buzz in the audio. With no
video connected, the RF power meter should read between
11 to 15 Watts blanking pedestal power. Sync tip, p.e.p.,
equals 1.68 times the blanking power.
Use a good resonant broad bandwidth 70 cm antenna
such as the High-performance 432-Mhz Yagi’s described in
the 1995-2005 ARRL Handbook Chapter 20 or commercially
made antennas like the Old Antenna Lab 5L-70cm, DSFO-
ATV-25, or circularly polarized KLM 435-18C & 435-40CX,
or Diamond F718L and NR-770H omnis. Do not be tempted
to just try it out with a rubber duckie, 2 meter antenna,
broadcast UHF TV or other antenna not specifically designed
for the video carrier frequency or 50 ohms. 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 4.
FRONT PANEL:
VIDEO INPUT. This input accepts any standard NTSC
composite video into 75
Ω
from cameras, VCRs, computers,
SSTV or RTTY converters, home satellite converters, etc.
Use RCA phono plug shielded cable (Radio Shack 15-1535).
Push RCA phono plugs straight in, but pull and twist off only
in the clock wise direction to keep jacks tight.
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. The level is
controlled by the line audio gain knob.
TX FREQ. Transmit frequency switch select, use only the 2
ATV frequencies per your local band plan to prevent
interference - check with local ATVers before first getting on.
The switch is 6 position, 4 are used as marked. The open
positions default to 426.25 MHz or special ordered frequency.
2
Keep 3”
minimum
clearance
around the
heat sink
and top of
box for
convection
cooling or
add a fan.