VIDEO GAIN control. This sets the white level or depth of
modulation of the selected video source. The knob should
be slowly increased clockwise just to the point of white
smearing or blooming as seen on an external video monitor,
and then backed down a little. The viewfinder in a color
camera can also be used if it can accept external video into
it, as some do for VCR playback. If you do not have any kind
ofmonitor, youmighttryhavinga distantATV receivingstation
describeyourpicturebacktoyouover2meters. Seemonitor
output paragraph.
LINE AUDIO GAIN control. Nominal input is .1 to 1 Vp-p
into 10K. Increase the line or mic audio gain controls to the
point that the green LED blinks off (indicating that the AGC
limiting at 25 to 40 kHz), and then back off a little. In the off
position, the whole sound subcarrier board is turned off.
MIC AUDIO GAIN control. Mic audio is mixed with the line
audio and its level is varied independantly. If you connect
the audio from your home VCR or camcorder, you can use
the mic input to voice over comment.
MIC jack accepts any low Z dynamic or low Z Amplified
electret camcorder mic in the range of 100 - 600 Ohms with
a mini plug. Mic audio is active at all times and mixes with
thecameraorexternalaudioinputstoenable greater pickup,
commentingwhilerunningvideo tapes, etc. Mikes must have
a shielded cable to prevent RF pickup hum and buzz. Some
electretandamplifiedmics are very susceptible to RF pickup
and may need the addition of a small 220 pF disc cap (RS
272-124) directly across the mic element. Radio Shack has
some replacementremote-controldynamicmicsforportable
recorders (33-2019) that work well and some provide the
“push to look” plug also. Awind screen over a unidirectional
micelement helpswhenusedforportableorwithcrossband
repeaters for full duplex to minimize speaker feedback. The
33-3021 is good for in the shack.
PTL submini jack. PushTo Look is like push to talk only with
video. Grounding the tip keys the transmitter.
RECEIVE TUNE control varies the varicap voltage in the
VCO in the GaAsfet downconverter between 420 and 450
MHz (0 to 10 on the knob) in receive plus some overlap to
accommodate conversion down to TV channels 2, 3 or 4.
434 is between 3 and 5 on the knob into channel 3.
XMIT/REC switch. It is in parallel with the PTLjack. The red
lamp above this switch will light whenever you are in the
transmit mode and the green is lit in receive.
POWER ON switch turns on the applied +12 to 14 Vdc to the
TC70-20Sa. If the green receive light does not come on,
check the internal 5 Amp 3AG fuse and the reason for it to
blow before replacement. If the leads were reversed or an
overvoltage condition and fuses keep blowing, check the 16
Volt 5 Watt zener at the power on swtich and/or 78L08
regulatorontheTVC-2G fora short andreplace if necessary.
RF POWER OUT 250 Ohm pot on the end of the TXA5-70
exciterboard controls the drive to the M57716powermodule.
This is used to reduce the output power when driving an
external amplifier. See the procedure on page 5 and 7. You
must unplug any video input then set the TXA5-70S board 1K
pedestal pot CCW for maximum output first before adjusting
the peak power output, then reset the pedestal pot to 60% of
peak. Your Transceiver may go as high as 28 Watts at full RF
pot CW, but decreased linearity and sound sync buzz may
occurabove20 Watts. Goingfrom 20 to 28Wattsp.e.p. gives
insignificant change at the other end anyway.
INTERNAL CONTROLS
Your TC70-20Sa comes to you all set up to operate, do not
make any internal adjustments unless you have the proper
test equipment, tools and experience. The power is set for
22Watts p.e.p.with13.8Vdcapplied - a RF powermeterwill
show the blanking pedestal setup of 11 to 13 Watts with no
video applied, or less under video modulation. Refer to the
board layouts on pages 6 & 7 for pot locations. 3
OPERATING NOTES: ATV practices are somewhat different
from the other bands and modes. Since we must use
directional antennas to make up for the 23 dB higher noise
floordifferencecomparedtoNBFMduetoreceiverbandwidth
(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. This is why many ATV contacts are initiated
by calling or listening on a 2 meter FM simplex ATV
coordinationfrequency(146.43for 434.0 &144.34for439.25).
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
experiencethecorrelationbetween2 meter simplex and 70cm
ATV DX. It is much easier for all local ATVers to monitor a
squelched 2 meter FM simplex channel than to try tuning
andswingingthe70cmbeamlookingforsyncbarsorlistening
to TV speaker noise. Once another ATVer comes up on 2
meters,youcan roughlyswingthebeamson eachotherbefore
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
thesametimeon 2 meters (full duplex) to comment on picture
content,etc. Otherslistening to the 2 meter channel are often
hooked into ATV this way. You can also run full duplex audio
and video with another ATV station on 33 or 23 cm.
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 meterATV freq. by publishing in local
radio clubs, contact your localARRLSCM, or pick a night and
timeto start anATV net. TheTC70-20S is portableenoughto
give a little demo at your local radio club or hamfest.
IF YOU BELIEVE THE TC70-20Sa ISN’T WORKING, check
all cables, connections, power supply, internal fuse and the reverse
polarity 16volt protection zener connected to it, board test point DC
voltagesand VSWR. Ifyou reversed thepower cable, appliedmore
than 16 Vdc or close by lightning strike, the protection zener may
have shorted before the fuse blew. You can replace it with a Radio
Shack15V 1W zener. Measure theRF OutTPDC Voltage on page
7. Ifyou can’t determinethetrouble, callusand describetheproblem
or ask any questions you might have. It will save us both time if we
suggest some things to try that may have been over-looked, or for
us to better evaluate the problem. The TC70-20S can be repaired
by us for $75 plus parts cost in a few days if we believe the problem
is customer caused, or only your shipping cost to us if we determine
thatit wasdue to ourworkmanship andmaterials withinareasonable
time and given circumstances. Include with the unit your name, call,
streetaddress -no PO box- VisaorMastercard numbers,expiration
dateand exactnameas onthecard, andadescription ofthe problem.
There is no other warranty expressed or implied. See our latest
catalogue for our full service and return policies.
