PC Electronics tc70-20 User Manual

P. C. Electronics 2522 Paxson Lane Arcadia CA 91007-8537 USA ©2001

Tel: [phone] m-th 8am-5:30pm pst (UTC - 8) Tom (W6ORG) & Mary Ann (WB6YSS) O’Hara

24 hr FAX order line 1-626-447-0489 Email: [e-mail]

Web site: http://www.hamtv.com

TC70-20 70 cm ATV TRANSCEIVER

USERS MANUAL

The TC70-20 transceiver is designed to provide over 20 Watt p.e.p. (sync tip), continuous duty, of
AM video modulated RF in the 70 cm (420-450 MHz) Amateur band in transmit, and downconvert the
whole band to TV channel 2, 3 or 4 to enable receiving on any standard NTSC television set. Any
licensed code free Technician class or higher Radio Amateur may operate this transmitter in accordance
with 47 CFR part 97 of the FCC Rules and Regulations. The TC70-20 accepts U.S.A. standard com­posite video (1 volt pk-pk) from any source such as color or black and white TV cameras or camcorders,
VCRs, or computers for transmission. Audio from these sources or a low impedance dynamic mic is
also transmitted on the 4.5 MHz FM sound subcarrier.

PLEASE read through this manual before plugging in any cables and attempting operation. Each
connector and control is described here to enable your proper hookup and operation. Also the unique
video practices associated with ATV and the 70 cm band are described.

1 ©2001

REAR PANEL:

Keep 3”
minimum
clearance
around the
heat sink
and top of
box for
convection
cooling or
add a fan.

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 TC70-20
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.
Y ou will need a 13.8 Vdc regulated power supply capable of
4 Amps continuous. There is a 5 Amp fuse inside the unit.

TV. The downconverter output is at this jack during receive
for connection to your TV receiver antenna input. Run a 75
Ohm RG6 coax with F connectors. Radio Shack has cables
made up in various lengths (15-1530). If your TV does not
have a 75 Ohm coax input, use a balun at the TV (15-1 140).
Tune the TV to channel 2, 3 or 4 depending on which ever is
weakest or not on the air in your area. With the TC70-20 still
off, fine tune the TV set, with the AFC off, for all snow and no
adjacent channel feedthru. Now turn on the TC70-20 and
slowly tune the REC TUNE knob for the best picture from a
known close by ATV station. It is set up for 434 MHz to be at
channel 3 with the knob between 3 and 5 on the dial. 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.

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 50Ω 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 1995-2000 ARRL Handbook Chapter 20 or commercially
made antennas like the Old Antenna Lab 5L-70cm, DSFO­ATV-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:

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

TX FREQ. Transmit crystal frequency switch select.
VIDEO INPUT. This input accepts any standard NTSC

composite video into 75Ω 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 of f 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.

2

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
of monitor, you might try having a distant A TV receiving station
describe your picture back to you over 2 meters. See monitor
output paragraph.

LINE AUDIO GAIN control. Nominal input is .1 to 1 Vp-p.
This control is independant of the mic audio gain. The mic
and line audio is mixed in the subcarrier generator. In the of f
position, the whole sound subcarrier board is turned off.
MIC GAIN control sets the level from the low impedance mic
jack. This 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
the camera or external audio inputs to enable greater pickup,
commenting while running video tapes, etc. Mikes must have
a shielded cable to prevent RF pickup hum and buzz. Some
electret and amplified mics 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. Presently Radio
Shack makes 2 different replacement remote-control dynamic
omnis for portable recorders (33-2001 & -1067) that work
well and some provide the “push to look” plug also. The 33­2001 has a wind screen which is preferred for portable work.
The unidirectional 33-3015 is used for full duplex to minimize
speaker feedback.

PTL submini jack. Push To 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 PTL jack. The red
lamp above this switch will light whenever you are in the
transmit mode.

POWER ON switch turns on the applied +12 to 14 Vdc to the
TC70-20. If the green 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 regulator on the
TVC-2G for a short and replace if necessary

INTERNAL CONTROLS

Your TC70-20 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
20 to 24 Watts p.e.p. with 13.8 Vdc applied - a RF power
meter will show the blanking pedestal setup of 11 to 13 W atts
with no video applied, or less under video modulation. Refer
to the board layouts on pages 6 & 7 for pot locations.

RF POWER OUT 250 Ohm pot on the end of the TXA5-70
exciter board controls the drive to the M57716 power module.
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 TXA5-70 board 1K
pedestal pot CCW for maximum output first before adjusting
the peak power output, then reset the pedestal pot to 60% of
peak. Y our T ransceiver may go as high as 28 W atts at full RF
pot CW, but decreased linearity and sound sync buzz may
occur above 20 Watts. Going from 20 to 28 W atts p.e.p. gives
insignificant change at the other end anyway.

OPERA TING NOTES: A TV practices are somewhat dif ferent
from the other bands and modes. Since we must use
directional antennas to make up for the 23 dB higher noise
floor difference compared to NBFM due to receiver 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. This is why many ATV contacts are initiated
by calling or listening on a 2 meter FM simplex ATV
coordination frequency (146.43 for 434.0 & 144.34 for 439.25).
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
experience the correlation between 2 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
and swinging the 70cm beam looking for sync bars or listening
to TV speaker noise. 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 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 meter ATV freq. by publishing in local
radio clubs, contact your local ARRL SCM, or pick a night and
time to start an ATV net. The TC70-20 is portable enough to
give a little demo at your local radio club or hamfest.

IF YOU BELIEVE THE TC70-20 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
voltages and VSWR. If you reversed the power cable, applied more
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
Shack 15V 1W zener. If you can’t determine the trouble, call us and
describe the problem 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
TC70-20 can be repaired by us for $50 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 that it was due to our workmanship and
materials within a reasonable time and given circumstances. Include
with the unit your name, call, street address - no PO box - Visa or
Mastercard numbers, expiration date and exact name as onthe card,
and a description of the problem. There is no other warranty
expressed or implied. See our latest catalogue for our full service
and return policies.

3