Icom IC-720 Instruction Manual page 7

3-8 CW KEY
For CW, a plug is supplied for connection to the key. Proper
connection is shown below. In the event the terminals have
polarity, be sure to connect with proper polarity. When
switching with semiconductors or relays with resistors in
the circuit voltage must be adjusted to below 0.4 Volts.
| Key Wiring Diagram
3-9 RTTY
Accessory socket pins 8 (ground) and 9 should be con-
nected to the teletypewrite through a high speed relay or
level converter to TTL tevel, the tones for the terminal unit
are available from pin 4. For further details see the chapters
on "Use of RTTY" and "Transmit and Receive."
3-10 EXTERNAL SPEAKER
The ІС-720 has an internal speaker, but if desired an
external speaker of 8 ohms impedance can be connected to
the external speaker jack on the rear panel. When con-
nected the internal speaker is disabled.
3-11 HEADPHONES
Any good headphone set, of 4 - 16 ohms impedance may be
used. When the jack is inserted halfway both the internal
speaker and the headphones will operate - a convenience
when recording, or others are listening. A tape recorder can
be connected to the headphone jack to record contacts.
When the plug is fully inserted only the headphones will
function. Stereo headphones can be used, but in the half-
plugged position only one earphone will operate.
3-12 ANTENNA
Any amateur band antenna of 50 ohms impedance can be
used with the IC-720, wire, vertical, or rotary beam. 1 is
highly recommended that a high quality antenna be used -
the few dollars invested will more than pay for the antenna
in the longrun. Careful installation will also pay dividends,
follow the manufacturers instructions explicity. Loose
connections or connectors will cause many problems.
Since the ІС-720 has a General Coverage receiver it is
recommended that a long-wire general coverage antenna and
an antenna coupler be used. The antenna's impedance
should be 50 ohms. To attempt to use the Ham band
antenna for general coverage reception could result in
mismatching, and attendant poor reception.
In mobile operation choice of antennas is as important as
fixed operation. Be sure that the antenna's ground con-
nection is made to the car body or chassis firmly, that the
impedance is 50 ohms, and that the match is good.
3-13 POWER SUPPLY
It is recommended that you use Ше ІС-Р515 as a power
supply for base operation. If you wish to use another type
power supply make sure that it meets the voltage, current
requirements. Note carefully the overvoltage protection,
for a runaway regulator can destroy the ІС-720; be espe-
cially careful that more than 16 Volts cannot be supplied to
the transceiver. Do not connect the power supply, antenna,
accessory plug, or microphone with the Power Switch in
the on position. Be especially careful not to transmit with-
out an antenna or dummy load hooked up. If the fuse
blows replace it with a 20 Amp fuse, only after fixing the
cause. Do not turn the Power Switch on and off repeatedly
for this way cause the readout to mis-display. Should this
occur, turn the set off and wait for approximately 30
seconds before turning it back on.
3-14 CAUTIONS
As the unit has already been closely adjusted with highly
sophisticated measuring instruments, never tamper with the
turnable resistors, coils, trimmers, etc.
C-MOS is used in the Logic unit as well as the PLL. C-MOS
ICs are very susceptible to excessive static charges and over
current and care must be used when handling them. There-
fore, avoid touching the Logic unit and the nearby circuitry
unless absolutely necessary. When it is necessary to check
the circuitry, observe the following points.
Ground all measuring instruments, the soldering iron, and
other tools. Do not connect or disconnect the C-MOS IC
from its socket, or solder it when the power is on. Do not
apply voltage of less than —0.5 or more than +5 Volts to
the input terminals of the ІС. DO NOT MEASURE WITH
AN OHMMETER.
3-15 COOLING FAN
The rear of the PA unit is designed to provide for adequate
cooling, but with 200 Watts input the final stage produces
quite a bit of heat, and its temperature may rise during
prolonged transmissions. The fan is connected to a tem-
perature monitoring circuit which monitors the temperature
of the final stage. The fan operates as follows:
1. The fan does not operate normally in the receive mode,
but will run all the time in transmit.
. When the temperature rises to a point detected by the
monitor curcuit the fan will operate during both transmit
and receive to provide additional cooling.
. If the temperature rises to a danger limit the fan will
run much more rapidly. At this time stop transmitting
and inverstigate the cause of overheating i.e. antenna
mis-match, etc. Correct the cause of the overheating
before starting to transmit again.