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Definition: Selcall
Definition: Telcall

SC2/SC3 HF Selcall/Telcall Microphone
SC51 Selcall/Telcall/GPS internal module


General Notes on Radio Modifications

(see below for radio specific modifications)

PTT operation

Most radios require and active low (ground) to operate the PTT circuitry.

The PTT line output from the SC2/SC3 does not go completely to ground as it is an output from a relay driver IC. The PTT line will go down to somewhere between 0.6volts and 1.6volts (approximately) depending on the radios internal PTT circuitry.

With most radios this does not constitute a problem while with some others strange problems can arise. If you find that the PTT is intermittent or that the transmit power output is low then we recommend that a small relay is fitted to switch the PTT.  The relay may be driven directly from the PTT output line on the SC2/SC3 - don't forget a protection diode across the relay coil !

There are also some radios (notably the Codan 7727 series) which use a POSITVE PTT (that is  12volts). In this case the use of a relay is required as above.

The SC51 has two PTT outputs. One for voice and another one for selcall. The voice PTT normally operates whenever the microphone PTT button is pressed, while the selcall PTT operates when the SC51 board is tranmitting selcalls, GPS locations, etc. Some radios can mute the microphone when sending selcalls by using the selcall PTT (eg. Kenwood TRC/TK80).

Mute and Scan lines

The comments that apply to the PTT line also apply to the mute and scan lines, but in general there a very few problems with either of these lines.

Out of the two the SCAN line is probably the line which may cause problems when used in SCAN PULSE mode. Some radios have UP and DOWN lines on the microphone socket. It is possible to pulse the UP (or DOWN) line to make the radio pseudo-scan. Try not to scan more than say eight channels as this may lead to calls being missed.

There are three major problems using the UP/DOWN lines to produce scanning:

1) Some radios use the UP/DOWN lines for other functions as well. These radios usually measure the VOLTAGE on these lines to determine what function is required.

2) Most modern radios have a large number of memory channels whcih are sometimes arranged in "banks" of say ten channels in a bank. Some radios will allow you to step through a "bank" of channels with the UP/DOWN lines while others make you step through all the channels.

3) Some radios will bypass empty memory channels when using the UP/DOWN lines while other radios make you step through all the empty ones.

Alarm Relay Driver

We acknowledge that the internal Piezo alarm in the SC2/SC3 does not generate a very loud alarm. It is possible to use the Alarm Relay Driver line to drive an external Piezo or other type of alarm such as a flashing light or vehicle horn. When doing this select LATCH ALARM or HORN ALARM in the programming sequence as required.

Better still is to use the Audio Alarm Output line to feed back into the audio power amplifier of the radio.  The audio alarm is fed down it's own line or alternatively can be fed back down the receive audio input line by making link LK1.

Microphone Drive Level

Newer versions of the SC2 and SC3 (PCB Version 3 on) have an adjustable in-built microphone pre-amp. This pre-amp is also fed with a fixed level of selcall signal so that setting the voice level output power to a suitable position should result in an adequate selcall output power level.

Radio specific modifications

Note: not all these modifications have been tested by us!

SC2/3 - Codan X2 modifications

SC2/3 - Icom M710 modifications

SC2/3 - Kenwood TRC-80 modifications

SC2/3 - Kenwood/Trio TS-50 modifications

SC51 - Kenwood TRC-80 modifications

SC51 - Kenwood TK-90 modifications

SC51 - Yaesu FT897D modifications


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Jenal Communications design and manufacture a wide range of radio related devices including units which use the standard HF selcall and telcall signalling as used by Barrett, Codan, Icom and Q-Mac. Thousands of these products are being used all around the world by Governments, NGOs, Commercial organisations and Amateur Radio operators. We also manufacture a number of devices which use the common DTMF (or touchtone) signalling system.
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Last modified: 23 June 2015.