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CTCSS Mod.

[Under Construction]

Modification for user-defined CTCSS with open receiver.

Note 1: Applicable to FM1100 only.

Note 2: Using PDP or CDP programming, user-defined CTCSS has to be enabled in the FM1100.

When user-defined CTCSS is used on the FM1100, the tone detection and generation system applies to the receiver and the transmitter, and can not be switched off individually for each of these. In many cases, it will be desirable to transmit user-defined CTCSS only, yet leave the receiver open to all received signals.

Two hardware modifications are required to disable CTCSS for the receiver only:

A. Pin 19 of IC501 has to be disconnected to permanently pass audio signals through this IC.

B.  Pin 6 of IC303 requires a pulse to be able to flag reception of a valid CTCSS tone.

Background to the modifications

A.  When pin 19 of IC501 left open-circuited, it is internally pulled to +5V, forcing the IC to always pass the audio signals. (If the control pin is pulled to ground, the IC only passes audio signals if a valid CTCSS tone has been detected).

B.  When IC501 detects a valid CTCSS tone, it supplies a pulse to pin 6 of IC303. Instead of this, a different pulse has to be found. The requirement for this pulse is that it is not constantly present, but comes back after the CTCSS settings have been changed (or switched on again).

Initially, the pulse controlling the 'key beep' function was employed. However, its disadvantage is that, after the radio has been switched on, no pulse is generated until a key is pressed.

The alternative was to employ the 'noise level' signal (pin 1 on SKTB) and apply it to an inverter (IC315, pin 3, 4). The inverter supplies a pulse as soon as a carrier either appears or disappears. The one disadvantage of this option is that the receiver is not opened when CTCSS is switched on during reception of a signal. However the system will start to work when the first new carrier appears. In practice, this shortcoming will be insignificant.

Please note: the above modification causes the receiver to totally ignore any CTCSS tone.

Hardware Modification Summary

Unsolder pin 19 of IC501, cut off pin

Cut PCB track to pin 6 of IC303

Cut PCB track to pin 3 of IC315

Install wire between SKTB:1 and IC315:3

Install wire between IC315:4 and IC303:6

 

 

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Last modified: February 13, 2002