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Since installing the 10m repeater VK7RHF, the receiver has at times suffered during sporadic E openings noise, and what seems like AM modulated signals. The assumption is that this is from north of Australia from pirate stations using AM CB radios.
Doing some further reading led me to VK6TQ's circuit for a Sawtron he had on 29MHz (similar to the KL). In a accompanying text document, he goes on to say that a lot of the noise he has seen comes from these pirate stations running odd CB channel splits. In most cases, they are 5KHz or so off most of the repeater inputs.
In short he made a circuit using a NE555 timer to use the DC voltage from the discriminator to detect when there is a negative voltage swing to switch an active high signal which could be used to disable the mute in the radio. In other words, if a signal comes in on the receiver and is more than 3KHz off frequency, the mute would not open. On frequency signals were fine. His original circuit is below -
I built this circuit up on some veroboard and it works quite well. There was only one drawback. In the KL receiver, -5KHz produces a negative voltage swing to about 2.5V. +5KHz produces a positive voltage swing. VK6TQ's circuit would only work with negative voltage swings.
I therefore thought of adapting VK6TQ's circuit to allow a positive voltage swing. I used a LM339 as a window comparator. Below is there circuit. Unfortunately it's hand drawn, but should be understandable.
+VCC is taken from the decoder IC, which in the KL is IC107 pin 4. Pin 9 of the same IC has our AF DC volts. The LM339 produces an active low output. I found it easiest to AND this with the existing NHRC-Squelch I use in the RX module with a simple NPN transistor. The dashed box has the other end of the circuit (in the squelch board) to show what it's doing. Inject a signal 3KHz lower (or whatever you choose) than what it should be. Adjust the VRef Low pot until the green LED just turns on. Now go up 3KHz above on frequency and repeat, only this time adjust VRef High. Now go to on frequency. The green LED should be off. Make sure it stays off over the +-3KHz range and the RED LED should now be lit. This indicates the mute is open. I've yet to test this on air, but hopefully it improves any slightly off channel interference during band openings.
**EDIT** I also added a 0.1uF cap between VCC and GND for power supply decoupling.