Instructions, schematic, and notes on the MS-25 MICOR Squelch Board

Custom Products from Masters Communications

MICOR Squelch Board
For Two-Way Radio Applications

Model MS-25

Actual Size MS-25 PC boards.
(Click image to enlarge)

Schematic Image.
(Click image to download a PDF copy)

Overview:
This board will provide quality logic signals for driving a repeater controller or other radio interface device like URI, RIM, or FOB. It requires discriminator audio and a source of DC power. It doesn't mute or control the audio path in any way. It uses the Motorola MICOR SC6709 squelch IC (also marked M6709 or M7716 depending on manufacturing date).

Assembly:
If you purchased our kit, refer to the parts list and make sure you have all of the components you need to build it.
Click here for Parts List.
Assembly can be done by personal choice, but you may find it easier to install the resistors first, followed by the diodes, capacitors (observing polarity on diodes and electrolytics!), then transistors (flat to flat on silkscreen). Install the IC socket, followed by the voltage regulator IC and squelch control. Install the LEDs and six pin header.

Installation:
This device is intended to be installed inside a radio or project box for protection. Refer to the schematic for the pinout of the 6 pin Molex connector where all of the logic signals, power connections, and discriminator audio input is listed.

Setting the squelch.
Once built, installed, and operating properly, the squelch can be set by turning the control counter clockwise until the "SQ" led illuminates. Then turn the control in a clockwise direction to close the squelch.

Explanation of the logic signals and other pins:
Active high and active low outputs for COS logic are available. The outputs are open collector and are available on the 6 pin connector.

A pull-down resistor is included on the logic high output. Obviously if you are using this board and need active low outputs you should be using the outputs connected to the NPN transistor, but, the on-board pull-down resistor can make circuitry respond correctly when using the active high PNP outputs.

All logic is sourced from +9.6V and ground. +6.8 volts is the maximum voltage expected at pin 6 of the header because of the current limiting resistor R8. A ground is expected at pin 4. Caution should be given to the use of this output as it is unprotected, and misuse will cause switching transistor failure. While the PNP transistor has the over-current protection of the 9.6 volt regulator, there is no protection for the NPN to ground. Be certain that your connection to these outputs don't exceed the capabilities of the board or switching transistors.

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