Modifying a Motorola MICOR Squelch Gate Module

For better operation in a base/repeater station.

By M. Scott Zimmerman N3XCC

Modify a Squelch Gate Module (SGM) for better/easier use when building a repeater from a station.

The unmodified squelch gate module has several drawbacks when using it as a repeater controller. The greatest of these is the use of an on board squelch control which is separate from the 'main' or local squelch control. On the station control module, the local squelch control is used in the audio gating section of the card. The on card squelch setting is what governs station keying.  The setting of these two controls is a tedious chore that takes much time and skill. The purpose of this modification is to disable the squelch control located on the squelch control module and rewire the keying section of the card to be run from the main squelch pot.

The second part of the modification changes the logic used to gate the audio signal. Typically the audio is only gated by the COS logic. In most cases, this is unacceptable. The TX audio path should be gated by the same logic that is being used to key the radio set. Doing the second part of the modification will insure that the Audio is being gated correctly for the card's mode of operation (COS or COS & PL)  

The following modification is assuming that you are intending to use the SGM in a typical MICOR station situation where COS logic is active high and a factory PL decoder is being used. Special modifications for use with a TS-32 / 64 type CTCSS decode deck can be found below.

A manual showing the schematic and parts layout is a MUST for any successful modification!

One Squelch Modification procedure:

  1. Remove Q6 and Replace it with a M9642. This transistor can be removed from Q1, Q2, Q3, Q4 or Q5 positions since they are no longer used.
  2. Add a 22K resistor from the base to emitter of Q6 to eliminate possible RF interference.
  3. Remove R28 and replace it with a jumper.
  4. Remove R24, R27 and C9
  5. Remove JU15. This is a long lead with a pin connector on the free end. Be careful that you remove the correct one.
  6. Add a 10K resistor from the collector of Q6 to A+. A+ can be found at one end of R27
  7. Jumper Pin 5 of the main connector (COS) to the junction of R24 and R25.

Audio Gating Modification procedure:

  1. Remove CR21.
  2. Jumper from the Anode of CR17 to the anode of CR20.

Jumper JU14 now solely controls PL / COS keying operation of the squelch gate module. The PL option is only available if an OEM decoder is installed in the station. If the card is run in the PL mode, AND squelch will be implemented. This lead (JU14) can be run to a front panel mounted switch if desired. Wire the switch so that in the COS position the lead is connected to the A+ line. In the PL position the lead is unconnected.

If you are using a TS-32 / 64 instead of a factory PL decoder:

  1. Remove JU3 and JU4.
  2. Replace JU3 with a 4.7K resistor.
  3. The card's PL input is pin 4. Connect the white wire from the TS-64 to Pin 4.

The TS-32 / 64 needs to be set for active high logic. This is the factory setting for a TS-64.

Optional Steps:

Several indicator LED's may be added to aid in troubleshooting or verify proper operation. A LED can be wired from the collector of Q6 to V+ through a 2.2K resistor to indicate when a valid COS is present. A LED can be wired from JU14 to ground through a 2.2K resistor to indicate when PL is being decoded. This indicator will be on when the card is being run in the COS setting.

Squelch setting procedure:
Setting squelch with a modified Squelch Gate Module is quite simple. You simply set the main squelch control as you normally would. The modified card will use this setting, including the famous MICOR Bi-level squelch.

This modification was prepared by M. Scott Zimmerman; January 5, 2006
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