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  Modifying the Motorola MICOR Station Audio and Squelch board (TRN6006 series) for better duplex service in a base or repeater station
By Kevin K. Custer W3KKC
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After working with several MICOR base/repeater stations and mobiles converted for duplex repeater service, it became clear that the mobile audio and squelch board worked better in duplex service than the one intended for the base/repeater station. Why? The mobile audio and squelch is internally wired differently. The biggest difference is with the circuitry associated with IC-202.  The original configuration of the Station Audio and Squelch board always seems to let some squelch noise through when the station is squelched.  The use of this modification will eliminate that.  This modification will configure the board to mute the audio just like a mobile board does.

The second part of the modification involves rewiring the COS signal to come from the RUI pin (10) of the famous MICOR Squelch chip (IC 202).  It is beyond me why Motorola didn't use this logic signal in the Station originally, as it's obviously the best place to get quality COS logic.

If you are having trouble locating a Station Audio and squelch board, or the one you have doesn't work properly, consider rewiring a Mobile Audio and Squelch board for use in your station. Go here for information on using a Mobile Audio and squelch board in the Station.

Just for information, the mobile audio-squelch board is model number TLN4310A or B. The station board is model TLN6006A or B). If you find a TRN5716APR you have a board that was configured for use in a 900 MHz MICOR mobile, and these boards are different as the 900 MHz radios use 2.5 kHz deviation where low band, high band, UHF and 800 MHz use 5 kHz deviation. Anyone that runs 900 MHz MICOR radios will be happy to trade you for that TRN5716 series board.


  1. Rewiring the shunt switches will allow more effective muting of the audio path.
  2. Modify board to use "And Squelch". And Squelch is very helpful in eliminating the squelch noise burst when using the Motorola PL decoder board.

Presented here are step-by-step instructions on how to convert a TRN6006A (the most common station audio and squelch board) for better duplex use in a base or repeater station. Here's a schematic of that board.
Do Not Attempt to convert the board without a Motorola manual explaining the TRN6006 board. Foil traces must be cut in order to redirect the signals on the board, so be careful.

Note: The manual labels the row of 18 pins along the one edge of the board as the P903 connector, and the PL Decoder connector (6 pins or 9 pins) as the P201 connector so these modification instructions use those designations.


  1. Cut the circuit trace on the component side of the board leading from IC 202 pin 6 somewhere between the IC and the edge of the board.
  2. Connect a 100 K to 120 K ohm resistor from IC 202 pin 6 to ground at a convenient location.
  3. Connect a jumper from IC 202 pin 10 to the trace that was cut leading out to P903 pin 8.
  4. Connect a jumper wire from IC 202 pin 6 to the intersection of C211 and C213.
  5. For "AND squelch" cut jumper JU204.   Nothing will be connected to IC 202 pin 8.     See this web page for an explanation of "AND squelch."
  6. Cut IC 202 pin 13 foil trace after trace connects to C235, a .22 uF capacitor. IC 202 pin 13 will only be connected through C235 to ground.
  7. If you don't want to have to run the Station Control Module in "PL Disable" mode to listen to carrier-only signals, cut the trace leading from IC 202 pin 14. This disables the PL Enable lead on the squelch chip allowing either PL or Carrier signals to be heard on the local speaker regardless of the position of the PL Disable switch on the Station Control Module.

COS will now be recovered from P903 pin 8 (The Receiver Unsquelched Indicator or RUI, now connected to IC 202 pin 10), logic is inverted (zero volts unsquelched, and +6 to 9 volts squelched. IC 202 pin 10 will have a 2.2 uF cap from this pin to ground.

If you need positive logic (zero volts when squelched and a open when unsquelched), you will need to add a simple single transistor inverter. Instead of doing step 3 above, simply connect a 10K resistor from IC 202 pin 10 to the base of any common NPN transistor (2N3904, 2N2222, M9642, etc). Connect the emitter of this transistor to ground and the collector to P903 pin 8. This can all be done neatly, 'dead bug' style, on the solder side of the circuit board.

Positive logic as described above can be used if you are using some of the OEM MICOR cards as these require positive logic to operate properly. This will allow you commercial guys to continue using your squelch gate modules and community tone cards with the added benefit of fully muting the local speaker audio.

There should be a single .22 uF cap. from IC 202 pin 13 (Channel Activity Indicator) to board ground. This cap (C 235) should already exist from the factory. DO NOT load that pin with any additional circuitry as it will upset the operation of IC 202.

Remember to cut JU201 if using the PL board, and want to filter the PL tone out of the audio path to the speaker.   Optionally, cut JU202 when using the PL board to correct for audio response.  This can be done to taste.

If you need a way to feed good audio to a repeater controller and don't want to upset the way the MICOR feeds audio to the speaker, check out the MICOR Muteboard web page. The use of the MICOR Muteboard allows the original MICOR audio circuitry to be used to feed a local speaker with no repeat audio level interaction.

This modification was developed by and this web page is Copyright © by Kevin K. Custer W3KKC, January 10 1998
Hand-coded HTML was rewritten and is © Copyright 2004 and date of last update by repeater-builder.com.

Page updated July 28, 2001 to include Step 7.
Page updated September 1, 2005 to explain the COS modification.
Page updated November 30, 2005 to explain the OEM COS modification.

Comments welcome, email kuggie /at/ kuggie /dot/ com

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.