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  Modifying a MICOR mobile Audio & Squelch board for use in a base or repeater station
By Kevin K. Custer W3KKC
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This article describes how to modify a common mobile audio-squelch board (TLN4310B) for use in a base or repeater station. The statio and in addition, can be rather scarce in some parts of the globe. This mod solves both problems.

After working with MICOR base or repeater station and mobiles converted for duplex repeater service it became clear that the mobile audio & squelch board was better suited for duplex service than the original one intended for the base or repeater station. Why? The mobile audio & squelch is internally wired differently. The biggest difference is with the circuitry associated with squelch IC (IC-202), and the interconnect pin configuration. The original station Audio & Squelch board always seems to have some squelch noise when the station is squelched.  The use of this modification will eliminate that.   I have used this modification for years.

1. The shunt switches are wired to more effectively mute the local audio path (speaker).
2. Some times the bi-level squelch feature does not work correctly on the base/rptr board.
3. Modify board to use "and squelch". And Squelch is helpful in eliminating the squelch burst when using the factory PL decoder board.
4.  The mobile boards are easier to come by.

Presented here are step-by-step instructions on how to convert a TLN 4310 B (the most common mobile audio & squelch board) for use in a base or repeater station.
Do Not Attempt  to convert the board without a Motorola manual explaining the TLN 4310 board, or download the gif files for assistance on conversion.

Audio & Squelch layout (click on it for a larger image)

Audio & Squelch schematic (click on it for a much larger image)

Because the interconnect pins are wired differently on the mobile board, foil traces must be cut in order to redirect the signals in and out of the board, so be careful.

Traces that are cut near the pins of P903 will be done at about 1/4 of an inch from the pins, then we will refer to the separated traces as inside and outside world.   Inside world connects to circuitry orparts on the board, outside world connects to interconnect pins leading off of the circuit board.

Note: The 18 pins along the one end of the board are referred to as the P903 pins.
The 6 pins along the side that connect to the PL decoder are referred to as the P201 pins.

1. Cut circuit trace leading from IC 202 pin 11 near the IC chip. Nothing will be connected to IC 202 pin 11.
2. Cut IC 202 pin 13 foil trace after trace connects to C229, a .22 uF cap. IC 202 pin 13 will only be connected through C229 to ground.
3. For "and squelch" cut unmarked jumper leading from IC 202 pin 8, to P201 pin 3. Nothing will be connected to IC 202 pin 8. See this explanation of "And Squelch."
4. Cut out CR 203, or the jumper wire leading to it.
5. Cut foil traces leading from P903 interconnect pins 5,7,10,11, and 14, about 1/4 inch from the pin.
Note that the P903 pin 11 foil will be cut on the component side of board.
6. Jumper P903 pin 5 inside world (C204 outside world) to P903 pin 11 outside world.
7. Jumper P903 pin 3, (unaltered), to P903 pin 7 outside world.
8. Replace C 203 (a .22 uF) with a 15 uF non-polarized capacitor.
9. If using a factory PL board, jumper P201 pin 3 to P903 pin 10 outside world. See PL note below.
10. If using a factory PL board, jumper P903 pin 1 to P903 pin 4, to supply unswitched 9.6vdc. to PL board.

Nothing is connected to P903 pin 5 outside world.
Nothing is connected to P903 pin 7 inside world.
Nothing is connected to P903 pin 10 inside world.
Nothing is connected to P903 pin 11 inside world(top side).
P903 pin 14 trace is simply severed as nothing connects to either side of the cut.

PL Note:
As modified, the PL decoder output is routed through the audio & squelch board, to the backplane board, this is done by the  use one of the unused "outside world" pins that has continuity to the backplane board. I have used P903 pin 10 outside world in several units with success. Insure that you cut the trace leading into the web of the backplane circuitry, as not to disturb something else. If you are not using the factory PL board just skip steps 9, and 10.

Explanation Of Reverse Burst & "And Squelch."

A good place to recover COS is on P903 pin 8 (RUI), logic is inverted. This pin delivers zero volts unsquelched and 6 to 9 volts when squelched. NEVER connect anything to Channel Activity Indicator, IC 202 pin 13, this will upset the operation of IC 202. There should be a .22 uF cap. from IC 202 pin 13 to board ground. This cap (C229) should already exist.

Remember to cut JU201 if using the factory PL board, and want to filter the PL tone out of the audio path to the speaker.

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. 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.

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This modification sheet was designed, prepared and is Copyright © by Kevin K. Custer  W3KKC, Jan 10 1998.

Some HTML by Mike Morris WA6ILQ
Page last updated 17-Aug-2004