Duplex Modification of the
MICOR® mobile "audio & squelch board"

By Kevin K. Custer  W3KKC


Concept:
Make the mobile audio & squelch board less susceptible to RF when using a mobile converted to repeater station, and allow audio & squelch board to operate properly in a duplex situation.

Description:
By adding some small bypass capacitors, this modification will allow the audio & squelch board to operate properly in the presents of strong rf field. This modification was done by Motorola in their Police and other mobile repeaters. I have found this modification useful. This modification is not necessary in basestations, or repeaterstations. Secondly, you may want to modify the operation of the audio & squelch board to suit your needs.

Benefit:
Improve operation of audio & squelch board in mobile converted to repeater station, and allow audio & squelch board to operate independently from other circuits when operated in a duplex situation.

Modification:
On V.H.F. models with TLN 4310 A & B circuit board: Add  bypass capacitors between the following places on the two ic's on the audio & squelch board. On IC-201 (M6708), add a 15 pF cap. between pins 3 and 4, and one between pins 3 and 5.  On IC-202 (M6709), add a 15 pF cap. between pins 5 and 9, and one between pins 5 and 13, also on IC-202 add a 33 pF cap. between pins 5 and 15. Radio Shack sells a package of 50 small disc capacitors that work very well for this, RS part # 272-806. The package sells for $3.29
Add the capacitors to the solder side of the board keeping all leads as short as possible.
Refer to this layout for detail in adding the capacitors.

Cut circuit trace leading from IC 202 pin 11 near the ic. Nothing will be connected to IC 202 pin 11. 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, & the 15 pF you just added, to ground.

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 explanation of "Reverse Burst & And Squelch."

Cut out CR 203, or the jumper wire leading to it.

Either connect a switch to ground, or ensure no connection, by cutting circuit trace, to IC 202 pin 14 as this pin controls the on / off action of the PL. If using an outboard controller to manage the functions of the repeater simply cut the circuit trace leading to IC 202 pin 14, near the IC.

If original PL decoder board is being used, connect jumper wire between pins 1 and 4 on the main interface pins (P 903) to supply voltage the the decoder at all times.  Some boards have holes near the pins for the jumper and some do not.

On U.H.F Models: Add bypass capacitors between the following places on the two ic's on the audio & squelch board.
Add a 100 pF cap. between P903 pin 5 and P903 pin 6. The P903 pins are the 18 long pins along the one side of the board that connect to the interconnect board.
Refer to this layout for detail in adding the capacitors.

On IC-201 (M6708), add a 15 pF cap. between pins 3 and 4, and one between pins 3  and 5. Also on IC-201, add  four 100 pF caps. between pins 6, 7, 11, and 13 , and ground. On IC-202 (M6709), add a 15 pF cap. between pins 5 and 9, and one between pins 5 and 13. Also on IC-202, add a 33 pF cap. between pins 5 and 15.
Radio Shack sells a package of 50 small disc capacitors that work very well for this, RS part # 272-806. The package sells for $3.29
Add the capacitors on the solder side of the circuit board keeping all leads as short as possible.

Cut circuit trace leading from IC 202 pin 11 near the ic. Nothing will be connected to IC 202 pin 11. 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, & the 15 pF you just added, to ground.

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 explanation of "Reverse Burst & And Squelch."

Cut out CR 203, or the jumper wire leading to it.

Either connect a switch to ground, or ensure no connection, by cutting circuit trace, to IC 202 pin 14 as this pin controls the on / off action of the PL. If using an outboard controller to manage the functions of the repeater simply cut the circuit trace leading to IC 202 pin 14, near the IC.

If original PL decoder board is being used, connect jumper wire between pins 1 and 4 on the main interface pins (P 903) to supply voltage the the decoder at all times.  Some boards have holes near the pins for the jumper and some do not.
 
 
 

Copyright©  1-25-98 Kevin Custer W3KKC

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