View Full Version : TS-140S / 680 squelch mod with pictures.

03-18-2010, 03:28 PM
Mod originally written by KC9BZX. I just wanted to post this with photos to help others with the mod because the board scans from the service manual are very difficult to read.

The mod enables Squelch control for AM/CW/SSB instead of just FM on the Kenwood TS-140S/680. Note: In AM/CW/SSB it does not completely silence audio when squelched, but reduces it significantly to a just barely audible level. It can be silenced completely by turning down slightly the AF/RF gains.

Parts required

1 each 100 uF 20 v capacitor
1 each 10k ohm 5% 1/4W resistor
2 each 1N914 small signal diodes


All modifications are to the signal board. Remove the signal board
to gain access to the bottom of the board.

Remove R178 100 ohm resistor. (I cut the land between C177 and R178)
(Removes FM chip control voltage) R178 is on the bottom of the

Add jumper with a diode in series between IC6 pin 14 and the base of
Q57. Cathode of the diode goes to the base. (This is the squelch
trigger control voltage) Q57 is on the bottom of the board.

Connect a diode between the 8V line (very near IC6) and pin 4 of
IC6. This activates the FM chip (IC6) all of the time. Cathode goes
to pin 4, anode to the 8 volt line.

Connect a 10 k ohm resistor between IC6 pin 4 and IC6 pin 14.

Connect a 100 uF 25 volt capacitor between IC6 pin 14 and ground.
(This slows this quick squelch down a little bit)
Tests have shown (using commercial communications monitor) that
activating FM chip IC6 does not degrade IF strip performance at all.


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