Modifications for the Kenwood TS-680

TS680s Squelch mods - adds squelch to all modes

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 board.

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.

I have had this mod in my 680 for a bout year and a half and is still working great!
TS-680 6 Meter power output increase

Adjust VR15 (50M) control on the SIGNAL UNIT board while monitoring 6 meter transmit power out put into a 50 ohm dummy load. I adjusted mine while also monitoring the ALC level to insure that I still had some ALC action. Using this method I have increased my 6 meter power output to appx. 25 watts.
Broadcast Sensitivity Modification for the TS680S / 140S

Between .500 and 1.6 MHz attenuation is added to reduce? front-end overload on the Broadcast band. Some of us BCB DXers find this a HIGHLY undesirable feature. If I want attenuation I will use the front panel switch. The following mod will restore the sensitivity -
Cut the land between IC1 pin 11 and R7 (SIGNAL UNIT, M54581P)

Cut the trace between IC1 pin 12 and R10

Add a small signal diode (1N914, 1N4148) between IC1 pin 11 and R7 (Cathode towards R7)
Add a small signal diode between IC1 pin 12 and R7 (Cathode towards R7)

Trace the lands from pin 11 and 12 of IC1 and you will find a good place (to the left of connector CN5) to cut these traces as well as pad to solder the diodes to.
TS680 Freq Extend mod

This mod will allow out of band transmit on the kenwood ts-680-s.
Remove the bottom cover. In the middle of the board is a diode marked d-31 just clip it and reinstall bottom cover.

This will allow transmit from 1.6 MHz to 33 mhz and 49 mhz to 56 mhz.

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