Kenwood rigs usually stand out for their audio quality. This modification slows the RC time constant of the AGC (automatic gain circuit) in the TS-450S and TS-690S family of radios to remove some of the noise and distortion that was dominant with the standard values.

With reference to the schematic diagram, resistor R146 on the IF board acts as a bleeder/discharge resistor for the various capacitor values switched in by IC7. Increasing this resistance value, slows down the discharge of the AGC, resulting in much smoother sounding receive audio.

You need appropriate tools to work on a surface mount pc board. Turn the radio upside down and remove the bottom cover. With the main tuning know pointing towards you, the board on the right side is the IF board. Along the left side of the IF board, find capacitors C114 through C119 towards the back of the rig. The surface mounted components are located on the foil side of the board underneath this section. Disconnect the 2 ribbon cable jumpers and the 8 screws holding the IF board in place. Carefully flip the board over and locate a small SMD resistor marked with 474 on the foil side underneath the cluster of electrolytic capacitors (C114 - C119).

With an Exacto type knife, carefully cut one of the traces in series with the 474 resistor (R146). Solder a 2 Megaohm resistor across this cut.

This will increase the RC value from 2.2 seconds to 11.6 seconds (not really that long). Some hams may want to use a variable resistor or a switch to vary or switch the RC time constant.

Re-assemble the board, jumpers and bottom cover and enjoy the modification.

Disclaimer: Perform this modification at your own risk, and be aware that it may void the warranty on the radio. I assume no responsibility for any consequences of performing the mod. I have performed the mod to my personal radio (TS-450S) and am very satisfied with the results

73,
Woody AK2F
ak2f@qsl.net"