TS-450/690S AGC Mod

Discussion in 'Amateur Radio News' started by Guest, Oct 15, 2000.

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  1. Guest

    Guest Guest

    AK2F writes:

    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.

    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


    Woody AK2F

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