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AGC to fast

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  • Dave Campbell
    Hello all, I wonder if anyone has experienced what I have with my 897 (1.5 yrs old) Agc fast and slow settings work but slow is not quite slow enough. Strong
    Message 1 of 2 , Oct 31, 2004
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      Hello all, I wonder if anyone has experienced what
      I have with my 897 (1.5 yrs old)
      Agc fast and slow settings work but slow is not quite
      slow enough. Strong signals are a little distorted. I have
      to use the attenuator and/or back off the rf gain
      considerably. If I A-B it with my IC706 the difference
      is night and day. The 706 with full rf gain and even with the
      pre-amp on produces very smooth audio.
      Is the agc adjustable through the service menu or perhaps
      an internal pot to tweak?

      Thanks,
      Dave
      VE3ZZY
    • Jochen - DG2IAQ
      Hello. Unfortunately my FT-897D hasn t arrived from eBay yet. But I used the sparetime and had a closer look to the FT-897 servicemanual these days.
      Message 2 of 2 , Nov 13, 2004
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        Hello.

        Unfortunately my FT-897D hasn't arrived from eBay yet. But I used the
        sparetime and had a closer look to the FT-897 servicemanual these
        days.

        http://dw.ham.dmz.ro/downloads/yaesu/ham.dmz.ro.sch_serman_ft-897.pdf

        As the circuits are similar on the models FT-8x7, we can use my FT-
        817 AGC-fix for the FT-897 too. It should be easy done on the FT-897
        as the parts are seen without the need to remove a PCB unit and the
        soldering should be done as easy too.


        Slower the "AGC SLOW" time constant
        -----------------------------------

        - C1503 (1µF) -> 4.7µF parallel

        and maybe
        - R1479 (10k) -> 1k parallel

        Servicemanual, page 23, upper middle (below socket J1005)


        You can locate both of these parts when you open the FT-897 and have
        a look to the MAIN UNIT. The VFO looks toward to you, the backward
        sockets are still on the backward side.
        So you have to look just below the PR DATA socket (J1005). You should
        see 2x SMD electrolyts on the lower left corner (left of them is
        crystal CD1002) and 2x 5-pin ICs (Q1100, Q1095) on the lower middle
        side of the PR DATA socket.
        So the right one of those both electrolyt is C1503. On it's upper
        right edge there's R1479 going horizontally to Q1100 on it's right
        side.
        Or in other words again. Locate the lower left corner of PR socket
        J1005. You should see the crystal (or resonator ??) CD1002. Now from
        left to right: CD1002 -> C1491 -> C1503 (!!) -> R1479 (!!) -> Q1100.

        Everything OK !? I think so....

        Adding a parallel capacitor to C1503 had solved the "too fast AGC" on
        my FT-817 well but sometimes I would like an even slower AGC. But
        here there should be enough room for experiments. A value of 4.7µF
        should be a good start (..or the final solution, hi.). Just monitor
        some "problematic stations" on SSB and hold a small electrolyt or
        tantalum parallel to C1503 while listen to the RX sound. Then you
        should hear the improvement. You can try a 10µF too of course, but
        the S-Meter would get slow too (never forget) !

        If the stations still would produce some audio distortions while you
        just have 4.7 or 10µF parallel you could hold a resistor of 1k
        parallel to R1479. This might reduce only the strong signals a little
        bit.

        When I will get my FT-897D I will do that mod too (as I expect the
        same bad SLOW AGC like on my FT-817). Then I'm able to take some
        digital pictures and write a better modification sheet.

        Please tell about your experiences here with that mod !!

        73,
        Jochen --DG2IAQ--




        --- In FT897@yahoogroups.com, "Dave Campbell" <wcampbell@o...> wrote:
        > Hello all, I wonder if anyone has experienced what
        > I have with my 897 (1.5 yrs old)
        > Agc fast and slow settings work but slow is not quite
        > slow enough. Strong signals are a little distorted. I have
        > to use the attenuator and/or back off the rf gain
        > considerably. If I A-B it with my IC706 the difference
        > is night and day. The 706 with full rf gain and even with the
        > pre-amp on produces very smooth audio.
        > Is the agc adjustable through the service menu or perhaps
        > an internal pot to tweak?
        >
        > Thanks,
        > Dave
        > VE3ZZY
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