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35406Re: [softrock40] Re: softrock lite - troubleshooting the flipflop

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  • tmac
    Aug 3 3:14 PM
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      I knew I could count on you guys!

      I had never thought of lifting a single pin. I'll do that. I did
      recheck the chip orientation (which would be number one). It's ok.

      I just lifted pin 3 of the flip-flop. the divider voltage is 2.53 v
      now. Guess I would replace the chip now.

      I have some of the fairchild 74AC74's coming. Dont know yet if they are
      exact replacement. But looks right.

      If y'all have any experience on that, let me know. I can canibalize a
      74AC74 from the RXTX kit I have waiting on the shelf.

      Where can I get replacement parts?

      Thanks
      Tom - WA4FYN
      See Me at the Huntsville Hamfest!
      (or....dont let that stop you. ha)

      g8voip wrote:
      >
      >
      > Hi Tom,
      >
      > The title threw my as I could not find SoftRock you were talikng
      > about, but by process of elimination, got to the v9.0 RX!
      >
      > Try removing C19, then measure on the junction of R9 / R10.
      >
      > If you still have 5v on it, then try lifting pin 3 and pin 11 of U6 to
      > eliminate the device itself pulling it up.
      >
      > I use some solder braid and then a sewing needle with the soldering
      > iron to carefully lift up individual pins on a smd devices. Make sure
      > that they really are clear of the solder pads, and do not use any
      > force or you can easily pull of the PCB pad.
      >
      > Try measuring again on the junction or R9 / 10 and see what it is. If
      > it is still 5v, then start looking for the solder splashes! If it is
      > correct at 2.5v, then carefully push pin 3 then pin 11 back down and
      > see if the junction changes. You might have cleared any short in the
      > process. My bet is a short pin 3 to 4 or 10 to 11 :)
      >
      > Let us know how you get on,
      >
      > 73, Bob G8VOI
      >
      > --- In softrock40@yahoogroups.com
      > <mailto:softrock40%40yahoogroups.com>, tmac <tmac@...> wrote:
      > >
      > > Hi,
      > > This is a sanity check with y'all before I lift the f-flop chip (U6- 74A
      > > C74) off the board for testing.
      > >
      > > The voltage divider (r9 & r10) is giving trouble. The voltage at r9-r10
      > > is almost up to the 5-volt rail voltage. R9 top end is 5-volts. R10
      > > bottom end is 0-v . I've pulled the two resistors and verified their
      > > values. I've replaced the coupling (c19) capacitor that feeds the
      > > r9-r10 junction. I've inspected every solder joint for bridges to other
      > > components, and scraped between them with a pick.
      > >
      > > The mid point of the divider should be 5 volts. However, there is that
      > > si570 signal feeding thru the cap. Is a dc offset of the signal pulling
      > > up the metered voltage? I really doubt it.
      > >
      > > My conclusion is that the flipflop clock input is pulling the voltage up
      > > somehow. So, I've ordered some fairchild chips (couldnt find Ti) to
      > > replace the one I will lift for test in case the chip is the source of
      > > the pullup.
      > >
      > > Just thought I'd ask for the symptomatic information before I end the
      > > life of a chip.
      > >
      > > "It's all fun." It builds character." And still having fun.....
      > > Have a great day!
      > >
      > > Tom - WA4FYN
      > >
      >
      >
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