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Re: SDR40 v6.1 TX/RX stopped working

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  • Len Warner
    ... Looking at the circuit diagram, with nothing connected to /PTT OUT then Q9 should be having an easy life thermally, even if it were partially on or
    Message 1 of 47 , Aug 2, 2007
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      At 3:11 am ((PDT)) Wed Aug 1, 2007, in Digest 1864, ian mcphedran wrote:

      >Hi, I have replaced Q9 and no improvement resulted.

      Looking at the circuit diagram, with nothing connected to
      /PTT OUT then Q9 should be having an easy life thermally,
      even if it were partially on or defective.

      >It is running hot too and the '5V' rail is 4.1V.
      > I'm still searching for the fault that's taking all that 110mA on the
      > 5v line.

      But it's getting hot, so is the 5V regulator U4 and the
      voltages are out, so there is excess supply current
      somehow, even though R35 measures ok so it
      can't be passing through that.

      Other candidates are an invisible and perhaps
      partial short to 5V, i.e. U4 pins 15-16 or a faulty U4.

      Was that 4.98K an in-circuit measurement?
      If so, then that should prove no short to the
      5V rail unless it's a voltage-sensitive one.

      It's time for some very careful inspection around the
      U9 collector circuit node, perhaps some resistance
      measurements to nearby voltage traces...

      ...and if all else fails, some judicious track-cutting
      or IC-lead lifting to isolate the possible sources.


      Regards, LenW
    • Len Warner
      ... Perhaps, but output near the +ve rail could be: a) inputs driven to output saturation b) output pulled high externally (i.e. by a pins 7-8 short) c) device
      Message 47 of 47 , Aug 3, 2007
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        >U8 86 F
        >U8 pin 7 reads 4.2V
        >
        >Ian
        At 2:44 am ((PDT)) Thu Aug 2, 2007, in Digest 1865 Tony Parks wrote:

        >Sure looks like U8 is a problem.

        Perhaps, but output near the +ve rail could be:
        a) inputs driven to output saturation
        b) output pulled high externally (i.e. by a pins 7-8 short)
        c) device failure which also emulates (a) or (b).

        Both (b) and (c) can explain the U8 heating
        but only (c) is that device's fault.
        (There's no source for the heating current in case (a).)

        Meanwhile, none of these explain why:
        d) Q9 ran hot and
        e) Q9 replacement ran hot
        This doesn't seem to be in a related part of the circuit,
        so was Ian just unlucky with two rogue 2N3904?

        [BTW, my apologies for saying that Q9 collector pad voltage
        3.63V with Q9 removed seemed too low. I recalculated as
        3.64V - within the DVM last digit uncertainty.
        (Vcc-Q8vbe) * R40/(R35+R40) + Q8vbe =
        (4.32-0.65) * 22.1/(4.99+22.1) + 0.65 = 3.64V ]

        >Let me know what components you need to restore your board
        >and I will send them right away at no cost.

        I continue to admire your outstanding support for this project;
        "May Great Leader Tony live ten thousand years!"


        Regards, LenW
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