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Re: SDR40 v6.1 TX/RX stopped working

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  • Len Warner
    ... 3.63V is too low for the potential divider U4, R35, R40, Q8 base, so the Q9 collector node is definitely connected to something else - I wouldn t have
    Message 1 of 47 , Aug 2, 2007
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      At 6:54 am ((PDT)) Wed Aug 1, 2007, ian mcphedran wrote:


      >I have removed Q9 again.
      >The 5V line is 4.32V, U4 is running hot, and the input current at 12.5V is
      >107mA.
      > Q9 collector pad reads 3.63V, base pad reads 0V as of course does the
      > emitter.

      3.63V is too low for the potential divider U4, R35, R40, Q8 base,
      so the Q9 collector node is definitely connected to something
      else - I wouldn't have expected the /OE inputs of Q3
      to represent a significant load, they are CMOS inputs
      and hence high-impedance, aren't they?

      When Q9 was hot, something was sourcing current to it:
      now it's gone, something is sinking a little current from the
      collector node but the high current is still flowing from U4.

      The only other stray thought I have is that you are testing
      in the aluminium box, the box is not isolated from supply,
      and there is a short. Unlikely, but...


      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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