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SDR40 v6.1 TX/RX stopped working

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  • ian mcphedran
    Hi all, I posted a message on this subject two days ago but since then have done more tests. Basically the problem is that only the tip line in channel is
    Message 1 of 47 , Jul 30, 2007
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      Hi all, I posted a message on this subject two days ago but since then have done more tests.
      Basically the problem is that only the 'tip line in' channel is now working on the RX, after the TX/RX had previously successfully completed contacts on both CW and BSPK with other stations.
      The TX/RX is drawing 110mA on receive which is way over what it should. It is mostly going into the 5V regulator, which is running hot and only giving out 4.15V.
      Q9 is running very hot with 3.51V on the collector and 0V on both base and emitter.
      No other IC, transistor, or resistor is running hot on receive. (I haven't checked TX)
      The TX/RX is built in an aluminium box with a lid so nothing dropped on it causing a short. Just what caused the loss of one channel is a mystery, and what is the link to a very hot Q9?
      Any thoughts would be welcome.
      Ian GM3GTQ


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    • 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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