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69599Re: [softrock40] Bad chip?

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  • Alan
    Oct 30, 2012
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      ----- Original Message -----
      Subject: [softrock40] Bad chip?

      > I'm running the tests for the tx opamp stage of a tx/rx ensemble build, per the instructions at
      > http://www.wb5rvz.com/sdr/ensemble/08_tx_opamp.htm
      > Everything checks out fine except for 2 voltages. Pin 1 of U6 is reading 5 volts instead of 2.5, and pin 7 of that same chip is
      > reading near 0, instead of 2.5. This is unbridged, with or without the USB connected, with power.
      > It has passed all prior tests, including receiving a signal in CW over the air. I have continuity-tested every pin of both chips
      > U6 and U7 to make sure each pin connects to each solder joint, and I have checked that no pin shorts to any other pin, except pin
      > 3 to pin 5 on both chips, as they should. Pin 1 to Pin 6 shows 10k resistance, as it should, on both chips. I have double-checked
      > the value of every resistor in the TX circuit. I even pulled R21 to do a direct measure because it was reading about half its
      > actual value plugged in. Pulled it read 2.21k as expected. Pin 1 and 7 of U7 both read correct values of 2.5 volts. So my question
      > is, do I have a bad chip? Something else I can check?
      > The voltage to R23 hairpin is 2.5, as is R15 and R16, R24, R22, R21, R20, R19 and R18. Only R17 reads 5 volts. I can't see how it
      > is coming anywhere but from the chip.
      > R25 is -1.2, R26 is 2.5, R27 is 2.5, and R28 is 1.6. The weirdest one there is R25, so I checked the other side of it and it
      > reads -.9. The last chip is uninstalled at this point, but I'm not sure where the negative voltage is coming from.

      Where have you got the meter negative clip?

      73 Alan G4ZFQ
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