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55151Re: Op amp troubles....I think?

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  • VA3XCD
    May 2, 2011
    • 0 Attachment
      The saga here continues and I am getting more and more confused as time goes on....

      I ordered a new LT6231 op amp chip from Tony and it arrived today.
      I soldered it in place, the correct way this time :), and went on to do the testing. I began to check the pins, both on the pads as well as at the pins. All the voltages checked out as per spec. I then went to check the values for R33 and R32. I checked the voltage on R33 first without the 10K jumper, and got a spec value. I powered down, installed the jumper, and checked the voltage, and got a value of .003 volts. I powered down, removed the jumper, powered up again and checked the voltage at R33 again and still have .003 volts. I then re-checked the voltage on the pins of U11, and got the following values:

      Pin 1 .002
      Pin 2 .218
      Pin 3 .241
      Pin 4 0
      Pin 5 .241
      Pin 6 .147
      Pin 7 .045
      Pin 8 5.018

      All units are volts.

      At this point I went back to check the values on U10 in the QSD circuit, trying to work my way back.

      I started to check the voltages on the pins. Here are the values I was getting:

      Pin 1 0
      Pin 2 2.503
      Pin 3 .129
      Pin 4 .108
      Pin 5 .101
      Pin 6 .230

      At this point I went back to check the ones I had just checked, and I dont know what happened, but they were all back in spec, just the way they were when I initially finished the stage.

      So I went back to check U11 again, and this time right up to pin 5, the values were in spec. I checked pin 6, and it had dropped back to .142 volts. I went back to check pins 1 through 5 again, and got similar very low values I mentioned above.

      So I went back even further, with intermittent spec, and bad voltages.
      I went right back to the LO circuit to check the values on the R12 and R13, and I noticed that now the si570 will not let me enter 24 MHz for a frequency for the tests. In fact it will not go above anything past 15.99 MHz now. I dont know if that is a function of the rest of the components being in the circuit, or if something is really wrong somewhere.

      At any rate, this is the stage I am at now. I think I have included all relevant info that I can think of.....

      As I mentioned in the initial post when this all started, all test values were in spec up to the point where I installed U11.
      Now the values are intermittently good and bad.
      I have also gone as far as checking all my joints, and also wiggling components to see if I could find a bad joint somewhere. I wiggled all the inductors, and most of the components and still could not nail down a reason for the intermittency.

      Any help is greatly appreciated, especially before I pull out any more of what little hair I have left :)


      --- In softrock40@yahoogroups.com, "Alan" <alan4alan@...> wrote:
      > ----- Original Message -----
      > From: "Bill
      > Subject: [softrock40] Op amp troubles....I think?
      > > I was hoping to not have to embarrass myself again as I did with the
      > > quadrature
      > > clock problem I had, but here goes :)
      > >
      > Don't worry about embarassment. We all run that risk, I've made some silly
      > mistakes.
      > > Where I run into trouble is when I check the voltages on R32 and R33.The
      > > I have the following values:
      > >
      > > R32 NO resistor : 2.570V
      > > R32 with resistor: 2.583V
      > >
      > > R33 NO resistor : 2.567V
      > > R33 with resistor: 3.85V
      > >
      > > R33 seems to be in spec, but R32 is only slightly higher with the
      > > resistor than
      > > without.
      > >
      > > I did have an issue when I originally installed U11, where the voltages
      > > were off
      > > (the 2.5V values were reading .2xxxx instead of 2.xxxx) so thinking I
      > > installed
      > > the IC backwards again, I desoldered it, flipped it around, and
      > > resoldered it.
      > This looks as if reversing the IC may have damaged it.
      > If all joints are good you'll possibly have to change it.
      > . I did run
      > > into a
      > > similar problem with odd voltages again on U11, and found that if I
      > > power down,
      > > and power back up again, the voltages on U11 came back to where they
      > > should be.
      > Note that the voltages can vary quite a bit from Robby's pages. Maybe +/-
      > 1V. They will possibly change with frequency so a restart will be on a
      > different frequency.
      > > PC, loaded HDSDR and although it seems to be working in the software, I
      > > get a
      > > rather broad carrier on the centre frequency, and do not hear or see
      > > anything
      > > else other than that carrier. I do have a screen shot of what HDSDR is
      > > showing,
      > If the op-amp is faulty you may not see any signals. The shot seems to show
      > problems, when you get the IC fixed lookat
      > <https://sites.google.com/site/g4zfqradio/sdr-with-soundcard-basic-faults>
      > for some clues, maybe mixer settings.
      > > which I will post in the files section is that will be of any help.
      > > The file name is VA3XCDHDSDRscreenshot.jpg and is located in the root of
      > > the
      > > files folder.
      > >
      > Best put these in the photos folder, the file section is 99% full.
      > 73 Alan G4ZFQ
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