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SDR II Lite Kits.

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  • David R. Hassall
    Dear Members, I hope I can get some help here for the SDR II Lite kits. I am a Mentor for a Group of Kids building a Soft Rock Lite II kit and we have run
    Message 1 of 1 , Nov 14, 2012
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      Dear Members,

      I hope I can get some help here for the SDR II Lite kits. I am a Mentor for
      a Group
      of Kids building a Soft Rock Lite II kit and we have run into a problem that
      just
      doesn't make any sense.

      On one Kit we are having a problem with the 2.5 VDC bias that is generated
      by Resistors
      R3 and R4 from the +5VDC Bus. It reads .24VDC. Which indicates that
      something is shorted.
      At first we suspected a bad OPAMP chip since the 2.5VDC went to pins 3 and 5
      to bias the
      + input of the OPAMP. I was able to open the trace of the bias feeding
      those two Leads
      but the Bias stayed at .24 Volts. So I went the other way towards U3 and
      the transformer.
      I detected about a .02 voltage drop on R1 and R2 that meant that U3 must be
      drawing current
      of about 2ma pm both gates. Since the voltage divider of the 2 1,000 ohm
      resistors (R3 and R4)
      only has 2.5 ma to make the 2.5 VDC bias... a current through R3 of 4.5 ma
      would create a
      voltage drop of 4.5 Volts. So, I thought the U3 might have been damaged to
      static and
      It was causing the extra current in the gates. I removed U3 and measured
      the voltage at
      the R3 test point. It measured 2.5 volts. I installed another FST3253 from
      an unopened
      kit. Measured the voltage at R3 test point and it was back to .24V.

      So, To keep from chasing my tail, I thought I would ask the group if anyone
      else has seen
      this problem and what you did to cure it. I looked at the board real good
      under the magnifying
      glass to see if there were something there that would cause this problem
      like a unetched trace
      or a short between pins that didn't belong but nothing was noticed.

      Any Hints or cures for this problem gratefully accepted. I am at a loss as
      to what to do next.

      73 Dave
      David R. Hassall WA5DJJ
      WEBSITE: http://www.zianet.com/dhassall/
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