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                                   BULLETIN
         FSB-104 - HOW TO PASS NBADCP/BBADCP/WORKHORSE RECEIVER TESTS
                            DATE/REV:  15NOV96 / 0
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ABSTRACT:  Explains what steps to take to ensure receiver tests do not FAIL
because of external interference signals.

INSTRUMENT/PROGRAM AFFECTED:  All ADCPs (Narrowband, Broadband, Workhorse)
  Narrowband - Preamp/Receive Test portion of DRTEST/SCTEST/VMTEST program
  Broadband  - Receive-Path Test (PT3), Receive Bandwidth Test (PT6)
  Workhorse  - Pre-deployment Test (PA)

DESCRIPTION:  Many users tell us their ADCP reports a FAIL condition during
the self-tests that check the ADCP's "receive path."  In most cases, the cause
of the failure is external interference.  This external interference is
usually caused by a "noisy" environment, such as in a lab.  You can take a few
simple steps to find out if the FAIL condition is being caused by external
interference or by a problem with the ADCP.

SOLUTION/ACTION:  The following procedure explains how to conduct the tests
listed above in the Instrument/Program Affected section to reduce the
likelihood of a false failure.

  1.  Turn off any nearby equipment (monitors, radios, etc.) that is not
      needed to conduct the test.
  2.  The pressure case should be on the ADCP to help shield the circuit
      boards from external electronic "noise."
  3.  The ADCP transducer head must be immersed in water.  Ensure there are
      little or no air bubbles on the transducer faces.
  4.  If the transducer is immersed in a bucket of water that is resting on
      the floor, noise can be coupled into the ADCP.  As such, you should
      shield the bucket from the floor by inserting a piece of hard foam
      between the bucket and the floor.
  5.  Run the appropriate receiver test for your ADCP.
  6.  If the test fails, "spin" the ADCP 180 degrees.  If the failure moves
      from one beam to another, it is "probably" safe to say the failure was
      caused by external interference, and it should be all right to deploy
      the ADCP.
  7.  If possible, you may also want to move the ADCP to a different room, or
      at least to a different part of the lab to see if the fail condition
      goes away.
  8.  If after following the above procedure, your ADCP still fails the
      receive tests, contact RDI for assistance.

NOTE:  Although rare, we had a case where a Vessel-Mounted (VM) system was
being tested in a lab.  The interconnecting VM cable was apparently picking up
interference from the floor.  When the cable was isolated from the floor by
placing it on a chair, the test passed.
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