PCM-2 Manual.pdf - Voss Associates

UTILITY menu. If this does not restore the affected channel, the high voltage
circuit of that MDB (Modular Detector Board) is likely to be malfunctioning.
High voltage failures are commonly caused by a broken anode wire in the detector
that is short-circuited to ground. Before replacing the MDB, use a continuity
meter to test the anode-to-ground conductance. If a conductive path exists, rewire
the detector (or replace it) prior to attaching a new MDB to that detector.
Refer to the appropriate sub-assembly section of this manual for more information
on MDBs and detectors.
NOTE: Whenever an MDB is replaced, the DETECTOR DOWLOAD routine
must be invoked to restore the correct parameters to the affected detector.
Comm Fail
Communications failures can frequently be overcome by forcing communications
to be re-established through the DETECTOR DOWNLOAD routine under the
UTILITY menu. If this does not correct the problem inspect the connection of
the wiring harness to the affected MDB (Modular Detector Board). If this is
secure, the failure is likely to be a faulty RS-485 transceiver IC on the MDB.
Replacing the MDB is the fastest way to put the instrument back into service.
NOTE: Whenever an MDB is replaced, the DETECTOR DOWLOAD routine
must be invoked to restore the correct parameters to the affected detector.
-ligh Background Fail
A high background failure is most often caused by improper setup of the
instrument. If all detector channels are counting properly, the condition is caused
by too restrictive a count time for the statistical constraints applied to the
measurement exercise. In PRESET ALL and FIXED COUNT TIME modes this
condition is overcome by increasing the length of the count time interval. In
MINIMUM COUNT TIME MODE, the maximum count time must be extended
to correct the condition.
Hardware induced high background failures exhibit the symptom of a noisy
channel evidenced by excessive count rates. The count rates can be observed in
the VIEW BACKGROUND AVERAGES, or VIEW GROSS COUNT RATES
selections in the test mode menu. If high count rates exist in only the alpha or
beta channel, but not both, the problem is almost certainly on the Modular
Detector Board which should be replaced. Otherwise, the problem could also be
caused by a poor high voltage connection resulting from the Modular Detector
Board not being properly seated onto the detector, or by an anode wire in the
detector that is arcing to ground. If the latter is the cause of the problem, the
detector must be rebuilt.
NOTE: Whenever an MDB is replaced, the DETECTOR DOWLOAD routine
must be invoked to restore the correct parameters to the affected detector.
PCM2.MANI REV AI April 1995 9-7