Arcing - GE Medical Systems PRODIGY Service Manual

A rough edge around the shutter aperture may be wearing away on the tube
head below. The lead filings fall off the shutter aperture and gather on the filter
below, causing a reduction in x-rays over time. Any wear should be easily
visible on the underside of the shutter paddle. If there is visible wear, the lead
dust must be removed from the top of the filter. The Collimator assembly must
be removed, the filter cleaned, and then the Collimator assembly reinstalled
and realigned.
II.A Change in Current Control Settings
Use the Signal monitor option of the service software program to produce x-
rays (Tools/Diagnostics? Scanner X-Ray). Select 76 kV and 750 µA operation.

4.10 Arcing

The X-ray Tube Head Insert is an evacuated glass enclosure. An AC current
is applied to the filament inside the insert. It glows like the filament in a light
bulb, and electrons are boiled off into the evacuated space. A high voltage is
applied between the anode and the cathode causing electrons to rush toward
the anode, striking it and creating x-rays. As long as the insert is properly
evacuated, there can be no internal arc. However, no insert can be totally
evacuated and impurities can be ionized creating a lightning like effect; arcing.
During the arc the resistance of the insert is dramatically decreased and a
large amount of current flows.
4-134Troubleshooting
• Verify that the SBC is properly controlling the current. TP11 of the MAX
board should be approximately 0.75 volts. This voltage is dependent on
feedback information from the power supplies, which makes it difficult to
troubleshoot by any means other than SBC substitution.
• Check the current through the X-ray Insert. The absolute value of the
voltage at test points 1 and 5 on the XORB board is proportional to the
current through the X-ray Insert. 1 millivolt is equal to 1 micro-ampere of
current. A current setting of 750 µA should give a reading of 0.750 VDC
at test points 1 and 5. If either of these voltages vary from the expected
by more than 10 millivolts, the MAX board could be at fault. If TP1 and
TP5 are more than 15 millivolts apart, substitute new high voltage power
supplies.
• The significant amounts of electromagnetic energy released inside the
scanner can cause problems with the electronics of the scanner system
and may result in abnormal operations of the scanner. An arcing scanner
will have one or more of the following symptoms:
• complete lock up of the system
• a diagnostic error code message which is undefined or inappropriate
• a vertical stripe or artifact in the image of the patient's scan (effects all 16
detectors at once)
DPX-NT Service Manual (Rev A- 1999)