Canon/Toshiba Aquilion 64: over-kV trip only at high tube current
What happened
On this Aquilion 64 with the Spellman high-voltage generator, the scanner reported an over-kV trip at 120 kV / 500 mA on the large focal spot (values as reported in the case), while lower tube-current settings worked without errors. Filament calibration and HV-side inspections did not clear it.
What was checked
- Filament (tube current) calibration was repeated — the fault remained.
- The HV cable, its silicone sealing ring and the tube housing insert were inspected for damage and signs of tracking.
- The 24 V power supply mounted on the multiplexer was checked and then tested under load against its reference output curves.
- Since the trip appeared only on high-power settings, the engineers looked at supply rails feeding the generator control chain rather than at the tube itself.
Cause
In this case the engineers found dried-out electrolytic capacitors in the 24 V power supply mounted on the multiplexer; its degraded output was the likely reason the generator hit the over-voltage threshold at high tube current while lower settings stayed within limits.
What was done
The capacitors in the 24 V power supply were replaced and the unit was verified under load against its output graphs. The scanner was reported to have run about a week afterwards with no recurrence, and a spare 24 V supply was ordered as a precaution.
Safety
High-voltage generators and HV cables store lethal charge — follow the manufacturer's discharge and waiting procedures from the service documentation, verify the HV is discharged before touching the generator or tube, and run the electrical safety test before returning the scanner to use.
Lesson: An over-voltage trip that appears only at high tube current can come from a tired low-voltage supply in the control chain, so test supply rails under load before condemning the tube or the generator.
Covered in the course: CT Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.