GE JEDI generator with MX200 tube (Hemit): repeated generator burnouts
What happened
As reported in the case: a second JEDI generator burned out within a month on a GE CT with an MX200 tube and Hemit transformer, at a stated tube usage of around 600 thousand scan-seconds. The tube was still alive and passed the no-load kV test, but the engineers found air inside the Hemit transformer although the Hemit itself was serviceable.
What was checked
- No-load kV test of the tube (passed in this case)
- Inspection of the Hemit transformer for trapped air
- Diagnostics of the burned JEDI generators
Cause
Probable, as described in the case: the expansion bladder in the Hemit had reached the end of its life, letting air in and lowering the oil level; combined with breakdowns in the worn tube at 140 kV during gantry tilt, this overstressed and burned the JEDI generators.
The cause is probable and was not fully confirmed.
What was done
Oil top-up and vacuum treatment of the Hemit — a procedure from the manufacturer's service documentation, done by authorised personnel — together with replacement of the expansion bladder and replacement of the burned JEDI generators.
Safety
Before any work on the generator or the oil-filled Hemit, isolate the system from the mains and verify that high-voltage capacitors are fully discharged, and complete an electrical safety test before the scanner is returned to clinical use.
Lesson: Air in an oil-filled high-voltage component like the Hemit is never normal — find and fix the root cause, such as a worn expansion bladder, before repeated overloads destroy expensive downstream electronics like 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.