Field reports have identified instances of filter PCB coil overheating and failure in Fujitsu LJBYL outdoor units, typically occurring after approximately one hour of operation.
This issue results from incorrect wiring of the reactor coil on the Filter PCB terminal block, causing improper current flow and eventual thermal damage to the PCB components.

1. Symptom

  • Filter PCB (power board) exhibits coil overheating or visible burn marks around the reactor coil terminals.

  • System operates normally for a short period (~1 hour) before failure.

  • May trigger protective shutdown or main circuit breaker trip.

  • Coil and surrounding board area show signs of discoloration, odor, or thermal deformation.

2. Cause

The root cause is the incorrect connection of the reactor coil on the Filter PCB terminal block.

Technical Explanation

  • The reactor coil (choke) must be correctly wired in the DC circuit path between the rectifier output and the ACTPM (Active Converter Technology Power Module).

  • Incorrect terminal placement allows AC voltage or unregulated DC current to flow through the coil, generating excessive heat and magnetic stress, leading to progressive insulation failure and PCB burnout.

In essence: DC voltage must run through the reactor coil and return to the ACTPM.

3. Visual Indicators

From the field images provided:

  • The filter PCB shows heat damage concentrated near the reactor terminals.

  • Coil leads (brown and white) appear connected to a terminal pair not designated for the DC loop, confirming a wiring deviation from standard configuration.

  • Adjacent components such as the input relay and PFC choke remain unaffected initially but can fail due to thermal propagation if operation continues.

4. Corrective Action

Step 1 – Verify Wiring

  • Disconnect mains power and isolate the outdoor unit.

  • Inspect the reactor coil terminals on the Filter PCB:

    • Ensure brown and white reactor leads are connected to the DC bus terminal block per wiring diagram.

    • Confirm wiring path:
      Rectifier Output (+DC) → Reactor Coil → ACTPM (+DC input)

Step 2 – Inspect PCB

  • Check for heat marks or discoloration on the Filter PCB.

  • If damage is present:

    • Replace the Filter PCB assembly and reactor coil together as a matched pair.

    • Confirm board revision and compatibility (use latest version with heat-resistant coating if available).

Step 3 – Function Test

  • Reconnect all plugs and wiring harnesses as per the service schematic.

  • Power up the system and monitor:

    • DC bus voltage stability (approx. 310–340VDC)

    • Temperature of reactor coil and PCB area after 30–60 minutes of operation.

  • Ensure no abnormal sound, smell, or surface temperature above 70°C.

5. Preventive Measures

  • During commissioning or board replacement, verify reactor connections against the wiring label printed on the filter PCB.

  • Use factory torque specifications on terminal screws to ensure proper contact resistance.

  • Educate field technicians on identifying DC bus vs. AC input terminals, as they are visually similar but functionally distinct.

Reference

Field observation and analysis of Fujitsu General LJBYL outdoor units, showing filter PCB and reactor wiring failure due to incorrect terminal connection (images provided above).