Kenwood KM800 spins too fast - replace triac

ID: 185050

Description: After about 18 years of regular use, the...

Steps:

  1. Symptom:
  2. The machine starts immediately at high speed (~0.5 seconds per revolution).
  3. The speed cannot be changed in the first 3/4 of the speed control.
  4. The control only works correctly in the last 1/4.
  5. Disconnect the device from the power supply!
  6. Remove the cover of the control circuit board.
  7. Visual inspection: No obvious damage to the circuit board.
  8. Defective components:
  9. Desolder the BT-137 600F triac from the control board.
  10. You can also desolder the BC-547 if you want to replace it at the same time.
  11. Replacement:
  12. Install a BT-138 (12A) as the replacement type in the same orientation.
  13. Solder the leads cleanly and check the joints for bridges or cold solder joints.
  14. Reinstall the control board cover and test the speed control.
  15. After the replacement, the machine ran correctly again and could be controlled as usual with the knob.
  16. The speed is not measured electrically, but influenced mechanically.
  17. A ring magnet with four north-south sections is located on the motor shaft.
  18. On the circuit board above it is a coil with two metal pins.
  19. The hand controller changes the distance between the magnet and the circuit board:
  20. Proximity = strong induction → slow speed
  21. Distance = weak induction → higher speed
  22. This regulates the current flow through the triac via the induced voltage.
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