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. Triac BT-137 600F (presumably defective)
  10. Transistor BC-547 (replaced as a precaution)
  11. Replacement:
  12. A BT-138 (12A) was used as a functional replacement type.
  13. After replacement, the machine ran correctly again and could be controlled as usual using the rotary knob.
  14. The speed is not measured electrically, but influenced mechanically.
  15. A ring magnet with four north-south sections is located on the motor shaft.
  16. On the circuit board above it is a coil with two metal pins.
  17. The hand controller changes the distance between the magnet and the circuit board:
  18. Proximity = strong induction → slow speed
  19. Distance = weak induction → higher speed
  20. This regulates the current flow through the triac via the induced voltage.
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