Repair Case
KA3843 Auxiliary Power: No 24V Rail
Use this page when the fan, relay, control board or display is dead because the auxiliary 24V supply is missing or pulsing.
Measurement order
| Node | Expected | If abnormal |
|---|---|---|
| Bulk DC input | Present at auxiliary supply input | Input rectifier or bus path fault |
| KA3843 VCC | Rises to startup threshold | Open startup resistor, dry VCC capacitor, shorted chip |
| Switch drain | Switching waveform after startup | PWM chip or MOSFET fault |
| Secondary 24V | Stable output under load | Secondary diode/capacitor/load short |
| PC817 feedback | Regulates without hunting | Reference or optocoupler fault |
Common repair parts
- Startup resistor.
- VCC electrolytic capacitor.
- KA3843/UC3843 PWM chip.
- Switching MOSFET.
- Secondary rectifier diode.
- PC817 optocoupler and reference device.