Repair case · no VCC
KA3843 / KA3845 No-24-V Case: Open Startup Resistor
This case shows how to stop at the first missing stage. The main bus was present, but the controller never received startup bias, so the missing 24 V rail was a downstream symptom rather than the original failure.
Case summary
This case is a reusable diagnostic pattern, not a claim that every board uses the same resistor value or reference designator.
Initial symptom
- No fan, display or relay activity.
- Main input fuse remained intact.
- No visible 24 V output at the control connector.
- No repeated ticking or fan twitch, suggesting the controller never entered a switching cycle.
Power-off evidence
- The DC bus and secondary capacitors were discharged and verified.
- No short was found across the 24 V output or the switching MOSFET.
- The startup resistor chain measured open across one section despite no obvious external crack.
- The VCC capacitor was checked for polarity, leakage and gross capacitance failure before replacement decisions.
Controlled live evidence
| Measurement | Reference | Observed pattern | Meaning |
|---|---|---|---|
| Main DC bus | Primary bus negative | Present under current limiting | Input and bridge path reached the auxiliary stage. |
| Controller VCC | Controller ground pin | Did not rise into the start region | Startup feed or VCC loading fault. |
| VREF / output pulse | Controller ground pin | Absent | Expected because valid VCC was never established. |
| Secondary 24 V | Secondary return | Absent | Consequence, not yet proof of a secondary failure. |
Diagnosis
The evidence stopped at the startup resistor chain: source voltage was present before the chain, absent at the controller VCC node, and no VCC short was found. The controller, MOSFET and transformer were not replaced because the converter had never received startup bias.
Repair action
The failed resistor section was replaced with a part matching the board's resistance, voltage and power requirements. Heat-damaged solder and adjacent series resistors were inspected. The VCC capacitor was retained only after its condition was confirmed; replacing it blindly would not have repaired the open startup path.
Verification sequence
- Power through the current limiter with downstream loads minimized.
- Confirm the VCC node charges once and the controller starts.
- Confirm VREF, oscillator and output pulse.
- Confirm secondary rails with the correct secondary return.
- Reconnect the fan, relay and control loads one by one.
- Perform staged full-machine power-up and inspect resistor temperature.
Stop conditions before another power-up
- The replacement resistor value, voltage rating or power rating is uncertain.
- A VCC or secondary short remains.
- The MOSFET or rectifier has not been checked after a previous fuse failure.
- The current limiter remains bright after the controller starts.
- VCC rises beyond the expected board range or the output becomes uncontrolled.
Technical sources
- Texas Instruments UC3845 product page — pin functions, UVLO, VREF, current limit and maximum duty-cycle reference.
- UCx84x current-mode PWM controller datasheet — controller-level limits; it does not replace the actual board schematic.
- Texas Instruments TL431 product page — secondary feedback reference when the board uses TL431/KA431 regulation.
- WelderData internal technical records — used for measurement sequence and repair-pattern organization; exact readings remain board-specific.
Related repair map
Follow the evidence path from symptom to measurement, circuit and repair case.