Repair case
WS-250 Gas Click No OCV No HF Repair Case
A repair-case style explanation of a TIG gas-click/no-output/no-HF fault path.
Quick Answer
This repair case shows why gas valve movement does not prove that PWM and HF start are enabled. The machine had gas-valve action but no open-circuit voltage and no high-frequency start, and the fault was traced through the SG3525/CW3525 shutdown path around PC817, CA3140 and a 5.1 k resistor path.
- Do not replace SG3525/CW3525 first just because pins 11/14 have no PWM.
- If the PWM clock exists but outputs are blocked, inspect shutdown/feedback inputs.
- PC817, CA3140 and SG3525 pin 9 evidence can explain no PWM even when control power is present.
Case evidence path
| Evidence | Meaning | Repair direction |
|---|---|---|
| Gas valve clicks | Torch/gas control path is alive, but PWM/HF may still be blocked. | Continue to PWM and shutdown checks. |
| SG3525/CW3525 pin 4 clock exists | PWM oscillator is not necessarily dead. | Do not replace PWM IC first. |
| Pins 11/14 no PWM | Output is blocked or shutdown path is active. | Check shutdown/feedback input. |
| PC817 / CA3140 path | Feedback/shutdown path can pull SG3525 pin 9 low. | Measure optocoupler and amplifier path. |
| R12 5.1 k open | Bias/shutdown path fault creates false shutdown. | Replace resistor after confirming path. |
Related records
Use these to continue diagnosis.