Fault
ZX7-250 Fault Light On and No Welding Output: Protection, Upper Board and Secondary Rectifier Checks
A ZX7-250 diagnostic page for machines where the fan runs, the fault indicator is on, and there is no welding output.
How this fault branch is different
When the fault indicator is on, the machine is telling the technician that a protection path is active or a protected stage is abnormal. This branch is different from fan-runs/no-output with the fault light off. The repair direction should include connectors, overcurrent feedback, upper-board power devices, the middle-board transformer and secondary rectifier devices.
Protection-flow diagram
Diagnostic table
| Check | Expected result | Fault meaning |
|---|---|---|
| Board connectors | Firm and not oxidized | Loose connector can create false protection or missing drive |
| Overcurrent feedback | Not latched during idle | Current transformer or feedback circuit may be driving protection |
| Upper-board MOSFET/IGBT | No shorted power devices | Damaged power devices can trigger protection |
| Driver board zener path | No shorted clamp components | A failed 5.1V zener or gate clamp can follow power-device damage |
| Middle-board transformer plug test | Fault may clear when isolated | Then inspect middle-board transformer or secondary rectifier |
| Secondary rectifier diodes | Not shorted/open | Shorted rectifier can reflect load back to inverter |
Why the plug isolation test matters
One useful repair branch is to isolate the upper-board main transformer supply path. If the fault light no longer appears after the related plug is removed, the problem may not be only in the control board. The middle-board transformer or the secondary rectifier path can be responsible. This prevents a technician from repeatedly replacing power devices while ignoring a downstream short.
Recommended sequence
- Confirm the fault light behavior before opening the machine.
- Inspect connectors and previous repair marks.
- Check upper-board power devices for short circuit.
- Check the driver clamp parts and small zener/protection parts associated with the failed branch.
- Use isolation testing to separate upper-board inverter faults from middle-board transformer and secondary rectifier faults.
- Repair the secondary or driver cause before reinstalling new power devices.