Combo welder reference

CT-312 MMA / TIG / Plasma Combo Welder Repair Reference

A redrawn English repair reference for CT-312 style combo machines, focused on board stack, gas, HF and arc-start routing.

Quick Answer

Use this page to separate CT-312 style MMA/TIG/plasma faults by mode, board stack and control path.

If MMA works but TIG or plasma does not, check mode switch, gas/HF and torch-control paths before blaming the inverter stage.
If gas or HF is missing, start at 24 V control and board connectors.
If no mode produces output, move toward the main power path and protection evidence.

What this combo-machine page covers

CT-312 style machines combine stick welding, TIG arc start and plasma/cutting behavior in one board stack. The repair risk is that a technician treats a TIG/HF/gas fault as a simple main-output fault.

Board-stack reference

CT-312 board stack map

Function-based board routing for CT-312 MMA / TIG / plasma combo machines.

1Bottom board

DC bus capacitors, 24 V socket, main relay, start resistor, gas / fan / HF sockets.

2Middle board

Main transformer, rectifier diodes, reactor, shunt, thermal switch and feedback socket.

3Top inverter board

Power devices, driver transformer, control module and board supply.

TIG mode

Torch switch, HF start and gas solenoid path.

Plasma mode

Cutting arc, air/gas path and pilot-arc behavior.

MMA mode

Main output, arc force and thermal protection.

Repair split

Separate board stack, mode switch, gas / HF control and real output evidence.

Diagnostic route

CT-312 no-HF / no-gas / no-arc routing

Use this route before replacing the whole inverter board.

1Torch switch

Hand switch and mode input.

224 V control

Auxiliary supply and control relay.

3Gas solenoid

Click, coil and connector evidence.

4HF board

Relay, HV transformer and spark path.

5Main output

OCV and loaded-current evidence.

6Feedback

Thermal, shunt and voltage feedback path.

Gas clicks, no output

Continue to PWM permission and HF command.

No HF

Check HF relay, HV transformer and spark gap.

HF exists, no arc

Check output path, torch, work return and consumables.

Combo-machine fault split

Symptom	First evidence layer	Do not confuse with
TIG no HF	torch switch, HF board, relay, high-voltage path	main output stage only.
Gas valve does not work	24 V control, gas relay, solenoid connector	output diode or IGBT fault.
Plasma no arc	air/gas path, pilot/HF path, work return, output path	TIG-only start fault.
MMA works, TIG does not	mode switch, HF/gas/torch-control path	dead power stage.
Protection light	thermal switch, feedback/protection input, output short	front-panel fault only.