Chinesium CUT50 Plasma Cutter doesn't cut...

[GarySlaghammer]

I got this in bits as a last chance saloon before the tip. Blown MOSFETs meant a new upper board had been bought previously. It's not quite the same so I've had to make some changes for it to work (a 24V switching circuit for the mains thermistor bypass relay for one).

I have a continuous arc across the spark gap inside the machine when I hold the trigger now. The air solenoid actuates and air blows through at the recommended pressure of 0.4MPa/50-60PSI. I can get a constant arc to the work piece, but the spark gap inside the machine is also running (I believe it's supposed to be off once the arc is established at the torch, though the physics of this escape me as the circuit diagram I have for a similar model shows the spark gap across the transformer primary so I can't see why it wouldn't always be on). There is some minor arc-pitting of the work piece, but no plasma jet or cutting action (first picture). Numerous consumables have been tried.

What else can I check?

If I turn off the air supply, as it runs down I can get a quiet flame between the workpiece and the torch without the spark gap inside running (second picture). Moving the torch too far away from the workpiece causes the connection to be lost and the spark gap starts arcing again.

I'm sort of au-fait with the operation of these machines (SMPS into flyback) but haven't the logic, training or experience to follow what could be causing this.