Marquette M12150 mystery solved....I got the wrong torch head! But can I make it work....?
Huge props to "Toplscuda" Carl for guiding me through some troubleshooting tips on this antique. Talk about no factory support, this is an orphan. I found that the 'blowback' torch design that the PO tried to replace the original torch head with ("sure its compatible", said the salesman...) is ***-backwards in the way it strikes an arc compared to the original torch. I can't explain how the original torch (High frequency start, pilot arc, scratch start, lift start?) worked as I have yet to see one, but this machine starts the air flowing first with the torch trigger to protect the original torch tip I presume. Once the air starts flowing this new torch has a 'floating' center conductor that is physically blown back off the tip (breaking contact) due to the air forcing its way around it out the little kerf hole at the end. Now the center conductor is electrically isolated from the tip and the cutter does not sense a short (connection between the conductor and the tip) so it never tells the main transformer to turn on, click click click...nothing. Im stuck with an electrically live blow gun. BUT if I turn off the air supply (unplug the air solenoid cable) , I can keep the tip in contact with the conductor WITH the trigger pulled, now the control board senses a short circuit and turns on the transformer (the heavy sounding relay operates a large contactor) but it will quickly overheat the resistor in the unit (imagine a stick welder sticking to the piece) unless I manually start the air pressure again (touching the lead back onto the air pressure solenoid). Now the air again lifts the center conductor off the tip (clearing the short) and a big fat plasma arc is lit and is blown out the hole by the same air pressure that lifted it, or "blow back" start. I will look for the original torch head but I think I can get this to work by putting a second momentary "AIR" thumb switch on the torch head and running the wires back to the air solenoid harness. Pull trigger to electrically activate transformer, then hit the AIR button to get the arc started and blowing out the kerf hole. Get off the trigger to stop the plasma jet and that will automatically stop the air too as the trigger activates all 3 relays. to start again, pull trigger for 1/2 second until you hear the relay/big contactor 'thunk' and hit air again, blaze on! If I want 30-60 second post air after a cut, I can just hit the AIR button before pulling the trigger and it will blow dead air for 30-60 seconds as controlled by some timing circuit somewhere. I didnt see and 555 timer chip on the board so it must do it via good old solid state style.
