I have a hobart handler 140. wire feeds ,no weld output. we were welding bodywork for a very short time and I heard a slight pop from the welder . now no welding . checked ground, replaced tip ,tightened thumb screw on cable, fan runs ,wire feeds , no weld.

Check the selector switch with an ohm meter. Unplug the unit, ground to chassis and probe the contacts while turning the switch on and off.

Newer welders as hobart and miller are set up from factory to use innershield wire therefore polarity is set up as inverse. Check inside your machine and check that ground cable is on negative side. Your gas must be mix argon/CO2 or pure CO2 (not recommended).
Other potential problem is wire not being a mild steel one. Check you have gas flow out of your nozzle.
That's all I can think of.

This happens to me all the time. Wear and tear has taken its toll and everything has to be maintained for it to work. Most likely the welding tip is worn and should be replaced. Lincoln electric tips work well and I will replace mine several times during the day. Also check your tension and make sure the wire will uncoil freely from the spool. You can remove the tip and make sure the wire will feed without it. If not, your liner may be worn out. Also a welding Jell will help keep the tip from cloging. It will still happen, but hopefully not on your first weld of the day.

Check the two main diodes. These units have a light duty cycle. About 20% if I remember correctly. I have one that I was welding with for a considerable amount of time, and burned up the diodes. You can test them by disconnecting the wires (with unit disconnected) and use a ohm meter. You should get a reading in one direction, but not the other. If no reading, then the diode is blown. You can order new ones from your local welding shop. Also check all conections on the secondary for any signs of overheating and loose connections.

Arc blow could be the cause! Put your ground on the work piece and weld away from it. If you weld toward a ground clamp the magnetic forces will blow your arc around sometime. When taking a cert this simple mistake can make or break ya on your cap or reinforcement beads.

Check your input line voltage. 115 volt Mig welders / plasma cutters need stable voltage / current. Test to see if the voltage drops to below 100 volts when welding. If so, find a more stable power supply.

I have welded countless hours with Linde & Lincoln & even a few Hobarts & the only time that I ever had the same problem was when the ground connection either to the object I was welding, or the welding cable ground wasn't good & clean & tight. Same on stick welders also. I did develop the habit of grinding not only where I was preparing to weld, but where I was placing my ground clamp & that always provided a better "frying bacon" sound as I went along the business of pushing a bead. In a welder, the only other thing than that is involved is the step-up transformer and your wire. Hope this helps.

If it is new i would not play around with the welder Just take it back to where you bought it get them to replace it under warranty

make sure rollers are clean and slots not wore out check alignment
do not over tighten wire feed so it smashes the wire
flux wire is hard to get a good setting
make sure if you can set it for gas welding it is not on this setting

Some welders control wire speed with the amps control, some control amps with the wire speed control. You might try turning things up until it runs smoothly without pushing back. If you're new at welding, it does take some practice until it feels right.