I purchased this NS Master Coffee used, began to refurb it. http://nuovasimonelliusa.com/images/Tec ... er_ING.pdf
Other than extensive cleaning, only found the 135 degree C thermostat with a broken housing (back of boiler), so I replaced it during the refurb. When ready to start the machine up for the first time, I connected the power, water supply and drain and turned on the breaker. Machine registered off on the LED display. With the internal-front-panel switch off for boiler heating, I pressed the on/off panel soft switch. The machine did a self-test, displayed "Ready" and then began to autofill. As expected, autofill timed out (I had drained the boiler during refurb). I pressed the soft switch to off, waited a few seconds and turned it to on. Same results, "Ready", autofill, time out. On the third cycle, after beginning to autofill, there was a loud "POP" and I switched everything off including the breaker and water supply. Local Nuova Simonelli technician doesn't remember this model and appears to be of no assistance. He did support my guess that the high-limit thermostat (which I had replaced) had blown, even though I thought the front "1/0" switch was to interrupt power to the heating element. My guess is the machine is not filling and, for some reason, the 1/0 heater switch is not doing what I would have thought. Here is my initial plan (please critique): (1) Verify tstat blown. (2) Verify if boiler is partially filled. (3) Use meter to verify 1/0 switch disconnects heating element. (4) If this does not work, disconnect pump plumbing and power, verify pump works by itself.

I think your next step is to actually determine if something is broken or if you're misinterpreting a pop sound for a relay clicking off sound. The reason I ask is that right now, we really don't know if the boiler reached its 'full' point and a relay clicked, turning off the pump....or if a capacitor popped, stopping the pump.

What happens when you turn it back on and try to dispense a shot of espresso (run water from the group head?)

I'm guessing there is nothing wrong with the 135 degree switch. If the boiler isn't filling, it's unlikely that the heating circuit is even powered up. Perhaps the pop sound came from a capacitor either on the pump (working to fill up the boiler) or from the autofill circuitry.

Do you smell any acrid odours from the machine's vents?

Thanks for the thoughts, Russ. It was a loud bang, definitely a fuse or breaker or thermostat. There was a tiny puff of smoke, very faint, that eased up from the machine a couple seconds later. After a while, I put the breaker back on and the display showed the machine was on, since I hadn't turned it off. I did NOT try another fill cycle.

Alright! Well, when the smoke escapes a compenent, it usually leaves a nice tell tail sign!
Now you have to open up the machine and have a look at the electrical componentry. You are looking for a component that is unusually dark, black or having a burnt warped appearance. Sometimes high heat appears on components as very high gloss or sheen. If the components are mounted on a green 'circuit board', sometimes the high heat discolores the green board and turns it brownish. Have a real good look.
Take some photos of the circuit boards, sniff each area of them since your nose can find the area of 'overheating' faster than your eyes sometimes. If you can't identify what is wrong, post those circuit photos. Fuses don't usually emit much smoke so I'm thinking maybe a resistor or small capacitor near the pump. Have a look at that area. We know it was working, maybe longer than its duty cycle shoud have been.