I get the same problem with the all in one nixie clock... but when I have changed th IRF840, it have burned again after four or five days... So now the board is also melted and I can't solder a new IRF840 anymore...
I don't know I am annooy cause i was an expensive gift which don't work ...
Can you help me please ?
So yes, your power supply is fine. If you changed any of the settings relating to the high voltage generator I'd suggest you do a factory reset to put them back to known good values.
If you have not already removed the burned MOSFET, don't try to unsolder it. Instead cut off the leads close to the body of the part so you are left with pins sticking up. Then solder the replacement on to the pins.
As discussed earlier in this thread there are really only a few things known to cause the MOSFET to burn up. One is that the MOSFET was just bad but I think you already proved it was not since a new one burned up too.
The other thing would be if the controller is stopping with its gate drive output on, The controller should never stop but it can if there is a problem with the crystal or its 2 loading capacitors.
Did you modify the firmware in any way? The firmware supplied with the kit is stable, hundreds of kits are running just fine on it, but if you made changes and somehow introduced a bug it could possibly be crashing the controller... maybe.
Look into it later when the dust is clearing off the crater.