vario trafo 4 terminals, trouble shooting
The terminals are unmarked and your photos are blurry and don't show the wiper or the knob. If the knob is on the right then we can make the following assumptions:


Black, blue and yellow are terminated on the coil.
White must be connected to the wiper. This will be the output terminal.
Anti-clockwise rotation of the knob will rotate the wiper towards the blue wire. This must be the neutral connection.
Black is at the fully clockwise position so this may be the live terminal. Note that there are a few more turns after the black and this suggests that the device may be capable of giving a few more volts out than the mains supply.
The yellow wire appears to be connected about 60° from the neutral connection. If this is a 120 V device I would expect about 20 V from the yellow.








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) A typical variable output transformer. (b) What you've got?


  So I tried 220 volts on primary and it smoked and made a loud humming noise, ...


There is nothing on the transformer to indicate that it is rated for 220 V. It may be a 120 V transformer so this was far from a good idea.


  But should I put a fuse in series with it ...


This implies that you powered up an unknown, untested device directly from a wall socket with no fuse. Again this is not safe practice and the amount of power released in a fault could cause you serious damage directly or indirectly in your reaction to it.


  ... it looked like there was some tiny scratches on the coil before i started testing so I gave it some floor lacquer, It did the job i think.


Floor lacquer is not an approved electrical insulator and may be a fire hazard when the device warms up. In addition the wiper needs to make contact with the wires on the left side where the insulation has been removed during manufacture.

You need to treat mains power with a lot more respect and understanding than you demonstrated in this investigation. Sorry to be harsh but the potential outcomes are not funny.

If you want to test again then get a low wattage tungsten filament bulb - say 220 V, 25 W and wire it between the fuse and the live terminal. First though, update your question with photos of the ends of the transformer and a corrected version of my Figure 1b.