2000W Step up & Step down Transformer malfunction

Hi, I hope this is the right corner of the forum ;-)

Anyways, like in the description I have a Trisonic TS-420-3P transformer for a couple of years now, and lately its been acting up. The problem is that when I turn it on, the red light in the front indicating that the transformer is powered on turns on normally, but I can't hear the core working, nor do I get anything at the output (checked with a multimeter, and I've got a big zero). Normally I would hear the core working (and put something moderatly heavy on the transformer to damp down the noise) and everything was fine. This happened once before and an electrician fixed it, after that I got into electronics a bit and now I think I could fix it (instead of waiting and paying for it). But I have absolutly no idea what could be the problem :sad: There was just this one loose cable inside that I soldered back on the fuse, but even with that the thing is not working. What could be the problem here, and it really can't be that complicated (I think) because last time the guy fixed it in under 20 minutes. I would be very gratefull if anyone could help me with this.

A transformer may have a thermal fuse present between the windings (that can or cannot be replaced). Because you don't hear the hum, it should be a primary circuit issue.

Remove the AC and measure the resistance between the two primary wires/terminals of the transformer (to rull out wire break or thermal fuse blown). If you measure less then 100 Ohms, this is OK. If high resistance (think of above 100 kOhms), there can be a wire break or the thermal fuse is blown. This fuse is in series with the primary winding to interrupt the current in case of too high temperature. If you can't replace a fuse, the transformer is really dead.

If you have low resistance between primary terminals of transformer, measure the resistance between the live and neutral mains terminal (on-off switch in on position). This rulls out blown fuse or bad contacts, bad power cord, plug, etc. You should read a low value. As the light comes on, the power cord should be OK.

Please come back for further advice.