Transformer trouble

Hi guys.
Stupid thing... I have a wallward transformer (the old kind, no switching PSU) for an inexpensive 1-in-4-out headphone amp that used to have 12VDC output. After I opened it on order to resolder the cable, it suddenly puts out 18VDC. This thing only has 4 diodes and an electrolytic cap! I testet the diodes and all are OK, switched the cap just in case. Any idea whats wrong with it?