It is generally caused by a faulty diode bridge located on the board next to the transformer.
The faulty bridge can sometimes cause the fuse to blow but this in the extreme case. Any way, if one of the diodes fails in the bridge rectifier, then there will be a AC component on the DC power rail. This will mean that the solenoids will buzz, chatter and will be actuating very weakly.
The bridge rectifier in my Carousel was a B125C 1500. I used a W04 as a substitute. The secondary rail is about 24v so it is within the 50v rating of the W04.