I fixed a shaker that was claimed to be broken. I had to cut the rubber membrane to open the head.
Once it was opened, the cause of the trouble was obvious. The soldering joint could not put up with the motion of the head.
It is interesting to see that the spring has the damping layer between the metal sheets.
After the repair the DC resistance was measured. It was 1.9Ohm. The side of the shaker chassis said "3.5Ohm, Max 15VA". So it can take more than 4A (wow).
I gave 2A DC from the bench top supply and turn the current on and off. I could confirm the head was moving.
I'll claim the use of this shaker for the seismometer development.