Last week my Kerry KC2 ultrasonic bath started to fail on me, so I have brought a new one to replace it.
My old one still works, but does not seem anywhere as powerfull as it once was.
I have not yet pin pointed what the problem is, so I don't know if the transducer is starting to fail, or a problem with the driver board.
With mine, I disconected the transducer, and powered up the unit, and using my DVM I would get a reading of around 600vac.
I used my picoscope 2202 to test the low voltage side of the signal gen for the drivers, and thats was switching at 40Khz, as expected.
As my picoscope does not support high voltage, I am unable to see what the wave form looks like to the transducer.

If you can open up your ultrasonic bath, and take some high rez pictures, we might be able to help you find what has failed.

Best Regards.