These guys have repaired my Branson unit several times. Either there is an open somewhere, possible a loose wire, or one of the FETS is blown. Thats all the circuitry that was on the PCB. At the operating frequency the transducers look like a large lossy capacitor. The 250 W unit used two transducers connected in parallel. After the transformer was a large choke coil to resonate the transducers.
My units used feedback from a small series transformer on the output to drive the FETS into oscillation. If you have B++ and no output then the FETS are the likely suspect. The unit first rectifies the AC so check the ouput of the diodes for about 250 VAC.
My units had a fuse soldered onto the circuit board so check this. Added capacitance on the output can cause spurious oscillations and damage the output transistors so don't probe there with anything but a high impeadance probe. The output forms a resonant circuit with the transducers. These units generate high voltages when on so be carefull. Mine were smaller 250 watt units but there could be similarities so I'll post what I know and see if it helps. I worked harder than I needed to figureing out the feed back paths in the oscilator as the it wasn't related to the problem but I finally figured it out. I just opened them up and traced out the circuits in my notebook. I fixed a couple of cleaners for my wifes work and they weren't hard.