I started with a ~200w 10 Liter ultrasonic. It was made by a company called prosonix. They no longer exist. I paid $300 for it. I've had it for close to 8 years now and it worked great for 4 of those years until the timer failed. Then the transducers gradually stopped working.
I took it apart and found out that it was a very cheap rebadged chinese unit
. This is the closest resemblance to my unit:
https://www.ebay.com/itm/162246817237 It turned out the unit was run by old school metal caned to-3 packaged transistors. Unfortunately, the numbers were rubbed off on the transducers. I tried repairing it with other transducers but they simply don't sell new genuine to-3 transistors of the right specifications to match the unit.
What surprised me is how simple my ultrasonic machine was. It used an off-the-shelf Winco stainless-steel food tray as the tank, and the food tray lid as the lid. The basket was a unit-specific folded sheet aluminum basket. The transducer boards were clearly repurposed general transducer boards like these
https://www.ebay.com/itm/372237002078 stacked and screwed to the bottom of the unit.
There are dozens of other companies who rebadge these chinese units as well. They will look remarkably similar.
Regardless, the unit served me well for cleaning engine carburetors, and it worked ok for cleaning circuit boards. The unit is quite loud though, and I have major doubts that it is truly operating at 35Khz. if you get a cheap chinese ultrasonic, leave it in a different room when operating.
In terms of solution, I originally started experimenting with various de-greasers + distilled water, but that would often not clean off flux residue. From my research, it still looks like the best suitable PCB solution is Branson EC + distilled water. However, I have not found any literature on how to properly dispose of Branson EC. Considering it is a hazmat-class chemical DO NOT POUR IT DOWN YOUR DRAIN. If you go this route, look up how to properly dispose of such a chemical in your area.
Some general pointers on ultrasonic machine operation:
-Don't ever run the machine without solution. This can crack the transducers. I purchased a used crest and the damage was pretty bad, likely from this reason.
-Fill up your tank to the level designated by the manufacturer. Mine was about an inch from the top, the part where there was a sheet metal bend. Improper solution levels can damage transducers.
-Avoid placing items directly on the tank bottom, especially heavy items.
-Don't move the object in the ultrasonic machine when the machine is operating.
-Most companies suggest not to put your hand in the machine while operating.
Some of the bigger names in ultrasonic machines are Branson, Sonicor and Crest. They seem to make decent units and have a bunch of literature on the subject of ultrasonic cleaning.
If you really want to go all in, research vapor degreasers