Don't use alcohol in an ultrasonic machine, especially if you are inside. (Risk of fire/explosion doe to the atomizing of the alcohol!)

Use a commercially available PCB flux cleaner instead. This stuff for example, "Elmasonic 800 0101 Elma TEC Clean A1 Ultrasonic Cleaner Solution Concentrate for Electronics and Optics- Powerful Cleaning Fluid for Industrial Use", is about $265 per 2.6 gallons. It a concentrate but that's still pretty pricy!

That said, if you clean a board ultrasonically that has potentiometers on it, you will have to properly relube them as that will be history also.

Is a brush (like the ones Alex sells) and alcohol looking better?

Usually the area needing a cleaning is small and a drop of IPA (aka Iso-Propyl Alcohol) is all you ll need. In cases of boards with heavy flux residue from previous work, dirty boards, oxidised (like when solder has a dark color, not shiny anymore),  or boards with extended corrosion from liquid spinning, i have found out that an automotive industry product is awesome. Its an American product, here in Greece it costs about $3 bucks a can. The name is "FLAMINGO contact cleaner" , and it is supposed to be formulated for auto-motive repair industry ( you can find the specs in the oficial site if you google it... )

What is does is nothing short of a miracle. I have used ultrasonic cleaning in the past, i have not found an ultrasonic fluid that cleans and restores board to a FACTORY NEW condition like this spray does in 10 seconds. Try it on some old boards, spray liberal the whole board for a few seconds, let me know what you think 🙂

i generally use mine for certain water damage jobs but you do have to remove shields or dry really well. normally clean fux from rework with brush and ipa, depending on the position on board of course