After my ultrasonic cleaner failed after not using it for two years, a teardown was in order. I once fixed a similar unit replacing two high-voltage transistors that drove the ultrasonic transducers. That didn’t work for this unit. I am still troubleshooting the problem. Meanwhile, here is the teardown.

This Fisher Scientific ultrasonic cleaner is made by Branson, the well-known ultrasonic cleaner company. This is a simple stripped-down model that does not have a heater or a timer. While I know that running these units dry may cause the electronics to blow up, that never happened with this unit. It was moved 3000 miles and then sat in a humid Florida garage for two years, and that seemed to kill it.

This model has a 2.5-gallon tank; large enough for my Harley carburetors. It’s also handy for eyeglasses, jewelry, and anything else that needs cleaning. The cruddy scale on the side of the tank is due to the evaporated cleaning solution, as well as the hard water I used in California. I now use distilled water and Branson MC-3 cleaner solution.

The unit has no forced-air cooling. Instead it has these slots on the side, as well as slots in the front and rear, so natural convection can cool the circuit board. Some of the stains were on the unit when I won it on eBay, others got added by proximity to my ferric chloride circuit board etching.

The front of the unit has these unused screw countersinks. Instead of a thru-bolt, the sides attach to this location from blind screws on the inside. The covers appear to be injection molded.

The rear of the unit comes off with four screws. The square nuts are captivated in the side covers, so the injection mold has some expensive slides to allow these features. Don’t forget to put these square nuts back in when you assemble the unit. Don’t let the nuts fall out and get lost in the carpet as you take things apart.

The rear panel has an aluminum plate that holds the EMI (electromagnetic interference) ac input jack, as well as a rubber pass-through for the tank drain pipe. The white label reveals the unit is a Bransonic B5200R. The 1.5A current rating is about right for a 185W unit like this. The red label is a multilingual lawyer-speak warning label.

The inside of the rear cover shows good mold design for an injection-molded part. I would guess the material to be ABS, or an ABS-polycarbonate blend.

The rear aluminum panel holds an EMI filter that keeps conducted electrical noise from passing out of the unit and into your house wiring.

The input EMI filter has a 5A fuse. It takes an hour to figure out that you have to pry this little tab out at the back of the plug recess in order to get the fuse out. This fuse blew after I replaced a 3A internal fuse on the circuit board.

Here is the unit with the back off. The ultrasonic transducers are older flat piezo-electric types. There is a circuit board, and an iron-core inductor mounted to the aluminum base panel. That panel slides into slots on the cover and is captivated when you screw on the back panel. Note the earth ground wire from the EMI filter to a stud on the bottom panel. That stud also has a wire that goes to the bottom of the drain pipe, providing earth ground to the tank.

The design is such that you can connect everything and operate the unit opened up. I added a jumper wire to ground the tank, the wire was not long enough to reach and I did not want the safety hazard of an ungrounded tank. Try not to defeat safety measures. It’s you that will get electrocuted working on things. You can see two of the internal screws that hold the sides to the front panel.

The PCB has two 450V FETs and a cross-connected transformer to drive the FETs into oscillation. The yellow inductor in the front is a common-mode choke. All the unused pins on the left are for the heater, timer, and other features not used in this stripper model. The axial fuse had blown with the FETs so I added an in-line fuse. Despite replacing the FETs and checking all the components, I could not fix the unit.

To better understand the circuit, I reverse-engineered the PCB and came up with this schematic. The 2.2µF capacitor is not nearly large enough to make a dc bus, the circuit must have its 40kHz output modulated by the 120Hz rectified ac input. This is a reasonable design tradeoff. It improves the power factor and eliminates large, expensive, unreliable aluminum electrolytic capacitors. The iron-core inductor provides an impedance that tends to wash out the wild impedance swings of the piezo transducers. Like a crystal, the input impedance changes greatly with applied frequency.

With this schematic in hand, I will start probing the PCB with my LeCroy oscilloscope to try and figure out what could possibly be wrong. All the diodes and resistors and capacitors check out. I replaced the FETs. I have a hard time believing an iron-core inductor is shorted; the magnetics all have continuity. In addition to reverse-engineering the schematic, I laid out a replacement PCB. If worse comes to worst I can get a new board made. These units sell for $400 to $700 used, so I would like to get it working.

Also see :