Elma Ultrasonic cleaner fading then failing - troubleshoot help

[Cykar]

Hi everyone.

I was given an Elma S30H ultrasonic cleaner - which would really help me in my part time job of cleaning up and polishing jewellery. It's an expensive machine, but the jeweller who gave it to me said it stopped working, and if I can fix it I can have it - it's worth over £600 so I'm eager to fix it if I can, since it would be a quality tool for me to use.

Symptoms:
So, I turn it on and initially it seems to work. I can here the ultransonic transducer doing its thing, the heater, degas, sweep and timer functions all work.
After about 20s the sound from the transducer starts fading in and out, then 10-20s later it stops, and the indicators on the front of the unit show a "running light" on the timer leds.

The service manual says "There is a fault on the PCB control, on the connection betweenultrasound >> transducer system or on the transducer system." (see the service manual page 19 here: https://www.manualslib.com/manual/1596103/Elma-Elmasonic-S-Series.html?page=19#manual). Since the transducer works, initially at least, I'm guessing there's a problem on the control board itself. Their advice somewhere else in the manual: send it back to be fixed - not what i want to do since i have little money, and anyway, where's the fun in that? Similarly, new control boards are available to buy, but at over £100 I don't want to do that either and so have been trying to figure out what is wrong myself.

As far as I can see visually, there is nothing burnt out or obviously disconnected. The solder joints seem fine, and I have tested the caps which are all ok.

There is a small 8 pin surface mount chip by what I presume are power transistors on a big heatsink which you can see on one of the images I have attached. It is an L6384ED, a high voltage half bridge driver.

Could this be the issue?

I could get a new one for a lot less than a new board and although I might not be able to solder it on neatly, I'm sure I could do a good enough job but I would really like some other opinions as what the cause of the problem might be, or what else I should be looking for, before I start ordering parts.

Thanks for your help.

[Cykar]

I have put a video on youtube to show what happens:

https://youtu.be/KkkGjhdqYwA

[JKKDev]

I would try to test each module separately. For example, start with only plugging in the PSU and the user interface board. If everything seems fine, add the ultrasonic element thingy. If that's fine add the heater and so on. At some point, you'll recreate the error. At that point try every other thing plugged in except the presumed faulty part. If the machine works fine then it's either the part or the circuitry that drives the part. To figure what's wrong at that point we'll need a bit more information.

If the machine just won't run without certain parts then you'll have to figure out what's wrong from the problems it's got. Just remember it's not always the part that seems faulty the one that's actually faulty (eg. failing heater might be caused by a faulty PSU).

Also... What's that shiny thing on the large black square component designated BFI29....?
And another thing... Check what's going on with the top right black square component with 4 pins. There are 3 dots on it one of which is a bit suspect. There should normally only be one indicating the part orientation.

[jogri]

Given the fact that those units create a lot of vibration (and heat), you could try to insulate the PCBs from the vibrating parts by extending the cables and running it partly disassembled. The fading sounds like something that controls the transducer is loose: no connection when the vibrations reach a certain point -> vibrations stop -> it makes contact again. I recommend touching up the solder joints even if they seem fine as you won't be able to see if a component is properly attached (it is at least somewhat attached otherwise you wouldn't be able to start it, but that doesn't tell you if it is still attached when the unit is running).

And keep in mind that ultrasonic vibrations generate quite a bit of heat (those things get hot even without a heater), so you should definitely check the ESR of those caps (if you haven't already) as they could have been operated at elevated temperatures for quite a while (also check other heat sensitive components).

[Cykar]

JKKDev, testing parts disconnected i don't think will work since it will just register other faults, the fact that parts aren't connected. And as the service manual says, the leds are telling me its between the ultrasound and the transducer system itself, or just the transducer system.

The "shiny" mark on the BF129 (which I think is some kind of filter) is simply a paint mark, no doubt to indicate orientation. And the square component you pointed out is an oscillator, the marks look perfectly fine to me (SG-303JF), just from manufacturing.

jogri, yes, I have tested all the electrolytics, out of circuit, and they are all fine. I have also reflowed all the through hole solder joints. Having done all of that, the problem remains exactly the same, so I'm still thinking there's something about it not getting the power it needs, and that it might be the driver.

Thanks for your replies, but without any other ideas I might just have to get a new driver and try that.

[JKKDev]

Could you point out which connector is for the transducer and maybe figure out the expected signals, then compare that to the actual signals (voltage, frequency, current,...)

[Cykar]

I've removed the mosfets, and tested them. They are ok.

Was wondering what the blue and red circled components are? I thought the ones i marked in red were diodes, but they don't test quite right in circuit.

JKKDev, Don't think I can test for signals other than just voltages and that would be very tricky given the short cables connected to the board, and the way it is installed in the unit. Not sure what I'd be looking for either...

[jogri]

I thought the ones i marked in red were diodes, but they don't test quite right in circuit.

Meaning? A suspiciosly low voltage drop? That can happen when you try to test them with a DMM, the current is just too small to get you meaningfull results.

At this point you should consider reflowing the SMD solder joints (at least the joints to heavy objects like ICs) as they are far more likely to cause a defect: through hole joints consist of a hole filled with solder (how should they lose contact?) while a SMD joint just has to lift up a bit to become loose.

And you should also try to determine if this problem is caused by vibrations (if it isn't you don't need to reflow the solder joints), just solder some extension wires to the underside of the connectors on the PCB and run it disassembled.

Btw, what's that ST chip on your last picture? It looks like a GK42-something, but ST doesn't list such a part.

[Cykar]

jogri, that chip is an L6384ED, a driver for the mosfets readily available (like here: https://uk.rs-online.com/web/p/products/0445652/). The mosfets are IRF840, 2 of them.

I'll try reflowing the the surface mounted components. Going have to wait till next week though now

[JKKDev]

Ok at least tell me how many pins does the transducer have. You could solder some wires to the pins and measure voltage on them. If it's stable when you have problems than that's not it, if it's falling with the performance of the transducer than something is probably wrong on the board itself.

[Cykar]

If it's stable when you have problems than that's not it, if it's falling with the performance of the transducer than something is probably wrong on the board itself.

That's a good point. I'll reflow, put it all back together with some extension wires and test some more on tuesday.

There are 2 wires that connect to the transducer from the board, so should be easy enough for me to check that.

Thanks for your help.

[JKKDev]

If it's only 2 wires the error explanation makes sense as it's not possible for them to know what's actually going on. All they see is an anomaly on the driving line and report the error. I would hazard a guess that if you disconnect the transducer the machine wouldn't know it's missing. I think it only sees a drop in amplitude and that makes it think something's wrong.

First, check which pin is ground. Then you should expect high-frequency AC with constant amplitude on the other pin (https://www.piezodrive.com/ultrasonic-drivers/intro-ultrasonic/). If you can figure out the exact wave used (frequency, amplitude, waveform) you should be able to drive the transducer on your own to test it (without a scope and possibly some high voltage probes - I'm not sure what amplitude to expect - this would be impossible as well as a source that can supply such waveform).

From this point on I'm not so sure if what I'll suggest is the best testing option but I don't see another way. Since you still haven't pinpointed the error to either the transducer or the board and you can't (probably) drive the transducer you are left with loading the board AC supply with a resistive load similar to that of the transducer (you could measure the impedance of the transducer as it's still just a coil of wire I think).

If the result of the above test is the same as when the transducer was connected then the problem is with the board. If that's not the case I'm out of ideas for more tests so at that point I'd say the transducer is faulty.

If someone knows of a proper way of testing transducers as well as loading the HF AC supply do share

[Cykar]

Well, done a bit more investigating as far as I can.

Reflowed surface mounted components, no change.

Attached leads to the point where the transducer is connected. Attaching my multimeter in AC mode, I saw 400V+ initially, when it works as it should, then as it starts to fail the voltage drops to 220V-300V (not a very fast multimeter so hard to be too exact but what is happening is clear). This confirms what I initially felt was the problem from the way it sounded, which is that for some reason the transducer is not getting the power it requires when it starts failing. And given that it fails, comes back, fails, comes back etc. does that indicate a problem with the oscillating circuitry or or the driver?

I suppose I have to work out where the oscillating signal goes into the driver to work that out?

 

[JKKDev]

I would follow the signal (or the supply voltage in this case) ftom the output backwards measuring at every stage. At some point the input will be constant and the output will be fluctuating. That is your problem (either a component or a section).

From your last post it's not quite clear... Does the output fluctuate up and down (from 400V to 220V and back up) in sync with the transducer or does the output drop and stays constant while the transducer oscillates?

[Cykar]

The output drops in sync with the transducer. When the transducer comes back up to its normal volume, and the activity that can be seen in the water, the voltage has gone up again. So yes, the voltage drop is totally in sync with how the transducer is behaving.

[aqibi2000]

Using a dual channel oscilloscope observe the rail voltage for the control circuits in comparison to the 400v. Is there anything to see?

Could be as simple as the rail voltage dropping off due to a failing capacitor —> leading to a drop off in the transducer’s chopper circuit


PS have you intentionally bridged the 2 lower pins on the transformer in the photo ?? Are they meant to be common

[Cykar]

Those transformer pins were always soldered together.

I will try to follow the circuit and check the rail voltages on the controller circuit, tho I cant do it today. I have previously tested all the caps out of circuit and they all tested good so, hmmm...

[TheMG]

You say you've tested the electrolytic capacitors... but what about the film caps? While typically more reliable than electrolytics, and despite not suffering from the same failure mechanisms, they can still degrade and fail, especially if the film cap is subjected to repeated transients, each time it "self heals" it loses a bit of capacitance.

Might not be what's causing the problem, but I definitely wouldn't overlook them. Quick capacitance check to make sure they are in spec.