Hi Folks,

Talk about diving in at the deep end... I'm the wrong sort of engineer (materials, and now IT sysadmin) so last time I did electronics was in 1st year circa 1988...

however, I have nothing to lose...

I have the following Ultrasonic cleaner purchased about a decade ago (~$425 from JayCar)
https://www.jaycar.com.au/ultrasonic-cleaner-commercial/p/YH5410

it stopped working recently, so it got disassembled. looks like several of the resistors are kaput, but most of the rest of the components (capacitors, transformers etc look OK - no obvious burnout or damage).

Cause of the failure is probably overheating/overload because forced damped load with uncontrolled variable load (I put something too heavy in) or excess moisture (I spilled some water once too often).

I'm content to replace the broken bits and see how it goes... however... I'm having trouble identifying the capacity of the burned out resistors...

Photos - raw board.




I've had a go at working out the circuit...



The broken resistors are R3 and R13.

White IC is a switch (http://pdf1.alldatasheet.com/datasheet-pdf/view/5044/MOTOROLA/MOC3083.html)

R13 is part of the power conditioning circuit


R3 is part of the oscillator circuit. That's the one that is bugging me...
It's old and damaged and it's hard to read the colours, but I think it's orange black gold (gold) - making it 3 ohms.


that's low... mind you, R18 & R19 are 0.22 ohms

There is a second board that connects to the L & N connectors in the middle of the board that basically repeats the circuit. R3 on that one is also destroyed, as is R18 & R19.

So, before I go out and buy some replacement parts, do those numbers - 0.22 ohms and 3 ohms - make sense? DO I believe my guess work?

 

red,red,silver is 0.22 ohms.
If you're sure that's gold, it's 3 ohms.

Somebody will be along soon to double check me.
Of course, I'm in America, so I could be reading those up-side down.

 

I'm as sure as I can be - which isn't very...

I have a knowledge gap - specifically;

how hard is it to analyse this circuit and work out what logical values for that resistor are likely to be?

how much does it matter if I get it wrong (on a scale from "meh" to "hellmouth")

 

I'm 99% sure. I even walked to the repair room, pulled out the resistor drawers, and looked to see that resistors under 1 ohm end in silver and 1-10 ohms end in gold.

Your transistors are labeled as bipolar. Not a problem to see if they are shorted or open. Not delicate and afraid of static electricity like mosfets.

 

fuse was the first thing I checked. Discharge near the inbound 240VAC N is almost certainly due to water - and was probably the last straw. (that's after I cleaned it up)

For the sake of simplicity, I've simplified the order of events... first one transducer stopped (daughter board and that charring wasn't present when I attempted to repair that half) then the second stopped (and the charring, and moisture was) - cascading failure? yeah, probably... but now I gotta try and work out what it was supposed to be...

in a perfect world, I would understand the circuit well enough to be able to model it's behaviour... but right now I'd settle for an inkling as to if the R values are reasonable as guessed from the fading paintwork...

 

What colour is the third band underneath, has it completely been destroyed?

Looks silver to me, so it would be 0.3 ohms.

 

it's pretty much the same all around - gold with a thin band of blue, which may be copper salts.

let me go grab a photo...

TBH though, the colour of the bands is either correct or way off - with the original colour long gone...


what I'm fundamentally after is an indication on if that value - 3 Ohms - is reasonable for this circuit... or if it's crazy talk...

 

 

as you can see the orange and the black are pretty clear.
but the next two - that faint blue band that is the width I'd expect for the colour band could be the remnants of some other long-gone colour... probably not blue...

or it could be gold, as intended by the manufacturer...

 

what I'm fundamentally after is an indication on if that value - 3 Ohms - is reasonable for this circuit

Of course it is. Why would I have drawers full of resistors in the range of 0.01 ohms to 9.9 ohms? Because they are used in high power circuits like this one to get NPN transistors to share a load equally.

As for the gold with blue? Any trace of gold color is almost complete proof that the intent was gold. Other colors don't degrade to gold. They degrade to gray or black or white.

 

Of course it is. Why would I have drawers full of resistors in the range of 0.01 ohms to 9.9 ohms? Because they are used in high power circuits like this one to get NPN transistors to share a load equally.

As for the gold with blue? Any trace of gold color is almost complete proof that the intent was gold. Other colors don't degrade to gold. They degrade to gray or black or white.

Awesome news. thanks. That's exactly the sort of response I was after (either don't be an idiot, no one uses that! or, don't be an idiot, of course it's sensible..)

now I can go ahead and repair it with only a normal risk of it melting...

 

Update:

well... that was interesting...

first up, make sure you actually attach power to the board before testing - it helps. (twice! no. really!).

for what it's worth, the control panel circuit works just fine. so powering it on, you get a nice beep and the LED's light up.

but, dialing up the timer and hitting go resulted in almost immediate failure. Fuse blew...

so now I'm wondering what else is broken...

(advice - I could only get 3.3 ohm or 2.7 ohm resistors - not 3 ohms... I chose 3.3 - was that a mistake?)

how do I test capacitors?

 

been hibernating for christmas - but back at it now...

seem to be an expert at destroying components... 3A fuse, then another resistor - 0.22 Ohms R18....

 

Can you take a picture of the foil side?

 

Can you take a picture of the foil side?

like this? (flipped and cropped - same orientation as above)

 

it occurs to me that I should probably include the other "half" of the driver...

the above board includes the power conditioning part of the circuit, the control panel and the high power switch, as well as the oscillator/driver for one piezo-transducer. this bit here is the other piezo circuit...

the two circuits are connected via the L and N leads. The + and - connect to the peizos...

 

It may be a trick of the light, but there are some dodgy-looking joints in the top right corner of the post #15 pic.

 

hey Alex - yeah, just went and had a looksee... there are two connections that lack full solder where the big inductor attaches. (circled in red - is that what you mean?) It looks like it came that way from the factory. I had a squiz with an eyeglass and there is a little contact between solder and wire, and a touch of corrosion. The rest of the joints are OK. I'll add that to the list of things to fix before my next pyrotechnic adventure.

so, plan of attack:

1: replace both 0.22J resistors
2: replace the 3.3 ohm resistor with a 3 ohm resistor (my choices were 2.7 or 3.3, so I'm going to put a 3.3 and 2.7 in series, in parallel...)
3: resolder that junction
4: get a box of fuses
5: try not to release too much blue smoke...

 

Yeah those joints look a bit suss.....best to give them all a bit of a touch up, I don't now if ti's the lighting or wot not but quite a few are looking a bit crook, in anything it'll look a bit more professional when touched up. May even placate the gods and save sacrificing a (insert something outlandish in this space) to make them happy

Hope you haven't blown any internal smoke valves.

 

up close and personal the rest are OK... but yeah - it's less well made than I'd hope for given the cost - purchased @ jaycar about 10 years ago for $425 - "professional" grade cleaner...

my main problem is I'm sufficiently inexperienced with LRC circuits that I'm stumbling in the dark rather a bit...