Ultrasonic Cleaner repair L&R T14

Hello,
I need some assistance with troubleshooting the Ultrasonic driver circuit in the L&R T14 cleaner I recently acquired.

When I received the cleaner, I was told it was non-functional. I opened the case and looking at the PCA, I saw that one of the
power transistors was damaged (hole blown in side and remnants of fire (soot) around the damage area.)
Also, a 2W carbon resistor was completely burned to a crisp. parts were missing and only the metal stubs sticking up from the PCB remained.
Lots of soot on board from what was obviously a 'flame out' as the resistor smoked.

I replaced the resistor (8.2 ohm) and the power transistor MJE13009 (NPN 12A 400V). At the same time I also replaced the 2nd power transistor
(same type) as it too was not functional.

I connected the Piezo drives/tank to the circuit, filled the tank with solution and applied power.
the circuit worked for about 20 sec, then the same resistor 'cracked' open (with a popping noise)
and the 2nd resistor (same value on the other power transistor) also cracked open.
I quickly removed AC power, but it was too late. Both of the NEW power transistors had shorted (base to collector).
and the collector to Emitter was open. :-(

I made a YouTube video so you can see the board and the schematic. Please see this video

If you can proved me with some ideas as to 'why' these power transistors are failing, please let me know.
You can reply here or comment at the YouTube video.

thanks

Welcome to ETO!
My guess is that you may have a shorted turn on the output transformer.
Check that all the diodes are ok if you can.
To make it easier for others to follow this thread, I've taken the liberty of grabbing a screen-shot of the schematic:

alec_t said:

Welcome to ETO!
My guess is that you may have a shorted turn on the output transformer.

Click to expand...

I would agree, a shorted turn often blows the drive transistors in any kind of transformer driven circuit.

There's very little else there that could go wrong, it's a pretty crude and simple circuit.

To make it easier for others to follow this thread, I've taken the liberty of grabbing a screen-shot of the schematic:
View attachment 92648

Click to expand...

Thanks for that

Thanks for the replies. Hmmm, that 'may be' the case. however, I do measure some 'resistance' on those transformers. While it's not an accurate way of measuring inductance, it gives you 'some'
idea if there is a short of not. Unfortunately, I don't 'know' what a 'good' transformer value should be, so just a guess on my part. :-(

a couple of directions I could go from here.
A) could I 'disconnected' the Piezo transducers and run the circuit 'unloaded' without damaging anything? removing the load could allow me to make some measurements
to see if it's operating correctly.
b) what would a piezo transducer look like (resistively' when measured with a DVM? open, Short, something in between?
c) what if I removed the transformer from the circuit, and replaced it with a resistive load? could I safely measure the circuit and see if all is OK?

thanks

The piezo transducers really look like a capacitor. I did some measurements on mine. I "Plan" on replacing the guts with a driver manufactured overseas. I have it, just didn't do it yet.

oilseed said:

Thanks for the replies. Hmmm, that 'may be' the case. however, I do measure some 'resistance' on those transformers. While it's not an accurate way of measuring inductance, it gives you 'some'
idea if there is a short of not. Unfortunately, I don't 'know' what a 'good' transformer value should be, so just a guess on my part. :-(

Click to expand...

Neither resistance or inductance would help you, you need a 'ringing' tester to detect shorted turns.

a couple of directions I could go from here.
A) could I 'disconnected' the Piezo transducers and run the circuit 'unloaded' without damaging anything? removing the load could allow me to make some measurements
to see if it's operating correctly.

Click to expand...

That could quite possibly blow it anyway, regardless of any fault.

b) what would a piezo transducer look like (resistively' when measured with a DVM? open, Short, something in between?

Click to expand...

Open.

c) what if I removed the transformer from the circuit, and replaced it with a resistive load? could I safely measure the circuit and see if all is OK?

Click to expand...

It wouldn't work, the transformer is the part that makes everything else work.

To Keep it simple...
I see that some companies (and even on Ebay), one can purchase new piezo transducers. However, I also see they are designed to resonate at different frequencies.
32Khz, 35Khz and 40Khz. how do I know which one to get? (assuming my Piezo's are shot)?
then, how does one 'tune' the circuit to the new piezo? put in a variable pot (dual) across R4 & R5? then adjust for maximum resonance?

to Nigel? thanks for the added feedback. I'll check the Piezo's. if I get any resistance measured, maybe that's the problem?

Cheers

I ordered mine from here: https://www.ultra-piezo.com/Products/ultrasonic-generator/ultrasonic-cleaning-generator/ Postage was the killer. It actually came with transducers. About $83.00 shipped to US. No service info is available for the board. The board was cheap, shipping was high,

The transducers usually crack or fall off because of operating them without water. For mine, I just thought the design was crazy and not robust enough. So, the plan was to keep the original transducers and attach another chassis on the bottom for the new controls.

My tank is pretty small- Estimate 8 x 4 x 3" deep.

I like yours better.

Again, I can't say if it works yet. PS: a.k.a KISS works.