Recently, Somebody Somewhere wrote these words
> Hi Declan!
>
> > BTW, These diodes are a mains full wave rectifier, but have NO
> > electrolytic smoothing it. That's pretty unusual on a single phase.
> > That's where I'll start. I know, I know, electrolytics are useless at
> > hf, but it might keep the voltage up across those diodes.
>
> This might be on purpose!
Yes, it was :-(
The Half/Full power switch has a diode there to rectify the AC _before_
it goes to the bridge rectifier as a form of power reduction.
> We have some ultrasonic baths at our institute and from all of them
> you at least hear an audible 50Hz (or better 100Hz) kicking - which
> might be intentional.
>
> At least I cannot see an electrolytic cap as a major factor when it
> comes to production cost. On the other hand it might just be that -
> another buck/quid saved on the circuitry... and for driving the
> high-voltage oscillation through the piezos (I don't think you really
> have "coils" under the bath, do you...) you surely don't need a
> filtered mains voltage.
>
> Did you have a look at the base and collector voltages with an
> oscilloscope? You might see some ringing there, but even if not, the
> faster, newer transistors (and I suppose we all agree on that) might
> just lead to a steeper switching of the transformer, which will
> increase the energy in the harmonics (there we are again...).
NO. This thing relies on
1. Board firmly bolted in place onto bottom
2. Case being fasteded up tight
to make the earth circuit, and the drive to whatever it is under the
tank. I've blown up a scope before, and have no wish to repeat it!
>
> I guess it's more tricky to kill off the oscillations by artifically
> slowing down the transistors without re-inventing the whole circuit,
> but a semi-decent line filter should do the trick.
>
> Now I am sitting in my office and I have one of those modern PFC power
> supplies, which has a discrete line filter, instead of those metal
> cans: line | | ( | | ) L1a( | | )L1b ( |_| ) | | | |
> o----o-| |-o----o | | C1 | | | | | | |L2a( | |
> )L2b| --- ( | | ) --- C2a--- ( |_| ) --- C2b | | | |
> o----o-| |-o----o | C3 | | | load
>
> C1 0.47uF 300V~ X C2a, C2b 2.7nF 250V~ ceramic C1
> 0.22uF 300V~ X
>
> L1a, L1b ~15 windings each on both sides of a toroid L2a, L2b ~30
> windings each on both sides of a toroid
>
> The inductors are of the "common mode, open former" type, probably
> around 0.5mH per coil....
>
My mind is working on a completely different line of thought. The
customer said "It [the U/S cleaner] used not to interfere [with the
radio] before".
/sound of 1c dropping
He _obviously_ did not have a mains radio, ergo he has a battery radio.
If he had a mains radio, he would have his ears blown off by the
radiation.
So what I have to kill is not the huge mains radiation, but the
transmitted stuff, which means that whoever told me to fix the
transistors (Fred Townsend IIRC) is forgiven :-D.
Now the neighbours will think all hell has broken loose again when this
thing goes back home, but that's not my issue.
/Goes searching for BUX 80s - and stupid little capacitors to put across
those diodes.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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