Recently, Somebody Somewhere wrote these words
>
> I'm having a lot trouble understanding what you are saying. Let me
> state my belief. The aluminum case TO3s that you replaced the
> existing transistors with probably have a gain bandwidth product
> (Ft) 10 to 1000 times greater than the old transistors. This means
> that instead of driving the transducer with a sinusoid waveform it
> is now a nice crisp square wave. This could cause ringing and RFI.
> Furthermore, the hfe is now greater while the parasitic capacitance
> is now reduced by a factor of up to 100. This now permits parasitic
> oscillations that are heard as RFI. Poor grounding, inductive
> spikes, and poor earthing are the red herrings. Don't get me wrong.
> These are issues to be dealt with but not the source of RFI which I
> see as the subject of this posting. Fred Townsend
>
>
I checked the gains. The Manufacturer's transistors died (my fault), and
I now have the replacements in place.
BUX 80 (The manufacturer's job) has a minimum hFE of 3
BU225s (The Replacements) have a minimum gain of 10.
Curiously, the BU225s are the older, as I am using old stock (93/94
Vintage) and they are steel, whereas the manufacturer's ones are
aluminium. As BU225s have a much higher VCE, (2250V vs 800V) I do not
expect great gain from them.
The entire thing, of course, as an ultrasonic cleaner is enclosed in a
steel casing which has this convoluted earth. I know better than to
think it is a perfect seal of all electrical noise - far from.
>
> You might be able to minimize the spikes by putting an RC damping
> network (snubber) across the primary of the transformer (between the
> collectors of the transistors). This is what is typically done with
> triacs and SCRs to lower the level of flyback transients. I also
> found a lot of power supply schematics that put the snubber across the
> secondary instead.
If there isn't one there, I will certainly try that and see what effect
it has. I'm not too worried about radiated RFI, as there is a stell
casing which (theoretically anyhow) should act as some sort of Faraday
Shield. I had pointed out
> >Moving the radio off the same wall socket lessened interferance
> >considerably.
>
Trying to tell people that when I moved the u/s clearer to a different
mains point from where the radio was connected, the interference was
lessened considerably. That would tell me that the interferance was
passing via the power lines, which led me to suspect diodes.
BTW, is there something funny with your box, Dave? A button stuck? I got
5 copies of your reply.
--
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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