RM> I suspect the "real fault" was poor layout.
 BL> It's never layout. These things take off around 1-10 MHz and
 BL> layout problems don't begin till the tens of Mhz. My guess is
 BL> that a large proportion of *all* those converters oscillated,
 BL> depending on the actual transistors fitted... a design fault.

 RM> It's a fault I haven't seen before, so I wouldn't call it a
 RM> "large" proportion

  He, he. I'm not fazed by that. I've designed sets with inherent
faults that no tech *ever* found. A tech's approach is to keep things
working; an engineer's approach is to make things fail.

  With a power supply, dc/dc converter, or even an audio power amp,
the load itself is the main variable. They can work perfectly until
you get one particular funny load... but I still count that a design
fault. The engineer should have found it.

 RM> So any Darlington pair is a potential HF oscillator?

  Not Darlingtons so much, which is the reason for their popularity.
It depends on the configuration. Adding a funny emitter circuit can
set them off.

 RM> I've seen amplifiers of all sorts take off, and the problem is
 RM> either poor design (read: layout)

  I see. I've just told you that it is *never* (read: never) layout,
based on extensive actual design experience of these things and you
choose to contradict me. Okay

  I supppose you've confirmed this by fixing an oscillating amp or
DC-DC converter by changing the layout, and then changed the layout of
a good one to make it oscillate? Pig's arse, you have!

  I don't think you understand design. If I put a design into
production (read: that thing you have *no* experience of) and it
fails, there is a sudden stockpile of 100,000 repairs! Therefore, I
can spend 200 hours (read: $24,000) making sure none of these things
happen... and I do. Engineers don't just get one working, we get them
*all* working, and to do this requires a rather deeper knowledge than
is possible for an amateur working at home or a service tech in the
field. It's just the way things are. You can't spend $24K on a
single set or a few dozen, but that's just a piss in the ocean of a
design laboratory's $2M budget and 100,000 units a year.

  Service techs just slap on a 0.01 and hope for the best - if you
can't fix it, modify it - and never find the real reason, never
understand the actual process, and accept wild guesses (read:layout)
as actual fact. You don't have to do any more. Most of the time, all
you guys have to do is fix something that an engineer designed right,
and when you find a bad one you use standard cures... slap a 0.01
across the collector base (0.01 is bit high, btw).

  Unfortunately, design engineers can't work like that. We have all
the time in the world to get it right. I offered you a little of that
experience but you know better.

  Pig's arse, you do.

 RM> I'm quite aware of the risks of sticking a low freq pole
 RM> partway down a feedback amp. In this case, however, it worked.
 RM> QED.

  If you understand it so well, why use such a ridiculously high
value? 1000pF would usually do the trick c-b, or 0.01uF b-e (at power
frequencies). All you have to do is move the already-existing pole,
and 20-times is usually enough.

  Of course, if you get it oscillating in the 10-100MHz range, you
need to look at layout, but I already said that, didn't I?

Regards,
Bob
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