On 28.02.2016 22:31, Cursitor Doom wrote:
On Sun, 28 Feb 2016 19:06:38 +0100, Dimitrij Klingbeil wrote:

Also, even with a dummy load connected, the stray capacitance of
an oscilloscope, when hanging off the loose end of a power circuit
with some 800 to 900 V worth of HF on it, would probably cause so
much undue capacitive loading that the power supply circuitry would
hardly handle it.

Isn't this just another example of the unsatisfactory nature of this
resonant converter design? If the thing is *that* fussy that a little
bit of stray capacitance can catastrophically destabilise it, then
AFAICS it's a fundamentally unreliable topology and it would be
better to have used one of the non-resonant forms of converter.
Unless there's some compelling reason I may be unaware of not to for
oscilloscope power supplies, of course.

That's definitely not "a little bit". By very far, not!

Muscling around a scope chassis (not the probe tip, but the probe ground
and the big scope chassis connected to it on the other end of the cable)
from zero to some 800 V in several dozen microseconds is no small feat,
much less doing that 20000 times a second repetitively.

Not many power supplies will do that on an internal node without running
into major stability issues (unless you have a very small
battery-operated "pocket" scope, sitting on a wooden table far away from
any earthed metal, thereby being a "light" load).

The probe tip is not the issue, but the scope itself, hanging from the
probe ground, that is.