On 28.02.2016 22:59, Dimitrij Klingbeil wrote:
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.

P.S. Since you indicated that you have some background with radio...

Consider the collector of V1806 as a signal source. As a signal source
that can basically drive a 800 V peak-to-peak square wave.

Consider the whole power supply board (including any cables and the
isolation transformer or variac that you are using to feed it) as one
half of a dipole antenna.

Consider the scope (the whole metal chassis) and the probe cable as the
other half of the same dipole antenna.

Consider the two halves connected in the middle by the probe ground
clip, at that overheating power resistor in the supply.

What you get, is a center-fed dipole, sitting on your table, and being
driven with a 800 V peak to peak fast square wave. Not a light load.

An isolated high voltage differential probe would "separate the halves",
so that the big (parasitic) dipole would no longer exist.

Regards
Dimitrij