On 28.02.2016 20:53, Cursitor Doom wrote:
On Sun, 28 Feb 2016 18:55:47 +0100, Dimitrij Klingbeil wrote:

[...]
I think that the most realistic test would be to sweep the resonant
circuit with a signal generator and watch the waveform. If the
resonance frequency looks right (in the 20 kHz ballpark) and a
signal generator is able to drive it from a high 600 Ohm source
impedance to a significant amplitude without much "sagging" (that
is, the resonant circuit presents little load to the generator),
it's probably OK.

Thanks again, Dimitrij. You're obviously an expert on the little
understood world of resonant converters so when you say try this or
that, I make a point of paying extra attention. I liked your theory
on the resistor heating due to this supply running out of resonance
as a result of component values changing over time; in fact I'm
currently pinning my hopes on it. It's a pity I'm stuck here for a
few more days with my revolting in-laws but it'll be the first thing
I do on my return!

Hi

Please don't rely in my advice too much. While I do design electronics,
I'm very far from being an expert in this particular field. I've never
actually designed a resonant power supply, unless you count one little
3W prototype based on a modified Royer / Baxandall structure.

It may be relatively easy to look at a ready-made schematic and try to
guess various upper and lower limits based on parts and topology (like
"signal X cannot be higher than Y volts, otherwise part Z breaks down"
or "ratio of transformer X cannot be above or below A:B, otherwise the
ratings of part Y would be exceeded"), but that's not expertise by any
stretch of the definition. A lot may be intuition, but that's no
expertise either.


Somewhere I have a big old valve/tube capacitor tester capable of
simulating realistic high voltage working conditions. It'd be
interesting to know what kind of checks it's capable of performing
if it's still in working order and if I can find it among the
towering piles of obsolete test equipment I have here (a couple of
million pounds worth of gear at new prices adjusted for inflation) I
may possibly hook it up and give it a shot.

How about those 'Octopus' component testers? They subject the part
under examination to sweeping test voltages over the expected
working range and you look for any signs of breakdown on an
oscilloscope in X=Y mode. I guess this method is about as good as it
gets?

I've had to look up, what an "Octopus component tester" is. Apparently a
transformer with some provisions for routing the voltage and current
signals of the load to an oscilloscope, making a simple AC curve tracer.

I don't think that you'll need one here. It can test for breakdown, but
in your case that's unlikely (the capacitor would be buzzing and arcing
and the supply sure wouldn't work "almost normally"). It won't see the
problems that are likely to be important in an LC circuit.

1. The cap must have the correct capacitance. Any LCR meter or any
common pocket multimeter with a capacitance function can measure this.
This is a basic prerequisite that should always be tested first and if
the capacitance is wrong, no further tests will be necessary anyway.

2. The foils inside the cap must have a reliable connection (deviation
manifests itself as ESR, ESL, and the general inability to supply high
impulse currents). This particular curse will sometimes plague the
trigger capacitors from photoflash units (the flash won't trigger or
will only trigger erratically while the capacitance value is still ok).

This is difficult to measure directly, but can be checked with another
capacitor as a reference. You'll need a known good capacitor with the
same value (in your case: 15 nF), but not necessarily with the same
voltage (you can use a known good, but lower voltage one for testing).
The test is only with a signal generator, so the cap won't be subject to
a lot of stress.

Connect the known good capacitor to the original inductor (transformer
primary) with no other loads attached. Sweep with a signal generator
(use as much voltage as the signal generator can provide without much
distortion, that usually won't be a very high voltage anyway) and look
for resonance on a scope. Note the resonance frequency. Disconnect the
known good cap and connect the original one instead. Check where the
resonance is. If it's in the same place and the amplitude has not become
lower, the cap is very likely good. If it disappears and you can only
measure the inductor's SRF instead, (if the inductor has more or less
the same resonance with or without a capacitor connected), then the
capacitor is basically open-circuit or very high ESR. If the resonance
has wandered away somewhere, especially upwards in frequency, then the
cap is most likely degraded and not a good candidate for full power
resonant use either. Same thing if the amplitude has dropped much.

If your resonant caps turn out to be good, that most likely leaves only
the snubber diodes and a possible frequency misadjustment as the likely
causes.

If you check the resonance with a signal generator and scope against a
known good 15 nF, and it suddenly wanders way, or the amplitude drops,
then you'll need to find replacement capacitors. Fortunately, if you put
"WIMA FKP1 33nF" into ebay search, there seem to be many available.

Regards
Dimitrij