On 07.03.2016 20:21, Dimitrij Klingbeil wrote:
On 07.03.2016 19:08, Cursitor Doom wrote:

I've just noticed that towards the bottom of (true) page 106, it
states the following:-

"The oscillator frequency is approximately 25kHz, determined by
network C1811, R1823 and is adjustable by means of R1824." ...

And before anyone suggests it: I've frequency swept the primary
circuit just in case there's a second resonance peak at around
25kHz and there isn't one.

I can only think of C1806 and C1807. They are in series. If one of
them has an isolation problem, that would leave the other one alone
in the circuit - and therefore double the capacitance.

Also there are the resistors in parallel - R1817 and R1818. They
should be 10 Megaohm. But if one of them is either shorted or
improperly replaced (maybe it formed an isolation breakdown from
over-voltage or someone put in a wrong value like zero Ohm instead),
then that would also short out the corresponding capacitor, and have
the same effect.

Usually resistors are reliable, but sometimes, some old ones of the
"carbon composition" variety, do form a "hot channel" and break
down.

Doubling the capacitance would almost halve the resonance frequency.
Actually it won't *exactly* halve it, because there is still a third
capacitor in parallel, namely the stray winding capacitance.

This looks quite enough to be realistic. If one resonance cap is
shot, the LC frequency will go down by a great deal...

P.S. There may be another failure mode that I did not consider right
away, that can make it a little harder for you to test the caps.

These high voltage impulse-rated capacitors are usually made with three
metal layers and two layers of isolation internally. Mains "X1" and "X2"
rated capacitors are also made in this way. They are like two capacitors
that are connected in series inside. When one isolator breaks down,
the whole capacitor won't be destroyed catastrophically because there's
still the other internal half in series.

But it can happen (depending on the construction of the capacitor, if it
has a continuous metal layer between the isolators) that the capacitance
will "double itself" instead.

If one of your 30 nF caps has failed in this way, it will "become 60 nF"
instead of becoming short-circuit. So you won't see it on an Ohmmeter.

But that would also be reason enough to detune the resonance a lot.

So, my advice would be, do not trust them, and do not trust their
parallel resistors too much either. Take a known working 15 nF cap and
sweep the transformer with it. If you get anything significantly above
17 kHz with a new cap, look for the main problem in this direction.

Dimitrij