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.

Hi

News indeed. Something must be amiss, and quite heavily amiss, that's
for sure.

It looks like the resonant circuit considerably out of tune.

Driving a 17 kHz LC with 25 kHz would not make sense to me, and it looks
like the circuit does not like it too much either, since it overheats.

If it was indeed tuned for 25 kHz, then the currently set 22 (or 20) kHz
pulse rate setting could at least make some sense. It would be slightly
below resonance, but probably not too far away.

I was assuming that the resonant circuit was ok-ish and the frequency
somewhat matched, but this turns out, now, not to be the case.

Now I've looked in the TDA1060 controller datasheet again, and checked
the adjustment range (with the trimpot set from 0 to 10 kOhm and the 11
kOhm fixed resistor and 3.3 nF timing capacitor) and the calculated
resulting range of frequencies happens to be from 17.3 kHz to 33 kHz.

With a 17.3 to 33 kHz range, that would put 25 kHz quite well in the
middle. A 17 kHz resonance frequency does not fit anywhere though. It
can't even reliably be adjusted (even if it were correct, which it
surely isn't) because it's simply out of the trimpot's range.

But it would not make any sense to pulse a 17 kHz LC at 25 kHz either.
The LC will be heavily capacitive, the power factor will be down in the
ditch, and that will overload (-heat) the driver (just as it happens).

Therefore I can only think that 17 kHz is wrong. The LC is out of
resonance. If it were OK, it should never have a 17 kHz SRF.

That leaves either a measurement error (now rather less likely) or a
heavy stray capacitance somewhere, that brings the resonance down.

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, and (assuming it was
initially slightly higher than the pulse frequency), a change from some
25 or 27 kHz to the 17 kHz that you are now seeing, could happen.

It could also explain the overload on the resistor - a heavily
capacitive out-of-resonance load would easily do that.

Can you get these two caps out altogether, connect a known good 15 nF
(or two of 30 nF in series) instead and sweep again?

Also, to avoid unforeseen measurement errors, can you do that out of
circuit? Just the transformer and the capacitor(s) on the primary. This
would also nicely avoid the resistors too, they too may be questionable.

You won't need a high voltage cap for sweeping - any good 15 nF one will
do. But don't power it up with "any 15 nF". To run at full power it
needs something like a FKP1 or MKP-4C with proper ratings (see my
earlier post, there are some suggested models that can work there).

Regards
Dimitrij