On Mon, 29 Feb 2016 21:47:38 +0100, Dimitrij Klingbeil wrote:

There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!"

As an aside, I'm just a bit mystified as to why anyone would want to do
this anyway?

Now, apologies for the delay, but I had the usual accumulation of
pressing things to deal with on my return so have only now got around to
carrying out the checks last suggested here.

OK, I measured the resonant frequency of the primary circuit (with the
chopper NOT disconnected, see notes below) by sweeping a frequency range
across the main tranformer's primary input terminals. It's not
particularly peaky, so there's a Khz or so on either side of Fo before we
get to the -3db shoulders. Fo, with no load connected came out as
17.35kHz.

Under power, with frequency counter connected between T1 and T2 with V1812
removed from circuit shows the PWM chip pulsing at 22.55kHz.

Unfortunately I have no idea what the factory figures should be and
whilst it seems like there's a big difference between the PWM chip's
output and the primary circuit's resonance, AIUI, they're not supposed to
be in sync at any time anyway. But are they supposed to be this far apart?

Notes:

1. I know somewhere it was stated that the chopper transistor should be
removed for the resonance test, but I couldn't see the harm in leaving it
in. If it invalidates the test, of course, then I'll whip it out and re-
do it. If you think it's relevant let me know.

2. I pulled V1812 as someone suggested because the noise coming back
down its collector from L1803 might have interfered with the frequency
counter's ability to read the clock pulses.