On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't
seem to tally up with the schematic. Or does it? I need a pint of
strong coffee to kick-start my head on this one. :-/ Anyway,
later...
...
... In one of your pictures there are a couple of diodes that look
messed up (V1809 and V1811) near the bridge. They are supposed to be
BY208-1000 (1000v rectifiers), I can see "40" on one, maybe 1N4007?

Well, if that is true then beware! V1808, V1809 and V1811 are supposed
to be very fast. Any slow (more than a microsecond) diode in these
positions will likely cause symptoms akin to a heavy overload.

Particularly V1811, if replaced with any 1N400x, is likely to render the
energy recovery circuit around L1806 as good as inoperative, thereby
dumping the entire energy from the switcher harmonics into R1814, which
will cause it to overheat fast.

Please recheck L1806 (both windings) for turn-to-turn shorts (with a
signal generator), and if any of the 3 diodes (V1808, V1809, V1811)
looks like it had previously been replaced (possibly improperly
replaced), consider replacing all 3 of them together, using the proper
parts.

Use fast soft-recovery diodes rated for 1kV here. If you can't find any,
use ultrafast ones. They're maybe not optimal from an EMI standpoint
here, but at least they should work well enough for testing.

If you can't find a BY208-1000 replacement, a MUR4100E should work.

Check C1806 for dielectric breakdown. It should be able to withstand at
least 500 V (or something in that ballpark). If it doesn't, replace.

Don't underestimate that L1806 energy recovery circuit. Although it
doesn't by itself transfer any power to the load, this supply heavily
relies on it for proper resonant operation of the main transformer. It
must be working properly before you can test the main transformer
waveform and have any chance of making correct measurements.

Besides, your description of heavy switching noise on V1806 (when you
tried to measure the base drive waveform), up to the point of the
waveform being unrecognizable in the noise, seems to indicate that the
L1806 circuit is shorted at high frequencies. This can be a result of
either a winding short in L1806 or a breakdown in one of its diodes or
some of these diodes being replaced by a generic slow silicon diode.

Dimitrij