On 21.02.2016 21:59, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
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?

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

Sorry, that may be physically too big to fit. A MUR1100EG or
something similar should work and fit in the available space too.

Dimitrij

Thank you. If you've had the chance to read my follow up to JC (I
think it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of
the diodes it uses then maybe it won't function properly as you
suggest. I can only imagine the technician who replaced it was
unaware of the critical nature of the part concerned. I'm kind of
unhappy with this design overall, to be honest. It was critically
appraised on s.e.d and found generally unsatisfactory. I'm strongly
tempted to just save the transformers, junk everything else and start
afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing
to do would be to bin it? :-/

Hi

As for the BY134, sorry, I must have overlooked that somehow, or maybe
it did not register in my memory right away. Anyway, it's just as bad a
choice as a 1N4007 and its ilk. It's designed for mains rectification
and doesn't even make an attempt at being fast.

No use in an active snubber or energy recovery circuit whose task it is
to "strip out" the high frequency components from a square wave.

Get rid of it, and while you're at it, consider the condition of the
other two (V1808+9) identical ones. Sometimes a person who does an
improper repair will try swapping nearby components hoping that another
one might be "less critical". So if you see signs of unprofessional
manual soldering on them, take that whole trinity and replace them.

Same with C1806. If it looks suspicious, does not pass a withstand test
at some 105% of its rated voltage or shows high ESR, change it too.

BY208-1000s are hard to come by nowadays, so here is a list of some more
modern candidates: MUR1100E, BYV26E, UF4007. They should fit, and even
though they are faster than the original BY208-1000, they should work.

There are also: RGP30M (slightly large, modest speed), UF5408 (slightly
large), MUR4100E (slightly large), STTH112U (smd), US1M (smd) BYG23M
(smd). They may or may not fit due to size and space constraints, and
the SMD ones would likely need some wire leads soldered on (won't look
professional, but hey, if others are too hard to come by, that's ok).

Once you've fixed that botched repair on the energy recovery circuit,
connect a taillight lamp to the 12.7 V output, test it again and tell
your results here (make sure you put all the proper parts back in,
before you switch it on, this supply may be unforgiving if any parts
are missing and it's powered on).

As for the design being "generally unsatisfactory", let me disagree.
Resonant converters do have a well earned place in the world of power
electronics, but the design of them is, in a way, a black art. They have
lots of pitfalls for the unwary and not so many engineers can actually
design them properly and they tend to use special components (inductive
ones in particular) that would be rather unsuitable for other topologies
too. Yet they do have certain benefits, low noise operation that is
suitable for sensitive measurement instruments, being one of them. They
are not so easy to understand, compared to "simple" flyback topology
supplies - so people go screaming "this is too complex" or "this uses
too many parts". In fact your supply's energy recovery circuit is
actually a little unregulated flyback converter of its own! But so far
(and considering the design's day and age), all the parts that I've seen
in that schematic seem to me to have a good reason for their existence.

Greetings
Dimitrij