On Mon, 22 Feb 2016 00:52:28 +0100, Dimitrij Klingbeil wrote:

As for the BY134,
[...]
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.

Will do. I'm guessing the tech who replaced that diode was solely
concerned with its voltage rating. In all honesty, I'd have been the same
before this speed importance was drawn to my attention in this thread.

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.

That one actually looks fine appearance-wise, but I'll test it
electrically of course.

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.

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).

Will do. I'll order the parts tomorrow if I can't find any in my spares
bin.

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.

I read somewhere that resonant converters are poorly understood by
engineers who don't specialise in them and that accurate, detailed
literature on them is not easy to find. So it's very valuable to have
knowledgeable people like yourself and others here who do understand how
they work; otherwise I'd have nowhere to turn for advice on how to
proceed with this!
I'm going to work through the steps you've outlined here and elsewhere
and hope they work. But if the problem remains, I shall definitely be
mothballing it for the foreseeable future. My patience isn't infinite!