On Tue, 09 Dec 2014 17:54:49 -0800, Phil Allison wrote:

Hi all,

recently I replaced a toroidal mains transformer in a 5 or 6 year old
Crown XLS602 power amplifier. This was a 240V, 800VA size tranny with
the secondary winding giving of 75-18-0-18-75 volts.

http://cdn.avsforum.com/3/38/3836566...ach143596.jpeg

The toroidal was made by a Chinese company - NRE Electrical
Manufacturing Co Ltd.

www.fsnre.com

The primary had shorted turns and luckily I had another very similar
toroidal on hand that did the job - after adding a small 18-0-18 tranny
as well. No other faults were found with the amp.

Failures of such large transformers is quite rare, so I began to unwind
the thing to have a look and also get some heavy gauge winding wire too.
It was a real bugger of a job cos both the secondary and primary had
been "bi-filar" wound. The shorted turns were found near the core, right
where the twin primary winding is split and series wired. No big
surprise there.

But then there WAS a big surprise underneath - the toroidal core itself.
Normally there are precision made, tape wound cores with molded plastic
caps covering the sharp edges. Not this one.

The corner edges had simply been ground off by hand and covered in cloth
tape. The core itself looked rough and uneven - then I realised it was
made from dozens of separate strips of steel, spot welded to each other,
end to end and of varying thicknesses and widths. The tranny was made
from scraps !!

Some dirt poor Chinese had hand assembled the core using rusty off cuts
and sweepings from the floor from some transformer factory. The centre
hole was perfectly round but the outside diameter varied by 3mm due to
all the joins.

One would reasonably suspect such a "Frankenstein" core of having poor
magnetic performance - but all my testing showed it was perfectly OK.

The Chinese are getting desperate.

But perhaps there are also rich Chinese who will pay good money for a
genuine hand-made artisanal artefact!

Mike.