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

That thing looks awful. nasty silicone-looking glue and sloppy cable ties parts buried under other parts. It is amazing fans cost less than real heatsinks these
days.

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.

fascinating.

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.

such a crappy transformer is slightly brilliant though. Somebody took the time to try that hack.