N_Cook wrote:
Phil Allison wrote in message
...

** Hi,

mainly due to the popularity of Ebay, electronic gear originally sold in
the
USA and Japanese markets is now regularly turning up in Australia. USA
items
are intended for a 120V, 60Hz supply while Japanese ones are for 100V,
50Hz.

Conversion to 240V mains operation involves internal inspection and
modifications OR simply determining what VA rating step-down tranny will
be
suitable. The modifications involve removing or shifting PCB links,
altering
QC connectors or rewiring the supply side leads to the AC tranny - PLUS
changing fuses and the AC plug to the Aussie version.

I had an EVENT active studio monitor (twin port model) in for such
conversion this week and it looked easy. There is an IEC inlet on the back
and markings that indicate the possibility of 120V and 230/240V operation.

The unit has a toroidal tranny fitted inside with four supply side wires,
so
it was of twin primary design allowing 120V and 240V operation. I
reorganised the four QCs according to a pattern drawn on the PCB and used
a
variac to raise the voltage.

At about 30V, the ( T4A ) PCB fuse blew - indicating the two primaries
were
wired out of phase. No real harm done and a few minutes later I had the
four
wires the right way around.

Variaced the unit up again ( to about 130V ) and this time it started
working, I verified the woofer and tweeter were both OK. Next I wound the
variac up in gradual steps to 240V and suddenly and there was a very loud
BANG !!

All sound stopped. WTF !!!

When I opened the unit again there was a smoke inside and black deposits
visible on the PCB and the nearby acetate filling used in the box. The
source was a 20mm varistor on the PCB that had exploded.

But why?

Markings on the part showed that it was rated for 270 volts ( 20K271) -
but
that is a DC voltage rating. So when exposed to a 240VAC supply with 340V
peaks it was grossly overloaded.

The stupid " Ying Tongs " had fitted the WRONG varistor.

The correct part is a 430 volt rated varistor OR one marked as being
275VAC,
as is the case with the class X2 cap wired in parallel.


.... Phil




Ever come across Crate (not some mickey mouse operation) USA "made" amps ,
for export to 240V countries , with mains switches + neons?
Neons glowing like lighthouses and melting all around, from the charring of
the droppers which are set for 110V including stating so on the switch
bodies

I've noticed quite a bit of dubious pilot lamps on equipment that should
allow 240volt export use. Luckily it's 120 here, so I've not have the
pleasure of experiencing what really happens with those.