"N Cook"

400 watt RMS amp for use on 240 V mains.

** Is that 400 watts per channel or 200 per channel ?


Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.


** Then the AC supply current did not exceed 8 amps by much for more than
a fraction of a second.


Shorted power trannies, burnt low power trannies


** Err - " trannies " = transistors ??

The mains transformer is rated at 2x 47V,5 amp.


** Say it is rated at 500 VA, for simplicity.

So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses
of
what rating ?


** From another post I see it is a toroidal type.

Couple of facts:

1. With secondary shorted ( before or after the bridge) primary current will
rise to circa 42 amps rms - so bye bye to any 8 amp fuse real quick.

2. At switch on, the peak supply current will regularly exceed +/- 100
amps for the first half cycle - diminishing to the idle value over the
next 10 - 15 cycles or so.

3. At 400 watts output, the AC current draw will be around 4 amps rms -
assuming this is a typical, low bias, class AB amplifier design. With hard
clipping the figure will rise to over 6 amps rms.

So - the maker's choice of an 8 amp AC fuse is not unreasonable, given the
above facts.

The knee jerk reaction of matching the fuse rating to the VA rating of the
AC tranny does not work in practice - just try using a 2 amp fuse if in
doubt.

Maybe try a 6.3 amp " anti-surge " fuse - the kind with spiral wound fuse
wire.

Keep a few spares handy.



....... Phil