Tim Shoppa wrote in message
oups.com...
N Cook wrote:
400 watt RMS amp for use on 240 V mains.
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.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs
of
added solder over the track for current carrying, the other polarity
trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other
rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added
fuses of
what rating ?

The power supply to your amp probably has massively oversize filter
capacitors.

Average current draw at 400W out might be only two amps, but because
of the rectifier-filter construction it doesn't draw that two amps
continuously. It draws it in very brief gulps 120 times a second.
Typical conduction angle might be 20 percent in a well designed
supply; with massively oversize filter caps it could be as small as 5
percent. Massively oversized filter caps have been all the rage in
consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

And remember, fuses aren't really there to protect your electronics,
they're in there to prevent fires.

If you want a supply that doesn't have such a small conduction angle,
you go to choke-input supplies, which for some bizarre reason aren't
nearly as popular as they were 50 or 70 years ago :-).

Tim.


The 2 reservoir caps are 6800uF, 80V rating with presumably about 65V on
each , when all is in working order.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/