On Mon, 10 Apr 2006 14:24:50 +0200, Wiebe Cazemier
wrote:

Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

Thanks in advance.

Seems very unlikely inrush current increased. I use torroids in my
amps and find they do use lots of inrush current (particularly with a
stiff power supply) I fuse them with slow blow fuses and had to
install two power switches - one for each pair of amps, or my clock
and modem reset themselves when I turn on the stereo.

Torroids have better regulation as a rule - so the current required to
charge the supply caps is reflected in the inrush current to a greater
degree than with E-I core transformers.

Does it require a slow blow fuse?

Sounds like you already eliminated the filter caps as a direct source
of the problem?

If you're using the correct fuse type and it still blows, and its been
working all along prior to this, check the amp. Ideally, look at the
current with a scope.. You may have a problem in the bias supply -
coming up unbalanced or very high current due to a bad cap.

A scope on the output terminals will show it coming on in an
unbalanced condition (and a loud speaker thump) - but depending on the
design, that could be normal.
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