Electronics Forum / Electronics / June 2003

Hi to all,

     When I upgraded my PC recently a pair of small, self powered speakers
were included in the deal.

     These were the Lepra brand model LS699.

      After a while I noticed a burning smell coming from the area of the
monitor but it was not that - the speaker with the PSU and amp inside was
the culprit.   On opening it I found the tiny PC mount transformer (no plug
pack is used) was running so hot it had blackened the insulation tape on the
primary winding and was much too hot to touch.

      The tranny would be about 2 or 2.5 VA from its size  ( core 28 x 25 x
12mm ) and powered a TDA2822M 8 pin  dual amp IC - but only just.   The
primary magnetising current measured 37mA and the resistance about 3500 ohms
when hot - that means almost 5 watts of heat loss in the primary alone -  no
wonder it was so damn hot !!

      The tranny has only *basic insulation* and no thermal fuse  - the
plastic bobbin melts like butter if you apply a soldering iron.  There is
only 0.7mm of plastic separating the windings.

      There is strangely no double insulation symbol on the unit, but there
is an approval number  ( N 10752)   - for all that is worth.

     I am concerned that some trannies will burn and fail with a breakdown
from primary to secondary and make the input plug live.

    Any one else seen this problem or another similar one ??

.............    Phil