Gareth Magennis wrote:
"Phil Allison" wrote in message
...
"Cydrome Leader"
Phil Allison
** Techs often get a sinking feeling when they discover a burnt AC power
transformer in a device. The unit simply blows fuses at switch on and
cannot
be seen to work at all - so maybe it has other serious faults. Also,
exact
replacement transformers are often not available or else involve a long
wait
and high cost.
Had one this week, a baby size 6VA tranny in the PSU for a new looking "
Mojave" valve microphone. The PSU has an IEC inlet and a switch for
115V
and 230V AC power - some goose had tried to use it with 240VAC and the
switch set to 115V !!!
The burnt tranny had two secondaries, 9V at about 300mA and 120V at
about
25mA - so nothing off the shelf would do. The local importer was blaze
and
any replacement would involve a long wait. The PSU case is very
compact, so
it was simply impossible to fit two transformers to get the required
voltages.
The tranny looks similar to the one in this pic:
http://www.chinatransformers.cn/User...5004507496.jpg
I decided to pull the old one apart and see if the burnt primary could
be
re-wound. There was over 2000 turns of hair fine wire that had merged
into a
solid lump since the enamel had melted and then reset. Eventually it
all
came off in clumps by using nippers and a sharp blade to cut it open.
Luckily, I had on hand a new ( 240V primary ) transformer with the
identical
core and bobbin size. So I pulled that apart too.
THEN it became clear that, with a bit of trimming and sanding, the half
bobbin holding the primary from the new tranny could be teamed with the
half
bobbin holding the ( good) secondary from the old one and it would all
fit
together nicely.
I managed to get all but one of the original lams back inside the two
bobbins ( now held together with Silastic) and fitted the steel cover
frame
back over the lot. Gave it all a squash in the bench vice for good
measure.
Result: A new transformer that works perfectly and now so does the mic.
Total cost $6 and about 2 hours time, it will be quicker if I ever have
to
do it again.
nice save.
** I thought so....
I'd never have sat there counting thousands of turns.
** I did no turn counting, remember the burnt primary was a solid lump.
The 2000 plus turns figure was an estimate based on the core cross section
( 0.4 sq inch ) and a rule of thumb about turns per volt. The wire was
0.12mm dia and that computed a similar number to fill the bobbin.
My working assumption was that for identical cores, 230V @ 50Hz primaries
ought to be all the same - especially so when the Chinese mass produced
both examples I had.
Turned out to be a very good assumption.
.... Phil
This bodes well for the future, perhaps the Chinese have indeed unwittingly
done us all a favour.
The transformer I have here in the SE Electronics PSU appears to be of
similar construction to yours. Likewise a pair of Tannoy powered monitors
also appears to follow the same design but obviously scaled up a lot.
I guess it would make perfect sense if, say, one factory was making all the
transformers, that they would have a modular system in place. i.e. a few
different VA sizes of cores with standard plug-in primaries, and producing
secondaries to order.
This is how it works for the most part. There are standard sizes of
cores/laminations and bobbins.
For high volume stuff, you might start to see customized parts like
bobbins and other mounting stuff.
I asked a transformer designer how he designs custom transformers. The
short answer was grab a previous design from the filing cabinet because
anything anybody wants has already been designed. Sometimes people want
different lead colors, and everybody gets a new part number.