Transformer repair

[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/UserFiles/Image/05004507496.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.



..... Phil

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a8be1rFoflU1@mid.individual.net...

** 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/UserFiles/Image/05004507496.jpg

Was this the PSU?

http://i47.tinypic.com/144ch4.jpg


Both PSU's for both Mics in this picture are identical internally, apart
from one has different value Zeners to produce the HT for the different
mics.

In this case the PCB's are both marked at 9v5AC and 200v AC at the TX
inputs.
The S.E. Electronics one has a shorted primary (One Zener dead also).

Just wondering if all these cheapo tube mics are all made in the same
factory.



Cheers,

Gareth.

 

[Phil Allison]

"Gareth Magennis"

"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/UserFiles/Image/05004507496.jpg


Was this the PSU?

http://i47.tinypic.com/144ch4.jpg

** No, this mic is an MA-200.

http://www.sweetwater.com/images/closeup/xl/1600-MA200_inbox.jpg

The AC tranny is smaller and mounted off the PCB.

Just wondering if all these cheapo tube mics are all made in the same
factory.

** Not so cheapo here in Aussie - the RRP is $1300.

I expect that most like it are coming from the same Chinese maker.

BTW:

The MA-200 uses a type 5840W sub-miniature pentode with wire leads.

http://www.radiomuseum.org/tubes/tube_5840w.html

Connected as a cathode follower and with a DC heater supply - it's as
QUIET as a mouse.



..... Phil

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a8c85sFbl8U1@mid.individual.net...

"Gareth Magennis"
"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/UserFiles/Image/05004507496.jpg


Was this the PSU?

http://i47.tinypic.com/144ch4.jpg


** No, this mic is an MA-200.

http://www.sweetwater.com/images/closeup/xl/1600-MA200_inbox.jpg

The AC tranny is smaller and mounted off the PCB.


Just wondering if all these cheapo tube mics are all made in the same
factory.


** Not so cheapo here in Aussie - the RRP is $1300.

I expect that most like it are coming from the same Chinese maker.

BTW:

The MA-200 uses a type 5840W sub-miniature pentode with wire leads.

http://www.radiomuseum.org/tubes/tube_5840w.html

Connected as a cathode follower and with a DC heater supply - it's as
QUIET as a mouse.



.... Phil

OK, cheers.

Both mine use ECC83. The HST had a cheap chinese one that sounded like a
mouse caught in a sandstorm.



Gareth.

 

[Cydrome Leader]

Phil Allison <phil_a@tpg.com.au> wrote:

** 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/UserFiles/Image/05004507496.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'd never have sat there counting thousands of turns.

 

[Phil Allison]

"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/UserFiles/Image/05004507496.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

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a8ea9uF4bqU1@mid.individual.net...

"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/UserFiles/Image/05004507496.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.


Gareth.

 

[Cydrome Leader]

Gareth Magennis <sound.service@btconnect.com> wrote:

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a8ea9uF4bqU1@mid.individual.net...

"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/UserFiles/Image/05004507496.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.

 

[Gareth Magennis]

"Cydrome Leader" <presence@MUNGEpanix.com> wrote in message
news:jvu3su$qkd$2@reader1.panix.com...

Gareth Magennis <sound.service@btconnect.com> wrote:

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a8ea9uF4bqU1@mid.individual.net...

"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/UserFiles/Image/05004507496.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.

Hmm, I'm not sure that in the real world you could easily take a transformer
from a random piece of equipment and replace the primary.


Gareth.

 

[Phil Allison]

"Phil Allison"

The tranny looks similar to the one in this pic:

http://www.chinatransformers.cn/UserFiles/Image/05004507496.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.

** The same idea may be also used when presented with 100V or 120V devices
made for the Japanese and USA markets and need conversion to 230/240 V
power.

Just gotta find a tranny that uses the same size SPLIT bobbin.


..... Phil

 

[Phil Allison]

"klem kedidelhopper"


This may sound like a stupid question, and at the risk of insult I had
to ask. You never mentioned it but I'm just curious Phil did you check
the mic out using separate power supplies before going to all that
trouble, just in case the unit supplies as well as the amp was fried?


** This was the scenario:

1. The mic and its PSU looked brand new.

2. The primary of the AC tranny had visible damage and smelt burnt.

3. The secondary winding look fine.

4. There was no fuse in the clip associated with the IEC inlet and a new one
blew at a low AC voltage.

5. The primary read 26 ohms when it should have read 700 ohms or so.

6. The secondary ohmage readings looked good at 3.5 and 450 ohms.

7. After I pulled the tranny apart and cut away the primary winding, the
secondary tested the correct 9:120 ratio.

8. The PSU consisted only of rectifiers, resistors and zeners - all of
which looked fine.

8. There was an IEC lead in the case with the mic fitted with a US style 3
pin plug.

The fact that over heating occurred ONLY in the primary and NOT in the
secondary windings proved that there was no overload applied to secondary of
the tranny.

It had to be an AC supply over voltage that caused the damage.



..... Phil

 

[klem kedidelhopper]

On Aug 9, 12:37 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"Phil Allison"











The tranny looks similar to the one in this pic:

http://www.chinatransformers.cn/UserFiles/Image/05004507496.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.

** The same idea may be also used when presented with 100V or 120V devices
made for the Japanese and USA markets and need conversion to 230/240 V
power.

Just gotta find a tranny that uses the same size SPLIT bobbin.

....   Phil

This may sound like a stupid question, and at the risk of insult I had
to ask. You never mentioned it but I'm just curious Phil did you check
the mic out using separate power supplies before going to all that
trouble, just in case the unit supplies as well as the amp was fried?
Usually what you described generally, (as you found) just opens a
primary and/or a fuse but you never know. Around here transformers get
blown up by lightning all the time. I've subbed transformers in
microwave ovens and othr equipment with some degree of success but I
usually try to check out the boards on the bench first.

By the way, while we're having this discussion, it seems like I recall
some time ago seeing a small article somewhere about how to determine
the secondary voltage or voltages of a transformer with an open
primary. Of course you can come close by looking at the voltage
ratings on the electrolytics that are closest to the secondaries but
this method didn't involve that or dis assembly either. In any event
Phil I applaud you you all your work in repairing that transformer.
Lenny

 

[klem kedidelhopper]

On Aug 9, 9:38 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"klem kedidelhopper"

This may sound like a stupid question, and at the risk of insult I had
to ask. You never mentioned it but I'm just curious Phil did you check
the mic out using separate power supplies before going to all that
trouble, just in case the unit supplies as well as the amp was fried?

**  This was the scenario:

1. The mic and its PSU looked brand new.

2. The primary of the AC tranny had visible damage and smelt burnt.

3. The secondary winding look fine.

4. There was no fuse in the clip associated with the IEC inlet and a new one
blew at a low AC voltage.

5. The primary read 26 ohms when it should have read 700 ohms or so.

6. The secondary ohmage readings looked good at 3.5 and 450 ohms.

7. After I pulled the tranny apart and cut away the primary winding, the
secondary tested the correct 9:120 ratio.

8.  The PSU consvolts isted only of rectifiers, resistors and zeners -  all of
which looked fine.

8.  There was an IEC lead in the case with the mic fitted with a US style 3
pin  plug.
The fact that over heating occurred ONLY in the primary and NOT in the
secondary windings proved that there was no overload applied to secondary of
the tranny.

It had to be an AC supply over voltage that caused the damage.

....  Phil

I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out. If that 40 volts happens to be the input to
a regulator then perhaps the amplifier can survive until the
transformer primary opens, but maybe that 40 volts might be rectified,
and unregulated and is then applied to the first stage. And then
that's all she wrote. It sounds like luckily you didn't have to deal
with any of that. Lenny

 

[Phil Allison]

"klem kedidelhopper"

It had to be an AC supply over voltage that caused the damage.

I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out.

** Totally wrong !!!!!

Try it yourself and see what REALLY happens.



.... Phil

 

[Phil Allison]

"klem kedidelhopper"


** Fuck off - you illiterate, bloody imbecile.

 

[klem kedidelhopper]

On Aug 11, 6:30 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"klem kedidelhopper"



It had to be an AC supply over voltage that caused the damage.

I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out.

** Totally wrong  !!!!!

 Try it yourself and see what  REALLY happens.

...   Phil

I'm not criticizing your deductions and how you arrived at them. Your
theories do make a lot of sense. I've never tried this, (over voltage
to a primary) knowing that it would be a destructive test. I guess
that I didn't explain this properly. I just thought that perhaps the
momentary 240V surge into the 120V primary might cause the 20V
secondary voltage to approach 40V before the primary opened, (unless
the transformer saturated first)? Even so I think there would still
exist the possibility of a significant over voltage before all this
happened. And if this AC supply was merely being rectified and
filtered but not regulated, I would suspect some possible damage from
over voltage to the electronics further down the line. Lenny

 

[spamtrap1888]

On Aug 11, 3:30 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"klem kedidelhopper"



It had to be an AC supply over voltage that caused the damage.

I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out.

** Totally wrong  !!!!!

 Try it yourself and see what  REALLY happens.

I'm confused. I would have thought that, at least for the first
instant, the turns ratio means there would be 40 VAC on the output,
until the primary's turns start shorting together as the enamel
combusts because of heating caused by the excessive input current.

If this is a correct analysis, then the question becomes, does an
instantaneous overvoltage on the secondary damage any downstream
circuitry here?

 

[klem kedidelhopper]

On Aug 11, 1:10 pm, spamtrap1888 <spamtrap1...@gmail.com> wrote:

On Aug 11, 3:30 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"klem kedidelhopper"

It had to be an AC supply over voltage that caused the damage.

I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out.

** Totally wrong  !!!!!

 Try it yourself and see what  REALLY happens.

I'm confused. I would have thought that, at least for the first
instant, the turns ratio means there would be 40 VAC on the output,
until the primary's turns start shorting together as the enamel
combusts because of heating caused by the excessive input current.

If this is a correct analysis, then the question becomes, does an
instantaneous overvoltage on the secondary damage any downstream
circuitry here?

I have to say Phil I can always count on you to liven up the
discussion if anyone ever has an opinion or even a thought other than
yours. It really is rather refreshing when it's all said and done.
We're all ignorant imbeciles, morons and fuckwits whom you've
tolerated. I truly don't know what we would do without you there to
continuously correct us. I suppose we should all be grateful for the
consideration and latitude you've so graciously afforded us. Lenny

 

[Phil Allison]

"spamtrap1888"

I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out.

** Totally wrong !!!!!

Try it yourself and see what REALLY happens.

I'm confused. I would have thought that, at least for the first
instant, the turns ratio means there would be 40 VAC on the output,
until the primary's turns start shorting together as the enamel
combusts because of heating caused by the excessive input current.

** Nope.

Magnetic saturation is an instantaneous phenomenon.

The cores of most small transformers operate well into saturation, even at
rated voltage - so doubling the incoming primary voltage causes complete
core saturation and the primary side current flow is limited only by the
resistance. The RMS current goes up by 5 or 10 times the normal level.

Crucially, the secondary voltage rises only slightly due to a large voltage
drop being caused by the primary's resistance.

A correctly rated fuse will blow immediately and the event is all over.

With the PSU in question, the AC fuse was missing - but very likely had
been replaced with a larger one at some stage.


.... Phil

 

[Phil Allison]

"klem kedidelhopper"



** Fuck off - you illiterate, bloody imbecile.