# Transformer repair

P

#### Phil Allison

Jan 1, 1970
0
** 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 !!!

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 P #### Phil Allison Jan 1, 1970 0 "Gareth Magennis" "Phil Allison" 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.

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

..... Phil

C

Jan 1, 1970
0
Phil Allison said:
** 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 !!!

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.

P

#### Phil Allison

Jan 1, 1970
0
Phil Allison

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

Turned out to be a very good assumption.

..... Phil

C

Jan 1, 1970
0
Gareth Magennis said:
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.

P

#### Phil Allison

Jan 1, 1970
0
"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

P

#### Phil Allison

Jan 1, 1970
0
"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

P

#### Phil Allison

Jan 1, 1970
0
"klem kedidelhopper"

** **** off - you illiterate, bloody imbecile.

P

#### Phil Allison

Jan 1, 1970
0
"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

P

#### Phil Allison

Jan 1, 1970
0
"klem kedidelhopper"

** **** off - you illiterate, bloody imbecile.

P

#### Phil Allison

Jan 1, 1970
0
"Peter"
"Phil Allison"
Well yes, it is a quite rapid action, but does not occur till the
current in the primary is sufficiently high. Saturation does not
occur over the whole input wave, and drops and reverses as the sine
wave input reverses. So during the non saturated period, the input
and output voltages are proportional to turns ratio.

** Strangely enough, the peak saturation currents occurs near to each AC
supply voltage zero crossing.

With a small transformer and double the rated AC input voltage, the (off
load) current wave is very peaky in shape and rises to about 15 times the
usual RMS value.

Shorted turns in the primary results in a few seconds, the resistance drops
suddenly and blows a fuse even 10 times the correct size.

The PSU in question contained only electros, resistors and zeners - all
operating well within their ratings in normal circumstances, so well able to
take a brief increase in voltage.

.... Phil

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