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Transformer repair

Discussion in 'Electronic Repair' started by Phil Allison, Aug 7, 2012.

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  1. Phil Allison

    Phil Allison Guest

    ** 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

    The tranny looks similar to the one in this pic:

    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
  2. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** No, this mic is an MA-200.

    The AC tranny is smaller and mounted off the PCB.

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

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


    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
  3. nice save. I'd never have sat there counting thousands of turns.
  4. Phil Allison

    Phil Allison Guest

    "Cydrome Leader"
    ** I thought so....

    ** 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
  5. 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.
  6. Phil Allison

    Phil Allison Guest

    "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
  7. Phil Allison

    Phil Allison Guest

    "klem kedidelhopper"
    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
  8. Phil Allison

    Phil Allison Guest

    "klem kedidelhopper"

    ** **** off - you illiterate, bloody imbecile.
  9. Phil Allison

    Phil Allison Guest

    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
  10. Phil Allison

    Phil Allison Guest

    "klem kedidelhopper"

    ** **** off - you illiterate, bloody imbecile.
  11. Phil Allison

    Phil Allison Guest

    "Phil Allison"
    ** 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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