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Whats gone wrong with this transformer ?

Discussion in 'Electronic Repair' started by N_Cook, Mar 24, 2009.

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  1. N_Cook

    N_Cook Guest

    Squashed profile toroidal to get in a 1U case of a 1992, Ampeg B1 amp,
    export version, internally set for 240 V use, blew (violently) the mains
    fuse. No burnt smell or obvious damage to the transformer.
    Disconnected and removed there are 2 seaparate primaries , >1000 Meg , 500
    volt isolation between them and the secondaries.
    3 wires to each primary , presumably 110V and 120 V each pair. Measures
    about 3 ohm over each "120V " pair of separate primaries which seems about
    right for a good 450 VA transformer.
    Connect either primary to a 20 volt ac variac supply and they draw 1 amp of
    current. I could understand a failure in one primary but both failed the
    same ? Could there be failed VDRs or something buried inside the transformer
    ?
     
  2. N_Cook

    N_Cook Guest


    Combining the priomaries and running varaic at 27 volt, again high current
    drawn, there is about 1 volt from 1 secondary and 2 volt from the other. One
    secondary is for main power rails and the other for HT supply for a pair of
    valves. Its a job to say which has some shorted turns.
    DC resistance of the secondaries is 0.5R and 4.1R
    I will try feeding low voltage AC in the secondaries and see what happens.
     
  3. Shorted turns in either primary will show up as a high current when driving
    either one since the non-driven one acts as a secondary.

    Or shorted turns in a secondary.

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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  4. N_Cook

    N_Cook Guest

    There is a light gauge secondary, presumably about 18V ac , full wave
    rectified to give 25.2 V dc for the seriesed heaters of 2 ECC83, CTs N/C.
    The main secondary winding supplies the TO3 o/p devices and a diode pump for
    the valve HT. Not determined where the opamp supplies are from.

    Putting 20V ac on the heater supply gave 50 V on main and 96V on each
    primary
    Putting 20V on the main gave 7.6V on the heater one and 38 V on each
    primary.

    I assume from this the primaries are ok and something wrong with the main
    secondary, perhaps 2/3 of the run is shorted somewhere, 1/3 only functional.
    Luckily they always dilute the epoxy infill with chalk or something so easy
    to chain drill out. Nothing obviously wriong with the outer layer of
    secondaries but I assume the problem is due to only one layer of polyester,
    half overlapped on the outside and only that separates squashed secondaries
    right up against the metal top of the 1U case. Rubber disk on the bottom
    face of the toroidal but nothing at the top other than the standard toroidal
    polyester wrap strip. At least they did not decide to pass the coach bolt
    through top and bottom of the case. If the secondaries are re-windable (if
    no primary damage) then some extra padding will have to go in over the Tx
    and some extra space found in the cab.
     
  5. My wild guess would be the HT secondary since that has the most turns
    of fine wire with the msot stress on it, but who knows? I would think
    that it will be rather difficult to sort it out in a well made
    transformer without knowing what the original specs were exactly,
    including resistances, or unwinding it.

    What are the spec'd voltages on the outputs?

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
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  6. N_Cook

    N_Cook Guest

    Don't know the specs. What is Murphy's Law - If something can go wrong it
    will go wrong at the most inopportune time or the most awkward place. I
    always thought the primary was buried on the inside of transformers -
    conventional and toroidal. Not this one, the primaries are on the outside
    and the main secondary is on the inside, presumably to save on the expensive
    large gauge copper wire. Basler Electric, Highland, Illinois
     
  7. N_Cook

    N_Cook Guest

    Can a toroidal former become permanently magnetised and cause excessive
    current draw. ? This one does not attract a small piece of iron. I don't
    know what the main
    secondaries are supposed to provide, but can give 300 watts into 4 ohms.
    I've wound off the primaries to try and
    understand what is going on with this transfornmer. Applying 70 volt ac ,
    from a variac, across series secondaries is ok but increase to 75 volt and
    the current draw goes up to 1/4 amp. Should be able to take twice that
    voltage. Absolutely no visible problem , to the wiring or the coating of the
    toroid and both secondaries measure DMM the same DC. Tomorrow will apply
    something a bit over 70 volt with temperature sensitive labels fitted
    around. I will even try a TV degaussing coil.
     
  8. No, it isn't magnetized.

    That fact that you can now apply 70 VAC to it without significant
    current drawn suggests that whatever short was present is now gone.

    How many turns were on the original primaries? Call that "n".

    Wind a few turns on the core, call that "m".

    Apply the rated input voltage x m/n to your winding and report back. :)

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  9. bz

    bz Guest

    The magnetic field on a toroid is contained within the toroid.

    You will have a hard time degaussing it unless you wrap wire through it.

    You will need at least a single turn of wire THROUGH the toroid to check to
    see if it 'remembers'.

    See how core memory works:

    http://ed-thelen.org/comp-hist/navy-core-memory-desc.html






    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
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