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Transformer question

Discussion in 'Electronic Repair' started by T N Nurse, Sep 25, 2004.

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  1. T N Nurse

    T N Nurse Guest

    I have a 50 watt valve amp which seems to have blown its
    output transformer. A new one is on order, but I was puzzled
    by some of the reading I got off it and wondered if someone
    could give an explanation. All the measurements were made
    with the transformer *_completely out of circuit_*, both primary
    and secondary.

    The transformer is from a push-pull standard guitar amp (Trace
    Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
    red hot before the fuse popped. Checking the bias voltages, I
    had around -46v on the grid of each of the El34s, so that was
    a reasonable figure. I then removed the output transformer
    and did some resistance measurements on it. When measure from
    the centre tap to either of the primary outer connections, the
    reading was 69 ohms for each. But when I measured across the
    primary outer connections, instead of the expected 138 ohms,
    I got an open circuit. I rechecked it numerous times but with
    the same result. I even removed the cables from their connector
    and measure across the bare wires, but still the same result,
    69 ohms from the centre tap to the outers, but open circuit
    between the outers.

    On the basis of these bizarre reading, I assumed the transformer
    is faulty and ordered a new one, but can anyone offer an
    explanation as to why I got such resistance readings? I have a
    vague recollection of similar results on a small 15 watt amp I
    repaired many years ago and replacing the transformer fixed it
    and it went on to give good service, but I would like to know
    what is actually going on. Anyone?
     
  2. philo

    philo Guest


    there are usually two wires in the center tap...
    if they are not touching when you take your reading ...there will be
    infinite resistance...
     
  3. John Fields

    John Fields Guest

    ---
    The only thing I can think of is that the primary is actually two
    windings with an intermittent connection at the center tap and that
    pressure from your probe/alligator clip/whatever when it was connected
    to the center tap was connecting the two windings so that you got the
    dual 69 ohm readings, but then when you disconnected from the center
    tap the connection between the windings was broken, resulting in the
    infinite resistance reading between them.

    Try resoldering the winding ends going to the center tap and see what
    happens. Maybe you'll wind up with a good spare!-)
     
  4. Ross Herbert

    Ross Herbert Guest

    On Sat, 25 Sep 2004 14:35:33 +0100, T N Nurse

    |I have a 50 watt valve amp which seems to have blown its
    |output transformer. A new one is on order, but I was puzzled
    |by some of the reading I got off it and wondered if someone
    |could give an explanation. All the measurements were made
    |with the transformer *_completely out of circuit_*, both primary
    |and secondary.
    |
    |The transformer is from a push-pull standard guitar amp (Trace
    |Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
    |red hot before the fuse popped. Checking the bias voltages, I
    |had around -46v on the grid of each of the El34s, so that was
    |a reasonable figure. I then removed the output transformer
    |and did some resistance measurements on it. When measure from
    |the centre tap to either of the primary outer connections, the
    |reading was 69 ohms for each. But when I measured across the
    |primary outer connections, instead of the expected 138 ohms,
    |I got an open circuit. I rechecked it numerous times but with
    |the same result. I even removed the cables from their connector
    |and measure across the bare wires, but still the same result,
    |69 ohms from the centre tap to the outers, but open circuit
    |between the outers.
    |
    |On the basis of these bizarre reading, I assumed the transformer
    |is faulty and ordered a new one, but can anyone offer an
    |explanation as to why I got such resistance readings? I have a
    |vague recollection of similar results on a small 15 watt amp I
    |repaired many years ago and replacing the transformer fixed it
    |and it went on to give good service, but I would like to know
    |what is actually going on. Anyone?


    Did you use a digital multimeter?

    I have encountered some strange an incomprehensible readings when
    taking resistance measurements on highly reactive components such as
    transformers when measuring with my Gossen Metrawatt MetraHit 25S
    digital meter (a not too cheap instrument), and when this occurs I
    always revert back to my trusty old moving coil analogue meter (AVO
    model 7) and usually the correct expected measurement is obtained.

    Ross H
     
  5. Asimov

    Asimov Guest

    "T N Nurse" bravely wrote to "All" (25 Sep 04 14:35:33)
    --- on the heady topic of "Transformer question"

    TNN> From: T N Nurse <>
    TNN> Subject: Transformer question
    TNN> Organization: Malathion Church of Holy Redemption
    TNN> Xref: aeinews sci.electronics.repair:39778 sci.electronics.misc:18316

    TNN> I have a 50 watt valve amp which seems to have blown its
    TNN> output transformer. A new one is on order, but I was puzzled
    TNN> by some of the reading I got off it and wondered if someone
    TNN> could give an explanation. All the measurements were made
    TNN> with the transformer *_completely out of circuit_*, both primary
    TNN> and secondary.

    TNN> The transformer is from a push-pull standard guitar amp (Trace
    TNN> Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
    TNN> red hot before the fuse popped. Checking the bias voltages, I
    TNN> had around -46v on the grid of each of the El34s, so that was
    TNN> a reasonable figure. I then removed the output transformer
    TNN> and did some resistance measurements on it. When measure from
    TNN> the centre tap to either of the primary outer connections, the
    TNN> reading was 69 ohms for each. But when I measured across the
    TNN> primary outer connections, instead of the expected 138 ohms,
    TNN> I got an open circuit. I rechecked it numerous times but with
    TNN> the same result. I even removed the cables from their connector
    TNN> and measure across the bare wires, but still the same result,
    TNN> 69 ohms from the centre tap to the outers, but open circuit
    TNN> between the outers.

    TNN> On the basis of these bizarre reading, I assumed the transformer
    TNN> is faulty and ordered a new one, but can anyone offer an
    TNN> explanation as to why I got such resistance readings? I have a
    TNN> vague recollection of similar results on a small 15 watt amp I
    TNN> repaired many years ago and replacing the transformer fixed it
    TNN> and it went on to give good service, but I would like to know
    TNN> what is actually going on. Anyone?

    The only thing I can think of is that (burnt) oxidized metal to metal
    contacts can form a rudimentary semiconductor and will not allow current
    to flow in reverse just like a diode. Try swapping your ohmmeter wires.

    A*s*i*m*o*v

    .... Are part-time band-leaders semi-conductors?
     
  6. Art

    Art Guest

    Having an open resistance reading on the primary in indicative there
    has been failure within the transformer, be it an open thermal protect
    device or actually a break within the actual primary wires. At that point it
    is not necessary to determine the condition of the secondary unless you have
    a specific reason for doing so, such as, determining what the shorted output
    valve may have damages along with the transformer.
    particular circuit. You may be able to compare the values with those on the
    other-side of the push-pull circuit to ascertain if indeed there is
    additional damages.
    characteristics of those particular valves. I would be interented also in
    the cathode current and plate voltages on eachof the devices. May be best to
    replace both of them since one obviously has failed, trying to get a matched
    pair, if available an reasonable cost. Cheers: Good luck in rectifying the
    initial failure and remedies.
     
  7. John Fields

    John Fields Guest

     
  8. I liked the two separate windings with two wires not making contact
    for the centertap. Has the OP retested or checked for this possibility?

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

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    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header is ignored.
    To contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  9. Jamie

    Jamie Guest

    its most likely your digital meter.
    my fluke i use at work will not properly
    read a field winding above a 100 ohms due
    to inductive reactions. it just sits there
    blinking now and then giving me an OL reading.
    try putting it on DIODE mode.
     
  10. CJT

    CJT Guest

    Any chance there's more in there than a transformer? Like a couple
    of protective diodes (perhaps as snubbers) that are back-to-back when
    viewed from end-to-end? It would be odd, but not completely out of
    the question.
     
  11. T N Nurse

    T N Nurse Guest

    Thanks to everyone who replied to this. The problem has now been
    solved - intermittent fault in the probe lead. Grrrrrr!!!!
    New probe leads now give the correct readings.
     
  12. T N Nurse

    T N Nurse Guest

    That's an interesting observation, and perhaps explains the previous
    weird measurement. I note that someone else has said that their
    expensive Fluke doesn't like transformer windings over 100 ohms.
    I tried today with a different multimeter (and leads) and got the
    expected readings. I assumed that it was the leads at fault, but
    perhaps not.
     
  13. At least that is a lot cheaper to fix than the cost of a new
    transformer.

    -john-
     
  14. John Fields

    John Fields Guest

     
  15. T N Nurse

    T N Nurse Guest

    {Centre tapped primary of a push-pull output transformer for audio amp}

    Looks like it. The two windings came in at 66 and 70 ohms respectively
    with combined reading of 135 ohms. Looks like a partial short in
    one of the windings. I measured it 3 ways just to be sure - using
    the multimeter, using a constant voltage source and checking the
    current and using a constant current and checking the voltage. All
    3 came in with the same readings. I'm waiting to check with the
    manufacturer to check if a 4 ohm difference is out of spec - I
    suspect very much that it is.
     
  16. David

    David Guest

    It is not uncommon for the DC resistance of a center tapped winding to differ on each half. For the same number of turns, one half could have more length of wire because that part of the winding is further from the core piece and one turn uses more wire. A better way to test is to put an AC signal on each half separately and see if the secondary gets the same output.
    David
    ..
     
  17. John Fields

    John Fields Guest

    It is not uncommon for the DC resistance of a center tapped winding to
    differ on each half. For the same number of turns, one half could have
    more length of wire because that part of the winding is further from
    the core piece and one turn uses more wire. A better way to test is to
    put an AC signal on each half separately and see if the secondary gets
    the same output.

    ---
    I disagree. Using EI laminations and winding the entire transformer on
    the center leg, it makes nore sense, if only for the sake of economy,
    to wind the CT primary bifilar and avoid using more wire than
    necessary. After all, an extra ten feet per transformer is about two
    miles of wire, 1000 transformers downstream. Putting down both halves
    of the primary at the same time also saves time and tends to make both
    halves of the primary look more alike than winding one half on top of
    the other.
     
  18. David

    David Guest

    John,
    That may be true, but this transformer may not have been wound with a bifilar primary. If bifilar I agree, if not, I stand by my original post and it should be tested by either measuring inductance or seeing if there is a difference in a secondary by feeding a signal into each half separately. I know a short on one side will affect both halves but a difference should still be seen.
    David
     
  19. John Fields

    John Fields Guest

    John,
    That may be true, but this transformer may not have been wound with a
    bifilar primary. If bifilar I agree, if not, I stand by my original
    post and it should be tested by either measuring inductance or seeing
    if there is a difference in a secondary by feeding a signal into each
    half separately. I know a short on one side will affect both halves
    but a difference should still be seen.

    ---
    I don't disagree with your measurement strategy except that I'd put a
    signal into the secondary and then measure the primary output voltages
    on either side of the center tap, the presumption being that if the
    winding on one side has shorted turns, the voltage out of that side
    will be lower than the voltage out of the other side.
     
  20. Jim Adney

    Jim Adney Guest

    I think this is only half right. Winding them bifilar would certainly
    make them more "balanced" in both turns and impedance, but the total
    amount of wire used would have to be the same.

    Think of it this way: I still need the same number of turns in each
    winding and the same number of total turns. Winding them bifilar
    (assuming the same wire gauge) will still occupy the same amount of
    the winding window.

    So instead of "an extra ten feet" of wire in the outer winding, we end
    up using five extra feet in each winding.

    I don't know how a real transformer manufacturer would look at it, but
    I suspect that the bifilar way has real appeal as long as the voltages
    are small.

    Once the voltages are large, we may want to spread out the ends and
    wind them separately, which most likely means that we're back to the
    original method of one on top of (and longer than) the other.

    -
     
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