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Power Transformer Replacment

Discussion in 'Electronic Repair' started by ll, Jun 4, 2007.

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  1. ll

    ll Guest

    I am in the process of replacing a power transformer in a musical
    instrument amplifier (solid state). As the existing transformer
    doesn't currently pass the continuity test across the primary, and the
    coil wire seems to be broken somewhere in the coil, I am thinking of
    just replacing it. How can I find the power rating of the existing
    one? I know that the amp is rated at 30 watts.

    If watts=amps x volts, then 30watts = 115v x 3.83amps. Would I need a
    transformer that was less than or equal to the 3.83amps with the same

  2. mc

    mc Guest

    Amplifiers are not 100% efficient, so there will be *more* power coming out
    of the power transformer into the amplifier than coming out of the amplifier
    into the speakers.

    Assuming you know the output voltage, I would suggest matching the overall
    size and weight of the transformer. What is the output voltage (and how
    many output windings are there)?
  3. Mike

    Mike Guest

    your best bet is to figure out the design of the amp. \

    is it a chip amp, or discrete amp? if discrete, try +/- 35V.

    if its chip amp, look for datasheet of chip to figure specs.
  4. DaveM

    DaveM Guest

    What's the make & model of the amp?

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
  5. Eh? If W = I x V then W
    I = ---

    so the current is 0.26 amps.

    However, when you say continuity test, what do you actually mean? The
    primary will not be a dead short to DC, and some continuity testers are
    designed to give a fail if the resistance is more than a few ohms. So if
    you can safely power it up, look to see what the voltage is or isn't
    across the output(s).
  6. ll

    ll Guest

    It's a discrete amp, and in looking at the amp, I neglected to include
    that it is rated at 10 watts (RMS). There are two secondary leads (I
    haven't dissected the transformer to see if there are any unused
    leads, etc).

  7. ll

    ll Guest

    I'll make a quick addendum to this: there are only two diodes on the
    circuit board, so the transformer is probably (?) centre tapped.
  8. Guest

    To calc what tf is needed would require at least:
    amp output power
    speaker impedance
    some idea of the output stage topology

    Its possible to come up with likely figure without those, but they'd
    be fairly hit & miss.

  9. Arfa Daily

    Arfa Daily Guest

    If the primary is really open circuit, there's a better than even chance
    that it's because a thermal fuse buried in the windings, has failed. Quite
    often, the wires from these fuse elements are brought out to tags on the
    transformer outside, so it would be worth looking to see if there is an
    extra tag on the primary, with a silver rather than copper coloured wire
    going to it. This will usually be one of the line power input tags. If you
    do manage to identify such, it is easy enough to just put a ( diagnostic )
    short across it, then repower, preferably via a variac, and measure the
    output voltages. I say via a variac, because if the the thermal fuse has
    failed, it may be for a genuine reason. These items do fail quite regularly
    for no apparent reason, but they will also fail if the tranny is subjected
    to a serious overload for any length of time.

    There are ways of making valid guesstimates of the secondary voltages, but
    last time we got a thread going on that subject a couple of months back, it
    descended into such a pissing contest, with so many opinions and
    cross-rubbishing of posts, that I really don't think that it would be a good
    idea to go there again ... :- /

    I take it that you can't obtain a genuine replacement tranny from the
    manufacturers? I've never been very happy about substitute power
    transformers, but that's another bone of contention that many disagree over

  10. ll

    ll Guest

    Thanks for your input on this - the stock replacement isn't available
    from the manufacturers, so I'll probably go with something the same
    size as the old one.
  11. hr(bob)

    hr(bob) Guest

    If the manufacturere is known, they might have a schematic even if
    they don't have the transformer.!!!

    H. R. Hofmann
  12. Marra

    Marra Guest

    I would get in touch with the manufacturer before you end up spending
    more on blown up components than the amp is worth.
    You could get a clue from the PSU electrolytics as most amp designers
    run the amp fairly close to the electrolytics capacity.
  13. ll

    ll Guest

    Is the secondary voltage from the transformer usually just below/
    around the value of the caps?
  14. Arfa Daily

    Arfa Daily Guest

    You don't want to go here ...

    This is exactly how the last argument on the subject started ... :-\

  15. neon


    Oct 21, 2006
    power transformers for the 120v ac range usualy the pimary is in the range of ~15 ohms now as far as the power of the amp of 30w usualy that means nothing because is music power peak power RMS power power to the speakers what? you and i both don't know. so my advice is to replace the power transformer with as close as possible to the physical type or more. the voltage that you don't know look at the caps after the rectifier and see what they are rated for 50vdc or 150 vdc. finaly i have never encounter an open primary before burnt transformers yes but that you can smell it hours before and hours after so make sure
  16. ll

    ll Guest

    Yikes! Point well taken.
    Well, I did, at long last, find a number. It says "Wuxi Radio
    Transformer Factory" with part number DBE-180 (E139334). Also, I
    found a schematic that lists the secondaries as 12VAC. As for the
    amperage, would 500mA-1A be sufficient?
  17. mc

    mc Guest

    Well, I did, at long last, find a number. It says "Wuxi Radio
    No. A 12-volt 500-mA transformer only delivers 6 watts (or less if the
    voltage and current are out of phase; I don't want to start that argument
    again). Your amplifier must have more electricity going in than coming out;
    it's not a perpetual-motion machine.

    Match the size and weight of the transformer and you'll get it close.
  18. Marra

    Marra Guest

    There is a good bit of loss in heat in the output transistors.
  19. mc

    mc Guest

    Yes, that's my point. The power transformer will be rated for considerably
    more watts (or volts * amps) than the output of the amplifier. Matching the
    size and weight of the original transformer is likely to result in a good
    match to the power rating of the original.
  20. Guest

    With 10w rms output youre looking at roughly a 12v 1.5A transformer.

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