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Dodgy step down tranny

Discussion in 'Hobby Electronics' started by Phil Allison, Feb 8, 2011.

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  1. Phil Allison

    Phil Allison Guest

    ** Hi all,

    a repair job arrived last night accompanied a step-down tranny - the
    repair is a US made amplifier that requires 120 volts AC at 60Hz - but
    copes with 50 Hz all right.

    The step-down tranny was supplied here in NSW by an eBay seller:

    See: www.rhinotools.com.au

    It is labelled as "2000W' and 240V to 110 V " plus the words "Korean
    Technology".

    The unit I have here is similar to ones on his eBay shop now, but not
    identical.

    Inside is a toroidal **autotransformer**.

    Problems:

    The unit cannot possibly deliver 2000 watts - the max continuous rating
    should be no more than 500 watts. The resistance of the 240 volt winding is
    5.6 ohms at room temp. Do the math.

    The unit does not output 110 volts under load - tested with a 500 watt
    resistive load, it outputs only 103 volts.

    The only protection for the transformer is via a fuse holder on the back
    fitted with a glass fuse rated at 10A - though the holder is not labelled
    with any value.

    Anyone smell smoke ??


    ..... Phil
     
  2. Sylvia Else

    Sylvia Else Guest

    Well, it would certainly get quite hot at its rated output, but I don't
    think I'd expect smoke. If it's the same size as the one on the website,
    it's reasonably large, and it only has to get rid of 50-60W.

    A buyer would have a justifiable gripe at the output voltage drop. You
    don't mention any apparent provision for tap switching, so I assume
    there's none.

    Sylvia.
     
  3. Sylvia Else

    Sylvia Else Guest

    Perhaps just two 10A plugs ;)

    Sylvia.
     
  4. Phil Allison

    Phil Allison Guest

    "Stupider than anything Else on Earth"

    ** Burn the witch !


    ** Do the math - you stupid fucking bitch.

    Try using " I squared R " maybe ......


    ** Wrong in both cases.

    ** The voltage dropped 7 volts ( from 110V) at 500 watts with the input kept
    steady at 240V.

    Do the math for 2000 watts.


    ..... Phil
     
  5. Sylvia Else

    Sylvia Else Guest

    Yes, you're right, I had a brain spasm. Just as well you've never made a
    mistake.
    I did make a comment about tap switching.

    Sylvia.
     
  6. Phil Allison

    Phil Allison Guest

    "kreed"

    The size of the case is irrelevant, they could shove a very small
    transformer inside
    you need to see the internal transformer to make such a judgement.


    ** Err - the weight might be a good guide.

    The unit I have here weighs just under 3.8 kg including the attached lead.


    .... Phil
     
  7. Sylvia Else

    Sylvia Else Guest

    I'm assuming the case is as small as they can make it, simply because
    that reduces the cost, and increases the potential profit.

    Also, it weighs 12 kg, so it either contains a metal brick, or has a
    hefty transformer.

    Anyway, Phil has correctly pointed out that I've made a mistake in
    calculating the heat to be dissipated given the resistance he measured.

    Yet I can't help thinking that something that heavy should be able to
    achieve more than 500W. Perhaps Phil could post its exterior dimensions
    - it may simply have been mislabelled.

    Sylvia.
     
  8. Sylvia Else

    Sylvia Else Guest

    Then it's not the same as the 2kW version on the website, which weighs
    12.5 kg. Indeed, it has a similar weight to the 500W version.

    Perhaps there's a QA problem with labelling.

    Sylvia.
     
  9. **The mass SHOULD be a giveaway. However, I've worked on more equipment than
    I care to think about, where the manufacturer (usually Chinese) has
    incorporated large pieces of non-structural steel to increase the mass of
    the product. I've even seen all-aluminum chassis devices, where steel has
    been used to increase mass. The first time I saw this done, was with an
    Onkyo (Japanese) CD player, back in the 1990s. The CD player weighed in at a
    hefty 25-odd kg. More than half that mass was the result of a large piece of
    steel bolted to the base plate. Without opening the player, an owner would
    never realise that it was the reason for all that mass.

    Mild steel plate is still less expensive than copper and laminated iron (in
    the shape of a transformer).
     
  10. Phil Allison

    Phil Allison Guest

    "Sylvia Else"

    ** Funny about the 10amp fuse being fitted ....

    Plus a 2kW version of the tranny would not fit in the box.


    ..... Phil
     
  11. Sylvia Else

    Sylvia Else Guest

    Seems an odd thing to do with a CD player, given that I wouldn't have
    thought a consumer would expect it to be particularly heavy - and
    certainly not 25kg heavy.
    Yes.

    In this particular case it's become apparent that there's a disparity
    between the mass of the example Phil has, and the mass of the 2kW model
    on the website. I suppose the website could just be mis-stating the
    mass, but anyone who paid attention to it on the website would notice
    the difference as soon as the transformer was delivered.

    Sylvia.
     
  12. Phil Allison

    Phil Allison Guest

    "Stupider than anyone Else on Earth "
    ** They are two different models.
    ** FFS - read the OP.

    The stepdown I have here is probably several years old.


    ..... Phil
     
  13. Sylvia Else

    Sylvia Else Guest

    That could just be a consequence of the mislabelling, or perhaps it's
    not the original fuse. Some people can be quite casual about ratings
    when replacing fuses - at least it's better than just covering the blown
    fuse with metal foil.
    It seems clear that this is not a variant of the currently offered 2kW
    model, and that it's not being used in an application that requires
    anything approaching that, suggesting that the purchaser probably didn't
    expect it to be a 2kW transformer either.

    Any chance of your posting a (link to a) picture of the offending item's
    front panel?

    Sylvia.
     
  14. Phil Allison

    Phil Allison Guest

    "Stupider than Anyone Else Alive or Dead.

    ** Burn the witch.

    ** Blah, blah blah,

    The fuse holder is NOT labelled at all.

    ** The unit is from the same supplier - that is all I ever claimed.

    FFS read the OP again and again til you GET IT !!

    ** So fucking what ??????????

    In the future it may have an owner who needs the whole 2kW.

    The owner may loan it to another to use with say 2kW of 110 volt lighting.

    Smoke will get out in about 1 minute if he does that.

    ** Yawnnnnnnnnnnnnnnnnnnnnn....



    ..... Phil
     
  15. **Absolutely. In fact, I've worked on a few Class D amps (that use SMPS)
    that are quite heavy, where there is absolutely no reason for them to be so.
     
  16. **High end audio is a different world to that which most people inhabit.
    Perception is more important than performance in most instances.
     
  17. Phil Allison

    Phil Allison Guest

    "kreed"

    Interesting too how the 3kw version seems to have (hard to read) a
    32amp breaker on the front panel.
    Unless they are fusing the secondary with it.

    **Bound to be in the secondary.

    The unit I have here has the fuse fitted in the primary cos at 18 amps
    nominal - the secondary side would need a fuse and fuse holder bigger than
    is cheaply available.

    The toroidal auto-tranny inside looks roughly made and carries no labelling
    at al.

    Dimensions are 120mm dia and 75mm high.

    There are just three light gauge, flexible wires coming out.


    ..... Phil
     
  18. Sylvia Else

    Sylvia Else Guest

    Did I say fuse holder labelling?
    I agree there's a concern about this specific transformer. But maybe
    that's all it is - one specific transformer that for some obscure reason
    has been mischaracterised. You did refer to the supplier's eBay shop,
    which implies that you thought that was somehow relevant.

    Presumably you'll replace the fuse with something appropriate.

    Sylvia.
     
  19. Phil Allison

    Phil Allison Guest

    "Trevor Wilson"
    ** That is true nearly everywhere.

    Especially politics.


    ...... Phil
     
  20. Sylvia Else

    Sylvia Else Guest

    Your point being? That the seller used to be a ripoff merchant, but is
    now selling quality goods?

    Sylvia.
     
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