AC transformer

Discussion in 'General Electronics Chat' started by Jonny2, Feb 19, 2014.

  1. Jonny2

    Thread Starter New Member

    Feb 19, 2014
    2
    0
    Hi, I'm a new member :) my history in electrical work is quite limited hence my question. I've opened up a radio / CD to try to find why it only works on battery power but not through the power input. The transformer is a sealed unit ? which has no identifying marks only that it has a "thermal fuse fitted internally". The DC needed is 12 V. The leads from the Tr are fed into a very small circuit board seperate from the main radio board. Am I right in assuming that this `unit` is a converter from AC to DC ? I ask this because the battery leads are attached at the opposite end of this small circuit unit.
    Also I will need information regarding the Tf to replace if necessay, specs that are not available to me and physical size is critical. Kind regards. :)
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    1) Yes.
    2) What physical size?
    3) If you can get a schematic, that would be excellent. If not, try some photos.
    4) There are several ways to do this.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,452
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    I have moved this to General Electronics Chat since this query is about power supply repair.
    Radio and Communications is for discussion of RF transmission and reception.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,452
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    Here is some information you can provide that would help get better answers:

    1) What is the make and model of the radio?
    2) On the radio, see if the power requirements (volts and amps) are written down.
    3) Measure the voltage from the power pack using a DMM. Show both DC and AC measurements.

    If you get zero voltage output, DC or AC, you can assume that the fuse is blown.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    This is typical of some imported devices and unfortunately often the fuse is buried inside the winding so repair is usually not possible, if you go to the Hammond Transformer site, you should be able to match up a suitable transformer, if you need one for 12vdc then you require a 9vac secondary.
    for power rating, match up one of equal physical size.
    Max.
     
  6. Jonny2

    Thread Starter New Member

    Feb 19, 2014
    2
    0
    Thank you for your promrt reply, I did manage to get inside the TF but there's still no way to find the supposed inner thermal fuse :confused:
    I've done a drawing of the sizes attached.
    The radio model is a mikomi KW 8 5W Compact disc Digital radio.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the thremal fuse in the transformer is probably just a lump in the windings. they put them in to prevent fires if the transformer gets too hot. I havnt had much luck replacing them unless it is on the surface.
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
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    The thermal fuse is almost always connected directly to a primary lead.

    Have you checked primary resistance? Infinity=bad.

    If you carefully cut and unwrap the paper, you will find connection to a hair-like wire on one lead. The other lead will be connected to the fuse.

    Temp. rating is usually marked.

    If not start low like 120°C.

    I'm waiting for one today for a humidifier motor. Only had a 150°C here.

    Unlike others experience. I rarely have trouble replacing them.
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
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