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Domestic amp keeps blowing 3.5amp fuses

Discussion in 'Audio' started by Oldampinthedamp, Sep 29, 2019.

  1. Oldampinthedamp

    Oldampinthedamp

    7
    0
    Sep 29, 2019
    Hi there - what an excellent community you have. This is my first post here and it won’t surprise you that I am asking for some help.

    My old AR60 amp keeps blowing both its 3.5 amp fuses on power up. It had been working well - it replaces my old Rotel which was also blowing its fuses.

    It blows its fuses with no connections in or out.

    Could it be my house? I have another AR60 which works great so maybe not that.

    Any guidance on what to look for would be great
     

    Attached Files:

    Last edited: Sep 29, 2019
  2. WHONOES

    WHONOES

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    May 20, 2017
    Who makes / made the AR60?
     
  3. Oldampinthedamp

    Oldampinthedamp

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    Sep 29, 2019
  4. Oldampinthedamp

    Oldampinthedamp

    7
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    Sep 29, 2019
    Could it be the two earths touching - I unscrewed the earth screw by mistake and a little disc fell out - maybe it was separating them?
    I don't think so but maybe am desperate looking for answers
     
  5. WHONOES

    WHONOES

    723
    147
    May 20, 2017
    I doubt if the earth connections have anything to do with your problem. What we really need is circuit diagram to aid possible diagnosis otherwise it is pretty much an impossible task.
     
  6. Oldampinthedamp

    Oldampinthedamp

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    0
    Sep 29, 2019
  7. Ylli

    Ylli

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    Jun 19, 2018
  8. Oldampinthedamp

    Oldampinthedamp

    7
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    Sep 29, 2019
    Hi Ylli - it is the supply fuses which are going everytime I turn on the amplifier.

    Thank you
     
  9. Ylli

    Ylli

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    46
    Jun 19, 2018
    Most likely you have a blown output transistor. Do you have reasonable soldering skills? It looks like these are TIP3055 parts in a TO-246 package. Either totally remove, or lift the legs of all four of these and check with your ohmmeter.
     
  10. Oldampinthedamp

    Oldampinthedamp

    7
    0
    Sep 29, 2019
    I do have a soldering iron but I am no great shakes, maybe too many great shakes, with it.

    I think I will have to take it to a repair shop in London

    Do you know what causes these parts to blow? As I have said my rotel amp suffered the same fate

    Thanks again for all of your help
     
  11. Ylli

    Ylli

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    Jun 19, 2018
    Even solid state devices do not have an infinite lifetime. Sometimes they just fail. ...Or it could have been a power surge, or an overtemp (too loud for too long). But we don't know for sure it is a bad output transistor, but checking these would be the first step.
     
  12. Oldampinthedamp

    Oldampinthedamp

    7
    0
    Sep 29, 2019
    I think it might be the overtemp as before it went it had been left on full with the record player plugged in and humming for 6 hours courtesy of a toddler
     
  13. WHONOES

    WHONOES

    723
    147
    May 20, 2017
    Hi. Finally got to see a schematic of the amplifier. The power amp stage is poorly designed. It has no thermal compensation for the output stage which is a sure fire recipe for thermal runaway which is probably what happened to yours.
    Being dc coupled from front to back, when one of the output devices goes bang it often takes some or all of the rest with it. But this does depend on the failure mode of the transistor concerned.
    You could try removing all of the power amp transistors and replace them with new ones and possibly add a mod to provide the missing thermal compensation or...…….chuck it away.
     
  14. skenn_ie

    skenn_ie

    24
    2
    Sep 7, 2009
    The most common faults in old electronics are the electrolytic capacitors. They will degrade at a fairly predictable rate, gradually losing capacitance, then go short circuit. Overvoltage will cause an immediate short circuit. They often, but not always, form a bulge on one end.
     
  15. WHONOES

    WHONOES

    723
    147
    May 20, 2017
    Old capacitors degrading as you suggest would cause a very noticeable hum from the amplifier. Also, an overvoltage large enough to cause a failure as you suggest would have to be quite substantial and is a very unlikely failure mode in this instance.
     
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