Stereo Power

Discussion in 'General Electronics Chat' started by Freyosa, Jan 17, 2006.

  1. Freyosa

    Thread Starter New Member

    Jan 17, 2006
    6
    0
    Hi
    I was wondering if any1 could help me out on my problem. I have this stereo bought from Japan (100V) and was plugged into a 230V socket without a transformer and it blew some parts of the stereo. I was wondering which part of the stereo would mostlikely be blown. I have opened up the case of the stereo and found that all the fuses are still fine. There is a burning smell so something is ovbiously burnt. There is a power supply part of the stereo which consists of a transformer, 4 diodes, 2 capacitors and 2 fuses.

    Any suggestions/help/tips would be greatful as to how I can test whether or not the parts of the circuits are broken or not.

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I hope you have a DVM with a diode check function.

    The filter caps are probably shot - too much voltage on them. The rectifier diodes will have to be checked. If you only have one bridge, then all voltages probably derrive off the main + and - DC voltages. Any or every transistor and polar capacitor is suspect. Check the output and driver transistors. If they blew, they may have pulled voltages down and protected the rest of the circuit. If lots of transistors are bad, it may be more useful as a doorstop.

    You will have to check just about everything, and that will be no assurance that something won't let go when you try powering it up after the obvious repairs. Bringing it up on a variac slowly would be a good idea.
     
  3. Freyosa

    Thread Starter New Member

    Jan 17, 2006
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    0
    Thanks for the reply.

    I have a Digital LCR Multimeter that can check the diode.

    But i'm quite new to this kind of stuff so i have a few questions.

    Is a bridge like a connection that the power is sourced into seperate components in the stereo. The power supply unit has a power plug(alike the ones in computers where u connect the power to motherboard etc).

    And where would i be able to get information as to the type of transistors i would be looking at. As i said i'm quite new at this and have limited resources. I guess i have to un-solder the transistors out and test them individually to see if they still work, but the Multimeter that i have can only test out the gain of the transistor.

    Thanks again
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    You're not too worried about gain in the transistors, but wheather they have good junctions and are not open/shorted. If the diode test shows open C - E, and you have junctions B - E and B - C, then the transistor is ok.

    As you have noted, the fun part is removing devices from the pc boards to get an accurate check. By the way, the bridge is the set of four diodes next to the transformer secondary that rectifies the AC into DC. There may also be a number of diodes in the circuit, including a couple of zener diodes for deriving lower voltages. Be nice if the mf'rs would include schematics with their stuff.
     
  5. Freyosa

    Thread Starter New Member

    Jan 17, 2006
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    0
    Umm, i have just taken out the 4 diodes and 2 capacitors and have no clue as to whether or not they are functioning properly.

    The 4 diodes all measured out to be approximately 0.533. The diodes seemed to be silicon rectifiers of the model number RL204. I notice this is off from the reading of 0.7 as a silicon diode is supposed to have, but I am not certain if that is true for this case.

    Also the capacitors seemed to measure out to be the same capacitance as well being 47.2nF. I don't have any information on the capacitor, other than it is marked M78 473J.

    After doing this i seem to be kinda stuck...as to which transistors i should be checking... There seems to be hundres...lol

    Thanks.
     
  6. Freyosa

    Thread Starter New Member

    Jan 17, 2006
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    0
    Is there anyway to test whether or not the transformer is blown?
    It seems to me that the burning smell is coming from the transformer

    The transformer seems to have 3 input and 7 output. I have tested the voltage on it and it does give reading, but it seems that only one input has voltage accross it while the other 2 does not have any reading.

    Thanks
     
  7. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    In my experiance, the mains transformer normally goes shorted or O/C, and in most cases, before any damage is done to the rest of the circuit.
    If the transformer Smells, then it most likely has sorted turns and will need to be replaced. Try to get one for that model, but with 230 volt, or multi input primary (Export, UK, AUS, NZ etc model).
    Before you fit it, make sure none of the power supply diodes/bridge rectifiers are shorted, and the capacitors have no Bulges, or brown liquid smudges at their bottoms.
    Quickly check that the output transistors have no Collector/Emitter shorts..(assuming its not using hybid output modules).
    Not a Job for the faint hearted, and I would recommend you get a pro to do the job.
     
  8. Freyosa

    Thread Starter New Member

    Jan 17, 2006
    6
    0
    Hi Gadget,

    Thanks for your reply. I see from ur profile that ur from New Zealand as I'm also from NZ. I was wondering if you know where i would be able to obtain transformers, or anywhere that you might recommend me to find one for my stereo.

    Thanks
     
  9. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    What brand is the Stereo..?
     
  10. Freyosa

    Thread Starter New Member

    Jan 17, 2006
    6
    0
    It's a sharp stereo.
    It's around 6 years old if not older.

    Thx
     
  11. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    your 1st choice is to replace the burned transfo with the same part #, but then again as "beenthere" has mentioned there could be other parts that have been damage that does not show any damage. even if these parts tested look good but when powered up they will start to show their true state.

    is this an output capacitorless power amp?

    moz
     
  12. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    The NZ parts wholesaler for Sharp is TESA. They have a branch in Auckland, and one in Christchurch.
     
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