Strange DC on output symptom...

Discussion in 'General Electronics Chat' started by noddyspuncture, Feb 21, 2014.

  1. noddyspuncture

    Thread Starter New Member

    Jan 15, 2014
    22
    0
    Hi guys,

    I am currently stumped on a couple of things I have here that are faulty.
    One of them is this Bang & Olufsen amplifier.

    I am getting DC on the outputs. The output transistors read OK and are not shorted - and there is a schematic included inside the unit (great idea!)

    I have attached a scan of the final output stage and also the headphone socket which leads on to the speaker sockets.

    I've circled the points in question in red. I was hoping someone could tell me why this is...? As you can see there is a 3000uF capacitor in series with the output to the 'phones/speakers and the circuit shows that there is 18.6volts DC on the 'other' side of it. What I am getting is that same 18.6volts on the side going to all the sockets...! On both channels.


    At first I assumed both caps were shorted in some strange manner (even though my meter told me they weren't)... so I replaced them anyway. It was the same... in fact no matter what capacitor I put in there I get that 18.6 DC voltage where I shouldn't.

    I even thought maybe this is how it should be (...?!?) and momentarily connected a speaker to check how it behaved... needless to say it didn't like it..!

    Does anyone have any idea please, why no electrolytic capacitor seems to block DC in this particular instance...?

    Cheers,
    Tom


    B&O.jpg
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    are you getting that voltage with or without the speakers connected? the output stage has two transistors in series as pushpull. the supply for the output stage is probably 36 volts, 1/2 of that is 18. with the speakers disconnected, there might be enough leakage to read 18 volts on the outputs.
     
  3. noddyspuncture

    Thread Starter New Member

    Jan 15, 2014
    22
    0
    Yes I am... no speakers connected - like I mentioned, when I connected a speaker 'momentarily' the cone jumped and it looked really unhealthy so I didn't want to risk cooking the speaker.

    So you think this might be 'normal' then...? Should I connect the speakers and just see if I blow them..?

    But also what I don't understand is how I can be reading DC through the capacitors...!?

    Cheers
    Tom



     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    first, try a resistor across the speaker output terminals, if it is just leakage, the voltage should drop.. also, if the voltage was negative, it is just part of the charging of the cap, and should drop. a lot of older radios of that era had quite a "pop" or thump when turned on. resistor less than 100 ohms.
     
  5. noddyspuncture

    Thread Starter New Member

    Jan 15, 2014
    22
    0
    Many thanks, I shall try that...!


     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You can also put a resistor in series with the speaker, which allows you to do audio signal testing without hurting the speaker (although it will be more quiet).

    1k will save the speaker from anything but might be too quiet, if it sounds ok you could reduce to 220 ohms for the second test.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    Have you notice this happening in the past or are you testing this amp for the first time without speakers?

    The exact purpose of the 3000μF capacitors in series with the speakers is to block DC. You will always get a "thump" from the speakers as the capacitors charge up when you first turn on the power. This is normal.

    If you want to test for leakage, measure the voltage across the speaker after you turn on the power. If you are concerned with testing with speakers, use 10Ω resistors instead of the speakers.
     
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