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VU meter not working. Dynacord Echocord Super 76

Discussion in 'General Electronics Chat' started by Rorut, Mar 2, 2018.

  1. Rorut

    Rorut Member

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    I bought myself an old Echo. The VU meter does not work. Measuring voltage over it in DC mode gives 9mv. Measuring Ohm over it turned off the needle goes up all the way smooth and back smooth (nothing happens in mega or kiloohm). Seems to be a circuit problem. Anyone worked with these machines before? Is the circuit inside located in a problematic position?
    Everything else seems to work fine.

    Schematic attached.
     

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  2. JimB

    JimB Super Moderator Most Helpful Member

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    A strange expression in English.

    If the equipment is otherwise working normally, then the fault is almost certainly in the fragment of the circuit around transistor T200.

    JimB
     
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  3. Rorut

    Rorut Member

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    English is not my native language. What I tried to say was is if the pcb where the logic related to the VU meter is located in a place that is difficult to reach whithout taking everything apart. Please tell me how to adjust my expression. Thank you
     
  4. dave miyares

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    What are the voltages at the collector and emitter of T200?
     
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  6. JimB

    JimB Super Moderator Most Helpful Member

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    Yes I realise that, and I did understand what you were trying to say.
    A better way to say it may be " Could the problem be in this part of the circuit?"

    JimB
     
  7. schmitt trigger

    schmitt trigger Well-Known Member

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    The schematic shows some voltage readings on the collector and emmitter. Start by measuring those.
     
  8. dave miyares

    Dave New Member

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  9. Colin

    Colin Active Member

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    The first thing I would do is replace the 5 electros. 1u 1u 1u 10u 100u This will cost you $1.00 The VU meter will now work.
     
  10. Rorut

    Rorut Member

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    Opened it up. A lots of cables attached everywhere. Blue circle measured between 13 and ground gives 33v DC. Green circle is from VU meter. Any ideas where the transistor is? Guess I need everything connected for making measurments and to reach the other side of that board I probably need to desolder a lot of connections.

    If it is not possible to make any measurments from this side as it is maybe one solution can be to desolder all cables, take the card out and replace all ellyt capacitors and/or any other components that may be old and then connect it all back and hope for the best? Dont know but that solution feels a bit difficult
     

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    Last edited: Mar 3, 2018
  11. JimB

    JimB Super Moderator Most Helpful Member

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    That looks OK.

    Unfortunately no.

    Yes.

    Try it first without disconnecting.
    It may be possible to move the board sufficiently to see the component side of the board.

    That unit is a fine example of something which was built with no thought given to faultfinding and maintenance.

    JimB
     
  12. Rorut

    Rorut Member

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    This sucks. Thinking is there any good 2 to 4 female/male connections I can buy and add to all places where a lot of cables go? Would make it easier to disconnect for future service aswell.
     
  13. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Looks like 4 screws holding the board in place. If you remove them and the cable ties you might be lucky and have enough cable slack to turn the board enough to get access to the underside.
     
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  14. Rorut

    Rorut Member

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    Checked again. That will not work. Some cables are to short. Measured nr 19 and that gave me 0v in both DC and AC if that could point me in any direction...
    Dc started at 4-5v and kept going down until 0.02v.

    Maybe I found where the transistor is. Not 100% sure but in the red circle I get measurements from top DC 0v middle 2.3v and bottom 14.9v. Also I get a resistance connected to the bottom 2.7K.
    Probably need to get it apart to be sure :(
     
    Last edited: Mar 3, 2018
  15. Rorut

    Rorut Member

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    Ok now I have found the transistor and made some measurements.
    Used a 1khz test tone and som music.
    Look at the image for results and measurments.
    transistor.jpg
     
    Last edited: Mar 5, 2018
  16. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Looks like T200 isn't conducting. C should be about 15V and E should be about 1.8V (I think. Difficult to read the schematic).
    Some possibilities:
    1) T200 is dead,
    2) R201 or R203 has gone high resistance or open circuit,
    3) bad solder joint at either end of those resistors,
    4) R202 or C201 or C205 is short circuit.
     
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  17. JimB

    JimB Super Moderator Most Helpful Member

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    I agree with alec_t.

    Just to confirm, measure the voltage between emitter and base, it should be about 0.6V.
    My guess is that you will see about 3v, in which case T200 is dead.

    JimB
     
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  18. Rorut

    Rorut Member

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    Thanks a lot. Will do that. Can it be broken because of other components that failed?
     
  19. JimB

    JimB Super Moderator Most Helpful Member

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    Unlikely in that circuit.

    JimB
     
  20. Rorut

    Rorut Member

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    I have 3.3V DC over Emitter and base.
    R206 is only 50K measured. Should be 68K.
    Is that a problem?
     
  21. JimB

    JimB Super Moderator Most Helpful Member

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    It is time to replace T200 then.

    JimB
     
  22. Rorut

    Rorut Member

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    Could I place it from the bottom side insted of taking everything apart. That would make it alot more easy. The old one must be desoldered first?
     

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