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No output left channel phono stage of Sony preamp mod. TA-E90ES

Discussion in 'Troubleshooting and Repair' started by Salva, Jul 13, 2018.

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  1. Salva

    Salva

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    Jul 13, 2018
    Hello everyone here. I need help to understand why in the attached diagram there's a voltage of -21V between source pins of Q114 and Q115 and all three pins of BJT Q113, the same as the drivers collectors of Q111 and Q112.
    I'm sure that the failure must be among those suspects but to my surprise, after desoldering all above mentioned transistors, Q111, Q112 and Q113 are correct! not showing problems when measured in diode mode with my tester Finest 503, showing the 0.7V that must be in the junctions.
    About the Q114 and Q115 final Mos-Fet's, I'm not so sure about their state, I think they are open but I could be wrong because, maybe my humble tester may not supply enough voltage between gate an source to turn them on.
    One thing I'm quite sure, all resistors involved are OK and both Q114 and Q115 kept cold after half an hour of turn the circuit on, while the other channel parts were warm to the touch.
    I'm totally sure that with all 5ha information, someone with the needed skills will have all the necessary to explain what's going on and probably why. The replacements are on their way, I ordered all and therefore I'm going to gather all active devices form input to output to be ready to face any complication I could find... oh no! I can't attach the Diagram!!!
     
    Last edited: Jul 13, 2018
  2. Salva

    Salva

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    Jul 13, 2018
    20180713_090156.jpg
     
  3. Salva

    Salva

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    Jul 13, 2018
    I could fix the Diagram problem, too much data to be send so a photo instead...
     
  4. kellys_eye

    kellys_eye

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    1,162
    Jun 25, 2010
    If measured with respect to 0V it indicates Q115 short circuit.
     
  5. Salva

    Salva

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    Jul 13, 2018
    Hi! Thank you very much for your quick response, this is what I thought from the enquires I made so it means rhat if I change the two Q114 and Q115 I would fix the failure?
    Yes, the measures were referenced to ground. 0 Volts
     
  6. kellys_eye

    kellys_eye

    4,307
    1,162
    Jun 25, 2010
    Changing them as a pair is your decision but you must set the bias correctly after changing either of them. Do you have the instructions for doing this?
     
  7. Salva

    Salva

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    Jul 13, 2018
     
  8. Salva

    Salva

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    Jul 13, 2018
    Yes, I have the service manual. If I've entered this forum is because I don't want to undertake this repairment on my own! I won't proceed until I receive advice to do so.
     
  9. WHONOES

    WHONOES

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    May 20, 2017
    Your output condition could also be caused by Q111 being open circuit which would allow Q112 to pull the gate of Q115 down to the - rail turning it hard on. Must admit though that Kellys_Eye's suggestion is more likely, but it would be worth checking Q111 just in case and may stop you going on a wild goose chase.
     
  10. Salva

    Salva

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    Jul 13, 2018
     
  11. Salva

    Salva

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    Jul 13, 2018
    Hi my friend! I already checked Q111, Q112 and Q113, the latter being the bias. After desoldering all three I found, to my surprise that they weren't damaged. I will double check them just in case one more time but...
    If the time arrives and I'm still unclear about the diagnosis, I have a loyal and reliable technician as a last (), but I'm scared to the bone for if in the process, the preamp suffers a dent or scratches. I'm OCD
     
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