Help understanding Onkyo amplifier circuit

Discussion in 'General Electronics Chat' started by cmayer, Oct 4, 2016.

  1. cmayer

    Thread Starter New Member

    Oct 4, 2016
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    Hi, I have a Onkyo amp A-8057 that I am troubleshooting and there is something I am not getting, see attached screen shot of the schematic. On the collector of Q513 I should get 1.43V and on the collector of Q515 I should get -1.35V according to the test points noted on the schematic. I am getting approx 50V and -50V since both transistor are conducting that makes sense to me. I have the same problem on both channels, orginally the power transistors were shorted, I replaced them and now the amp does not come out of protection mode. I don't not get why I should get 1.43.and -1.35V. It's been a while since I have worked with transistor I am probably a bit rusty. Note the schematic is for the A-8067, it is a very similar circuit to the A-8057 but this schematic has more test point on the schematic for debugging.

    Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Have you checked those diodes and resistor R527?
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    This suggests that there is a problem around Q517 which sets the output stage bias current and defines the 1.43v and -1.35V shown on the schematic. Q513 and Q515 are configured as current-sources.
     
    Last edited: Oct 5, 2016
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    What do you read at the base of q601 and q 603?
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Check Q517, D513,D515., what is the voltage at the output stage inductor L601?
     
  6. cmayer

    Thread Starter New Member

    Oct 4, 2016
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    Ye
    Yes, I actually had a bad diode, it is now fixed. I wi
    I did a bit more debugging after reading your comment blocco, it helped me understand how this should work.

    I removed Q517 and R533 to isolate and verify the functionality of Q513 and Q 515. Both are conducting and I am getting 50V at Q613 collector and -47.2 at Q115 collector. With Q517 removed I am getting 48.8V at it's base wich is the 50V from Q513 collector minus the voltage drop accross D515 and D513. To me it looks like Q517 is not doing, it's job it looks OK on the meter (no shorts, diode mode on meter looks ok)?

    Thanks
     
  7. cmayer

    Thread Starter New Member

    Oct 4, 2016
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    Yes, they are Ok. One diode was originally bad.

    Thanks
     
  8. cmayer

    Thread Starter New Member

    Oct 4, 2016
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    I was reading +50 on both I believe, this is what I am getting on channel B now, I took a few parts of channel A to further debug.
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Check Q601, Q603, Q605, and Q607 for shorts
     
  10. cmayer

    Thread Starter New Member

    Oct 4, 2016
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    Thank you, I found a few shorted transistors on both channels, I had the parts to get one going and the parts for the other are on order. Appreciate the input from everybody.
     
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