Blowing fuses in 919.....Any ideas?

Discussion in 'Exclusively Sansui' started by ghamilton, Mar 31, 2017.

  1. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Hard to say what would cause something like that, by the way - I can't remember, has this AU-919 been reconditioned?
     
  2. ghamilton

    ghamilton Super Member

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    Oh yes! I did it myself with the help of AK. It's been doing great for a few yrs. All electrolytics and black flags. I believe I did a good job. I may have to go back to my SHURE M97/SAS instead of my grado if I can't figure out that noise. It's clear that it's some kind of shorting/Arcing noise.
     
  3. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Considering the amount of gain in that phono section I would be looking real close at the transistors. Even things like dressed lead length, proximity to other components and cleanliness of the mounting pads might (might) come into play.
    As you narrow it down you (may) consider replacing the mc stage resistors with 1% metal films. Also check the entire ground plane for any cold solder joints, I would probably reflow all the ground points. Gonna take time to get it perfect, with that low a background noise even good scope work may not show it up.
    -Lee
     
  4. ghamilton

    ghamilton Super Member

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    What kind of connections are these that go to the MC board? In other words,...Are they suspect of the noise? The blue splits into a black and white and so does the gray. 20170415_135749.jpg
     
  5. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Those are center conductor with outer braid cables, verify the braid not touching the center conductor, otherwise doubtful
     
  6. ghamilton

    ghamilton Super Member

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    Ok,..I got it back together and I used a dim bulb tester with a 100 watt bulb. When I turned it on the build got real dim but the amp never came out of protection. I have the 25 turn bourn's pots in there and I don,t want to adjust them until I consult with you guys. What do you think?
     
  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Check the DC offset before the relay contacts, see if you can adjust it - same with bias, we need to know if the amp is 'DC stable' first. You can check the DC offset adjustment for THIS test only, using one the same points that you use to check the bias. If successful remember to turn down the bias after this check, leave the DC offset as adjusted - then we can consider applying full mains.
     
  8. ghamilton

    ghamilton Super Member

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    OK,...I'm trying to figure out HOW to check DC offset when the relays are not kicked in. Please walk me through this. I don,t quite understand. Thanks
     
    Last edited: Apr 16, 2017
  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Pins 27 and 28 are the Power Amp outputs - on the edge of the F-2839 Driver PCB


    upload_2017-4-17_8-47-8.png

    Here is a bottom view from the SM

    upload_2017-4-17_8-53-30.png

    The pins mentioned are in the areas marked in red. - note this is a Bottom view with the Right channel at the front (or bottom of the view seen here).

    Obviously the reason you need to measure here (pin27 or pin28) is because in protection mode the speaker terminals are disconnected. You need your meter set to 'Volts' - +probe to the appropropriate channel pin on the board (Right or Left), and -probe to chassis ground. Hopefully the reading will only be 1 volt or so, in which case - try adjusting the DC offset trimmer for that channel, - report back with what you find. If the voltage is say 20 Volts or more, you still have a component issue somewhere on the driver board most likely.

    Honestly ghamilton, I am trying to direct you, but after the event this amplifier suffered it should really go to someone who doesn't need their hand held like this. The possibilities for someone inexperienced making a mistake are enormous, actually they are not insignificant even when you know what you are doing. :(
     
    Last edited: Apr 17, 2017
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  10. ghamilton

    ghamilton Super Member

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    Well,... I set the DC offset trimmers to center position and bias trimmers fully counterclockwise as sm says. It now comes out of protection! So I took out the bulb tester and tried it again. Beautiful!!! I then was able to set DC offset and bias. Wonderful!!!! Now to find time to hook it up and listen. I'm so happy. Thanks!!!!!
     
    Last edited: Apr 17, 2017
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Wow - well done! - quite an escape. Please do keep us updated with how it sounds now.... just the same?.... better?
     
  12. ghamilton

    ghamilton Super Member

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    Well,...It sounds awesome!! There is still that noise in the MC section. So, I'm now using my M97/SAS. I'm very happy. There is also no drift with bios or DC. There use to be. But not now. Thank you so much. This is what AK is all about.
     
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  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Good news.

    Perhaps in the future, and when it is convenient you can arrange someone to have a look at the MC input, to look for that noise.
     
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  14. rasenthiran

    rasenthiran Well-Known Member

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    Hamilton,

    Great to know your precious AU 919 is working again. Congrats on getting it done by your self.
     
  15. EchoWars

    EchoWars Hiding in Honduras Subscriber

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    Sorry I wasn't around to help...things have been a bit of a mess around here. Glad it's singing again. :rockon:
     
  16. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    It's great to see you back Glenn. :thumbsup:
     
  17. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Yes, glad to have you back sir :)
    -Lee
     
  18. ghamilton

    ghamilton Super Member

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    Thank you Glenn. I'm sorry about your Mother.:(
     
  19. EchoWars

    EchoWars Hiding in Honduras Subscriber

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    Thanks for your thoughts. Last couple of months have been a trial.

    I'll pop in and out when I can, but things are still, and probably will remain, a bit of a clusterfuck for a while.
     
  20. Helios

    Helios Active Member

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    Hamilton, glad to hear the good news!
     

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