AU 717 problems

Discussion in 'Exclusively Sansui' started by drbacklash, Mar 6, 2017.

  1. drbacklash

    drbacklash Active Member

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    309
    Location:
    Shallotte, NC
    Here are the results of the tests:
    F-2722 right channel driver
    All connections to F-2721 disabled.
    With Molex connected, ranges from about 250-280 mv but will not settle.
    With Molex not connected, increases from 0 to 400 mv and then recycles.

    F-2721 left channel driver
    With Molex connected, 0 to 400 mv and then recycles.
    With Molex not connected, 0 to 400 mv and then recycles.

    Throughout all tests, the unit relay clicked and the protection light stopped blinking and went to a steady on status.

    Thoughts?
     
  2. Overundr1

    Overundr1 AK Subscriber Subscriber

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    2,041
    Location:
    Mississippi
    Yes, with those wires disconnected the relay circuit is not responding to the offset issue and the click indicates the relay circuit is ok. The values climbing positive then resetting indicates a closer look at the resistors you replaced around the sk97 input FET. Something is upsetting the voltage (current) at the fet input pins. One good thing is that both channels are mis-behaving in much the same manner so careful checks on the surrounding resistors might show up on both boards rather than an fet failure. Also check the two multi-turn pots to ensure you used the correct values in the correct locations. Did you replace (as I recall) ZD01 and ZD02?
    -Lee
     
  3. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    The R09 's look wrong to me - they look like 39 ohm (orange, white, black, red) not 3.9K ohm? - clutching at straws :(

    Forget that - it seems to be right but, goes without saying check all replacement resistor values.
     
    Last edited: Mar 11, 2017
  4. drbacklash

    drbacklash Active Member

    Messages:
    309
    Location:
    Shallotte, NC
    Ok, my plan of action is to recheck all resistor values on the driver boards, checks replaced diodes, and then transistors. I sometimes read where the value of components were listed incorrectly in the service manuals. Are there any known errors in the 717 manual?
     
  5. drbacklash

    drbacklash Active Member

    Messages:
    309
    Location:
    Shallotte, NC
    I checked all resistors and diodes again and found some I will replace. I know that you sometimes have to pull a component from the board to get an accurate reading but I figure that if you have to pull it you might as well replace it. Any resistor out by more than 5% gets a permanent vacation. R16 was interesting. It is a 27K and the initial reading is in the 24K range but it keeps increasing (just like the output readings have) to 26K +. I stopped testing because it took so long to get to 27K. This particular resistor is between the FET and R45 which tested at 4.1 and 9.8 instead of .33. In fact, all of the cement resistors were off by more than 100%.
    ZD01 and ZD02 tested around 700 but when I reversed the leads, they increased from about 0 to 2v and then recycled, also like the output tests. I replaced these originally with BZX85C22 but then I read that the replacement should have been 1N4748A-T. Which one is correct or better than the other one.
     
  6. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    BZX85C22 & 1N4748A are effectively the same part. = 22v, 1W, 5% zener.

    R16 is absolutely crucial to amplifier operation and should be a very good quality resistor. (if I replace these I use a 1% metal film)

    Make sure you have a good contact on those cement resistors (the leads oxidise) - but they need replacing (obviously) if the values are as you say.

    Lastly
    Static readings with an ohmmeter, if the component isn't disconnected, like the zeners you measured, don't mean anything much.
     
    Last edited: Mar 13, 2017
    Overundr1 likes this.
  7. drbacklash

    drbacklash Active Member

    Messages:
    309
    Location:
    Shallotte, NC
    Thanks John, useful information.
     
  8. drbacklash

    drbacklash Active Member

    Messages:
    309
    Location:
    Shallotte, NC
    I have now completed replacing all questionable resistors on both driver boards including the Zener Diodes. This work did not correct the problem. The unit still comes out of protection but the DC offset cannot be adjusted. On both left and right channels the readings start at 0 mv and increases incrementally to about 400 mv and then starts all over again. When I switch to speakers A, I get a solid 6 V reading. This is beyond my level of skill to understand and correct so I am giving up on this project.

    I want to thank everyone who chimed in with suggestions and guidance on this.
     
    Hyperion likes this.
  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Sorry to hear this drbacklash, unfortunately it's just too difficult & time consuming to work out what is wrong 'online' when almost everything has been changed. :(

    Something learnt here - do one channel at a time. :thumbsup:

    And lastly, I would LOVE to have that amp on my bench to find out what is wrong. :)

    Good luck with it.
     
    Last edited: Mar 20, 2017 at 10:26 AM

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