AU 717 problems

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

  1. drbacklash

    drbacklash Active Member

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    312
    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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    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

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    312
    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

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    312
    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
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  7. drbacklash

    drbacklash Active Member

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    312
    Location:
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    Thanks John, useful information.
     
  8. drbacklash

    drbacklash Active Member

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    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.
     
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  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
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  10. Overundr1

    Overundr1 AK Subscriber Subscriber

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    DR sent me the 717 for a closer look at the problem. First I must say that the quality of both his parts selection and soldering skills are great, next time I get one in with glue removal issues I know who to invite over to remove it lol :)
    Anyways, a simple error in wiring when he replaced all of the power supply wiring after removing the board for rework was the culprit here. Accidentally flipped the pale green/purple wires with the dark green/purple wires sending 6v to the speaker switch, oops. Once that was corrected offset and bias dialed in just fine. Listening to some fine Bach while I let things settle in.
    Without pulling the power supply and tracing wiring there was little chance the board could have figured this one out.
    -Lee
     
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  11. drbacklash

    drbacklash Active Member

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    Location:
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    Many thanks to Lee for being so smart and patient and willing to fix this unit for me. And thanks to Hyperion for his interest in saving this nice piece of gear. You can never be too careful in documenting your own repair work!
     
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  12. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I am absolutely delighted with this result - Well done Lee :thumbsup:

    Experienced 'eyes on' can be everything in this game.

    Congratulations drbacklash. ;)
     
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  13. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Hooked her up to my L100's for a real run. DR indicated that he wanted me to give it a listen as he was not sure the tonal qualities were where they should be. OK, fire up my vinyl copy of Bach Fugue in D Minor, wheeee. Bottom end solid, mids are right there, high notes clear and strident, just as a 717 should be. Call it a wrap.
     
  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I looked back through the thread to see if I made any mistakes and to see if I could have resolved this, and found that I could have picked up on something but was not employing 'lateral thinking' at the time. In the first post it was said "I pulled both driver boards and the power board" - when I do one of these and work on the power/protection board I only disconnect 2 or 3 wires, loosen the main power capacitors, and slide the board down and out, and work on it that way. It never occurred to me that the power/protection board had been completely disconnected and removed from the chassis to work on, if I had picked up on this I might then have picked up on the mysterious 6V on the speaker outputs caused, as it turned out, by a re-wiring error. I mistakenly thought that the unit had worked up until recently, and only the driver boards had been worked on in this new phase of component replacement work, using Leestereo's thread as a reference.

    Although I organised it in the background I am feeling a trifle guilty about drbacklash having to send it to Lee, (Overundr1) (bless him) to find the problem.
     
    Last edited: Mar 30, 2017
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  15. Overundr1

    Overundr1 AK Subscriber Subscriber

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    The older we get the more those pesky wire colors all look the same. Anyways, the tip off on this one was that the unit had 6vdc at terminals only when the speaker selector was engaged. By pulling the amp cards you remove offset from the protection circuit. If 6vdc (or some value other than @0.00 still appears then one knows the issue is in the circuit itself. Will admit to doing lots of transistor checking on that silly board until the light bulb came on and started tracing wires.
     
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  16. drbacklash

    drbacklash Active Member

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    312
    Location:
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    I am very careful when I replace components. I draw diagrams, make notes and even tie various colors of knitting yarn on wires to make sure I don't do exactly what I did this time. I did it in stages, over time so I may have been tired or it may have been late at night. Anyway it happened and I would never have been able to spot the error myself. A new set of eyes (and more experience) made the difference. I had reached a point where I was going to give up but I was reluctant to reduce this vintage amp to storage or a parts bin. Hyperion saved it by putting me in touch with Lee. I am happy that this particular Sansui AU-717 survived and will bring joy to a future generation of audio enthusiasts. At age 70, maybe it is time to fish more and leave this hobby alone.
     
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  17. HH-Bham

    HH-Bham AK Subscriber Subscriber

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    I was in M.O.N. Mississippi Tuesday and saw the amp that is subject of this thread on the bench, and Overundr1 mentioned that there was a thread about it. All I can say is that several times Overundr1 was so complimentary of the restoration of this unit that I became convinced that I was looking at the Holy Grail of amps. In my brief visit, that amp was the star in the room and it's not like the rest of the stuff there was BPC.
     

    Attached Files:

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  18. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Always a good time when an AK member stops by :)
    HH was granted his request to audition some operatic music on the 717 which increased the drool factor over the 717 substantially. Also was able to see TWO rebuilt sx-1980's at the same time, a rare thing these days. Drbacklash's unit will be on it's way back to his abode next week once I get back from district manager road trips. I was sorely tempted to talk him into parting with it, of the three 717's I have owned over the years his outdid the others sonically. Goes to prove what others have stated, careful selection of 1% resistors, Wima film caps and Nichicon gold tune main caps along with selected power supply cap value increases really bring this amp to life. Reading between the lines of DR's rebuild it looks like he followed LeeStereo's most excellent restoration thread which should serve as THE guide to the 517/717 series.
    Egads, looks like I need to drag out the vacuum cleaner and chase the carpet around a bit.
    -Lee
     
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