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AU-717 Hard to restore...

Discussion in 'Exclusively Sansui' started by ELEKTROSTORE, Mar 21, 2016.

  1. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    Hello AK Folks,

    Hope all is well. I'm in the process of troubleshooting a AU 717...as the subject says...hard to restore.

    I'm in the tests stage, but wanted to pick some minds of people who may have experienced the 717.
    The problem at the moment is on the left channel: can't adjust BIAS and DC offset.
    R channel is alright, can adjust bias to 20 mV and DC offset in 0 vDC, but the measurements on the L channel is bad. Can't adjust anything through VR1, VR2 and VR3

    What I have checked so far on the F-2721:

    • Power suppy : +54.2 V | -54.3 V
    • Zenner reference : +22.4V | -23.1 V
    • Full recap. Glue removed.
    • R23/R25 150 ohms Fuse Resistor replaced for metal film resistors
    • ZD01, ZD02 replaced
    • D01,02,04 replaced by 1N4148
    • New complementary pair of MJE15030/31 and MJ21195/96
    • TR06 replaced by KSC945C (center collector)
    • Some Resistors damaged by the glue, replaced
    Protection circuit it seems to work well (oscilator, relay, etc)
    F-2663 was full checked. Fully recap, Glue issue, zenners, resistors, etc.

    can anyone help me if they know of any problematic transistors, test point to check or components to verify ?


    [​IMG]
    [​IMG] [​IMG]

    I will be sure to document the build on AK, and any help will be greatly appreciated.

    Thank you all, and thank you in advance for any info shared.

    Thanks all,

    GABRIEL from Argentina
     

     

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  2. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    Help Please! :bye:

    I have replaced all transistors according to the equivalences below

    TR12 2sd382 MJE15030G
    TR13 2sb537 MJE15031G
    TR 07-08 2sc1904 KSA1220AYS
    TR 06-09 2sc945 512-KSC945CGBU
    TR 05 TR11 2sa899 KSA1220AYS
    TR 03-04 2sa750 KSA992FBU
    TR 01-02 2sc1400 512-KSC1845FTA

    I also replaced the power TR's

    Measuring according to the test points of the schematic, I find these differences:

    [​IMG]

    Can it be the dual FET?

    Thanks in advance!
     

    Attached Files:

  3. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,040
    Location:
    Hertfordshire, UK
    Gabriel, As I understand it your AU-717 was faulty before you started performing a restoration on it, you should NEVER do this, always always correct the faults first, and only change components to correct the fault.

    What you have done is gone ahead and changed many many components, any one of which could have been installed incorrectly, or of the wrong value. People are understandably a bit reluctant to help you (over 200 reads), because the problem you are seeking to resolve has now become lost in the vast number of component changes you have made.

    The original fault was 'unable to set DC offset and Bias', it would be quite unusual for it to be caused by the Dual FET - but still possible.

    Get the DC offset situation resolved first, search for threads on AK where help was provided for such faults.

    Then move on the the 'cannot set bias' problem.

    Here is a thread where I helped someone with a bias problem on an AU-717
    http://audiokarma.org/forums/index.php?threads/au-717-no-bias-on-right-channel.755478/#post-10276722

    It contains a description of how the bias adjustment circuit works, and what you should look for.

    Good Luck.
     
    Last edited: Feb 11, 2017
    slimecity, ELEKTROSTORE and Overundr1 like this.
  4. _mano

    _mano Active Member

    Messages:
    453
    Location:
    London, UK
    Hi Gabriel,
    Assuming you have replaced the same list of components on both channels. Suggest:
    1) Using your Good Channel, visual check of all the components to make sure they are the correct value and of the same pin orientaion.
    2) Using bright light and magnifying glass check that there are no solder splashes, topside and underside.
    3) Check all the component pins are soldered correctly for good joint. (This has had me a few times in the past)
    4) If all checks out OK, put both boards on a table and compare resistance values on the trackside, slow , tediuous, but will allow you to narrow the issue to a physical area.

    If you had the bias issue, before you started changing components, check the diodes, transistors you removed if, they have been damaged, this will also, give you an idea if the fault is still on the board.

    Goodluck.
     
    AUD101, ELEKTROSTORE and Hyperion like this.
  5. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    Many thanks John.
    I agree with you and that is exactly how I started this work.
    (I clarify that I'm fond of some knowledge in electronics, but I never worked with these things). And I know...I need to improve my English!

    I bought this amplifier at Internet, and soon, the left channel died.
    I started trying to investigate what could have happened. Already in the optical inspection I noticed that there were components that were not original in both driver boards (F2721/22)

    - Output TR
    - Darlington Exitters
    - Some TO-92 TRs
    - And the Dual FET DIL-6 in the F2721 (Replaced by 2 unrecognizable TO-92 stuck together)

    [​IMG]

    I started with the measurements according to the schematic, and while trying to measure on the TR11, I generated a short circuit that ended up flying the fuses.
    Checking each transistor, I discovered some components that did not work:

    - TR702 (in Short circuit, blow up the 2 fuses on the PSU)
    - TR11 - Damaged
    - TR12 - Damaged

    - TR13 OK (strange...)

    In addition to the glue issues in both boards, I decided to start to restore symmetrically the 2 boards with the components recommended here by you.
    F2722 worked properly, DC offset and BIAS settings, OK, sound was excellent, but the left channel remained dead.

    F2663 it was fully reviewed, with a full Recap, new Omron relay, glue removed and components affected by the rubber, replaced.

    I was able to get it in Dalbani (an electronics component supplier in Miami) 4 Sony 2Sk97. I used 2 of the new ones in the restoration process.

    I will read carefully the information of the link that you indicated to me, and surely I have to begin a comparison with the 2 boards together, point by point.

    Thanks a lot !
     
    Hyperion likes this.
  6. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    So I found the F2721 initially

    See a lot of non-original parts, resistor, TRs, Dual FET, etc.
    In TR06 there was also another replacement that I do not remember the number

    [​IMG]
     

     

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  7. Overundr1

    Overundr1 AK Subscriber Subscriber

    Messages:
    2,515
    Location:
    Mississippi
    What a mess to start with, I see why you had to wholesale rebuild to get close to original factory condition.
    Suggest starting with measuring b-c-e voltages referenced to ground on tr-06 on both the working and non working boards.
     
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  8. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,848
    Location:
    Las Vegas Nevada
    I would like to add that this no bias/offset adjust issue, And the common fix is related to all the checks "Hyperion" and " Mano" suggest.
    I've found the quickest method to fixing the bias issue is to go through the board completely checking every component (Resistors, diodes, transistors and caps). Piece-meal, checking the voltages can be helpful but they can throw you off the critical path.
    Finding the single culprit is difficult as the circuit works in a cymbionic relationship.
    First make sure you have the correct voltages to the board and all the grounds are connected and accounted for.
    Check the signal shielding for a good connection to the driver board and the associated preamp board.
    Don't forget to look closely to the traces between the components.
    Those boards are easy to access, easy to check all the required components in a short time.
    When this issue crops up after a good working board is recaped the problem is almost always a poor connection between the traces. Poor/missing or cold solder connections.
    If the set was inop from a natural condition, it usually ends up as a out of tolerance resistor causing the no adjust condition. (May of had a transistor blow out to start the event)
     
    Last edited: Feb 11, 2017
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  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,040
    Location:
    Hertfordshire, UK
    I have to say, now that I have read your account of how messed up this was to begin with, well done to you Gabriel for the work you have done so far.

    Great advice given so far.
     
    Last edited: Feb 12, 2017
    ELEKTROSTORE likes this.
  10. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    Thanks Overundr1
    I think that's the point. I have 0V referred to ground on each pin of TR06 (B, C, E).

    Following all the advice they left me, which I appreciate very much, I'm going to start checking again, traces, poor/missing or cold solder connections, and of course a visual check for the components, correct value and pin orientaion.
    If I still find nothing, I will start with the comparison of measurements and check every resistor value again.
     
  11. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,848
    Location:
    Las Vegas Nevada
    This might the slow down the repair.
     

     

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  12. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    What a shame of me!
    This demonstrates the AK members' knowledge experience in all their advices.

    I started by doing a quick visual check, component-by-component check, and then I detected the error.

    [​IMG]

    [​IMG]

    There is no R15 ! (10K 1/4 5%)
    This resistor is affected by the GLUE, only in the F2721 ! Things that happen ... Sometimes its hard to find. I swear, I checked the boards several times.
    But...Just like your advices ! It's always necessary to check one more time...

    I can't find where my resistors are stored. Tomorrow I will go to buy and do the tests again.
    It is very sure that the problem comes from there.

    Thank you all
     
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  13. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,848
    Location:
    Las Vegas Nevada
    Nice work!
     
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  14. ELEKTROSTORE

    ELEKTROSTORE New Member

    Messages:
    22
    Location:
    Buenos Aires - Argentina
    Unfortunately they are erased.
    Both are glued together, with some kind of Loctite.
    At least in the test they measure well. However they were replaced by the original part Sony 2SK97.

    [​IMG] [​IMG]
     
  15. wozniak

    wozniak New Member

    Messages:
    1
    Hi, Did you solve the amplifier problem? Comment if possible.
     

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