Sansui AU-719 buzzing

Discussion in 'Exclusively Sansui' started by Des Ibels, Mar 24, 2011.

  1. Des Ibels

    Des Ibels New Member

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    4
    I recently acquired a Sansui AU-719, cosmetically it is in lovely condition however once it warms up there is a constant humming in one of the channels. The repair shop is struggling to trace the fault any ideas / guidance would be really appreciated? I am based in the UK so it is a 240v version
     

     

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  2. nosirrah

    nosirrah Moderator Staff Member Moderator Subscriber

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    Welcome to AK.
    Sounds like a failing transistor to me, also those have the corrosive glue on the driver board etc. Is your repair shop familiar with vintage gear?
     
  3. inductor

    inductor Super Member

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    yes the 719 uses separate regulated supplies for each channel , doesnt it !
     
  4. Des Ibels

    Des Ibels New Member

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    Unfortunately I do not think he is, so I am trying to find some clues.
    Does anyone have a recommended tech in the UK for AU repairs?

    A recommendation would be very helpful

    thanks
     
  5. LeeS

    LeeS Member

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    Location:
    South Wales, UK
    Whereabouts are you Des?
     
  6. Des Ibels

    Des Ibels New Member

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    In the South of England, Hampshire .... can't seem to find anyone familiar with these amps yet. Been searching AudioKarma and the web in general for a recommended tech.
     

     

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  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    PM sent.
     
  8. nosirrah

    nosirrah Moderator Staff Member Moderator Subscriber

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    Perfect!
     
  9. Des Ibels

    Des Ibels New Member

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    Oh Wow!

    Well it has taken a while .... mainly because I took so long to deliver my amp to John (Hyperion). I was not willing to ship it, as it is too good to be damaged, so eventually when in John's vicinity I delivered the unit to him.

    What followed was the most professional courteous service I have received in a very very long time.

    John kept me appraised all through the process and we had several conversations relating to both fixing and upgrading the amp.

    I collected my AU-719 yesterday and its sounds absolutely wonderful and is emitting that "new" smell from within the chassis .... mmmmmm

    As well as fixing the initial problem, John also went through the entire unit refreshing the power boards, tired components and recapping for me.

    I am sure John will chip in here with a thorough description of the buzzing fault .... Nosirrah was right it was a transistor .... but it held out right until the last before discovery. Hopefully John won't be shy and will show you some pics of his handywork, it is very professionally executed.

    Anyway thanks to all the folks on this forum and a real big thanks to John ... if anyone in the UK is looking for help with a Sansui, you will be hard pressed to find a better place to go.

    Back to my listening chair :D:D:D:D:D
     
  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Simon (Des Ibels) and I had some discussion about the repair and re-cap, I persuaded him to adopt my current favourite choice of capacitors for this. Namely Panasonic FC everywhere except the PSU, and there use Panasonic FM, and for critical audio path locations - Nichicon FG. I also used Nichicons (not FG) for the 6 x 470uF 80v replacement capacitors as I could not get these in Panasonic FC.

    My approach to a re-cap is to visually check the amplifier over - looking for damaged components - traces of heat damage and so on. I then reset the bias and offset, a soak to test the stability of these adjustments. Then a listening test, using music I know very well - in order to give myself a sound quality baseline. This AU-719, I found was hard and even harsh, in sound quality - the lower frequencies were OK and the imaging was acceptable.

    Here are some of the pictures I took during this re-cap/repair, with some explanatory text, which I hope might help someone.

    The first 4 pictures are simply 'before' record shots - so I can see how everything was before the re-cap.

    Number 5 is a picture of one of the partially done driver boards - I have removed some of the electrolytics and have been cleaning the glue. In this case the glue was still very rubbery - and was extremely difficult to remove - as evidenced by some damage to the topside silk screening caused during this process.

    Number 6 is a picture of the driver boards side by side - the RHS one is finished and the LHS one is completely untouched.

    More posts to follow (I have reached my attachment limit for this post)
     

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    Last edited: Sep 24, 2011
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Number 7 is a picture of the re-cap of the PSU board - classic "4 dangly smoothing caps" fashion. Strip out of old components and glue removal in progress.

    Number 8 is after having replace the series regulator transistors - I have just begun putting in film caps in place of low value electrolytics (1uf, 50v - little red ones).

    Number 9 is a shot of both driver boards complete - notice more red film caps and the new zener diodes mounted using fish-spine beads to stand them away from the PCB for better ventilation.

    Number 10 is PSU protector board complete - but there is an error - which I didn't spot until it was almost too late - can anyone see it.... anyone who has done one of these will know where it is... (the answer later on...)

    Number 11 is a simple record shot for my use - so I get the leds for the input switching back in the right place.

    Number 12 is a picture of the phono amplifier - just released and ready for me to start removing old parts.

    More posts to come...
     

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    Last edited: Sep 24, 2011

     

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  12. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Number 13 is a picture of the completely re-capped phono amplifier. Note that I did not use 'open frame' type trimmers and opted for Bourns multi-turn types. This is in spite of the fact that it makes adjustment of the phono offset a bit more difficult. Notice that I have pitched up the front of the lower trimmer so that it can be reached with a long trimming tool (instead of through the hole in the board from underneath as with the old open frame trimmer)

    Well that's about it - I did take a few other pictures but not of much interest really.

    Lastly
    I added 4 x 2.2uF film caps located across the 4 main smoothing capacitors - in the same style as with AU-717/517.
     

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  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Now the error in picture Number 10 - Have a look at C615 in the upper left corner - see anything wrong ?

    It is installed according to the silk screening on the PCB...... But this is wrong !!!

    I took it out and put it back in the correct way round (but wrong way round according to the PCB marking).

    Lastly The 'Buzzing' Fault
    I had tried without success to even actually hear it - and was beginning to think I wouldn't ever hear it..... how wrong could I be. During one of the post re-cap listening tests, I was standing next to the amplifier - when I heard a rustling sound in the left speaker, I hit the -20dB mute switch and it didn't get any quieter - it got louder - then the protection activated !!! I powered off the amplifier - FAST !

    So, at last I had seen and heard the fault. Armed with the knowledge that I had already replaced many of the components that might cause this, I set about finding what was wrong.

    Back to the bench - right channel driver board removed - tone and phono power disconnected. I started measuring voltages - 10v of DC offset !!! - tapping various parts of the amplifier, I could not get the fault to correct itself even temporarily. I hooked the scope up and monitored the jittery voltage on the output. I substituted the MV103 diodes in the input section of the driver board - same. I was turning over in my mind whether I would swap the 2 dual FET's and risk damaging them OR.... should I shotgun the driver board small signal transistors...... I have never liked shot-gunning a problem - so I decided to gently prod some of the driver board components - when I got to TR04 - there was a familiar click and the protection light was steady !!! - further checks eliminated everything except TR04 - which I then changed - and the problem was gone. I was especially pleased to find and resolve this fault as another tech had tried and could not fix it.

    It appears that over the years one leg of the transistor had become 'socketed' and the tiniest bit of movement would disconnect the leg concerned. I was especially pleased to find this as when DC coupled circuits fail - everything seems wrong - so its hard to find out where the problem is.

    After re-assembly, a reset of bias and offset - extensive soak and listening tests there has been no return of the problem.

    You could not meet a nicer person than Simon (Des Ibels) - I was very pleased to have worked on his amplifier and resolved the problem to his satisfaction.

    That's all folks. :music: :smoke:
     
    Last edited: Sep 24, 2011
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  14. nosirrah

    nosirrah Moderator Staff Member Moderator Subscriber

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    We like happy endings.....:yes:
     
  15. addr

    addr New Member

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    Thanks for the info.
    Few months ago I've recapped my AU-719 and trusting the screening made the same mistake. I don't use the preamp so I haven't noticed anything wrong yet.
    The cap looks OK but the question is could it do some harm to the preamp.
     
  16. petehall347

    petehall347 the brandy coffee man Subscriber

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    what number was the transistor ? i had one last week with same problem not same amp but just curios
     

     

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  17. rasenthiran

    rasenthiran Well-Known Member

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    hi John,

    if i have a choice betwen au-717 and au-719?which does you recommend in term of sound quality.Does au-719 uses identical parts with au-717?

    thanks
     
  18. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    The cap in question is on the PSU/Protector PCB, on the output of the -ve regulated supply - you should make sure yours is in the right way round.

    TR04 = 2SC1845. - and note that it was not a bad solder joint causing the problem, the problem was inside the transistor case.

    They are both very good, and very close in sound quality - but to my ears the AU-719 is better - or at least better than MY AU-717. The build quality of the AU-717 is quite a lot better than the AU-719 - which definitely does not suffer sonically for this.

    As for identical parts - many of the individual component parts are the same - but all the PCB's are different, and the output transistors are different. Driver Board, Head Amp and Regulated PSU circuitry is significantly more complex, the Control Amp less so - when compared to AU-717.
     
    Last edited: Sep 25, 2011
  19. petehall347

    petehall347 the brandy coffee man Subscriber

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    TR04 = 2SC1845. - and note that it was not a bad solder joint causing the problem, the problem was inside the transistor case.



    exact same problem but was a 2sa841
     
  20. pete_mac

    pete_mac Super Member

    Messages:
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    Location:
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    Great thread! Very interesting regarding the error on the PCB screening!

    I've got a 719 undergoing repair, and once this is sorted out, it will be recapped. Looking forward to comparing it to my recapped AU-517 in the near future! Agreed 100% on the build quality comments - the 517/717 are noticeably better in this regard.
     

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