Recapped Sansui AU-719 Protection mode help!

Discussion in 'Exclusively Sansui' started by RR--, Mar 3, 2017.

  1. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Yep, KSC1815 is similar to 2SC1313 voltage wise, still unsure why changing it would have the effect of burning R635. :dunno:
     

     

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  2. Hipocrates

    Hipocrates Anti-Muppet

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    Last edited: May 31, 2018
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  3. RR--

    RR-- Active Member

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    I swapped back the new KSC1815 with the 2SC1313 but then accidentially put the KSC1815 on the right channel where I should have put a KSC1845, I think that sent way too much current to that poor resistor.
    I've been reading/googling the pin layouts before soldering them in, most on the PSU board went in fine flipped backwards.

    Edit: also the KSC1815 replaced the 2SC1313 just fine, the protection roblem persisted but the amplifier functioned as usual with good quality sound, when I accidentally substituted a KSC1845 with a KSC1815 that's when the burning started.
     
    Last edited: May 31, 2018
  4. RR--

    RR-- Active Member

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    So something still isn't right.
    I installed the new KSC1845 and KSC992 transistors on the driver boards in place of all originals and replaced both the KSC1815 transistors on the power board for good measure, I checked that the orientations were all correct, Bias and DC offset seem to be ok.
    The amplifier consistently powers on now without any perceivable protection issues, however one of the newly installed 1W, 68Ω resistors glows red hot and begins to smoke.
    Are you sure these are the correct resistors? They seem a bit small to me but I'm not sure if that matters with new components.
    All I can seem to think is that they're not able to cope with the current.
    Thanks again for the advice.

    [​IMG]
     
  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I think so, I went back through my orders and checked, they are small but are rated as 1W which should be plenty. (and is double the rating of the originals).

    Even when everything is running right they do get very hot, I suppose it is possible you have something pulling a bit more current, but the only time I have seen that resistor burn up is when there has been a misplaced connector or other fault (like a reversed electrolytic). The two connectors on the EQ or Phono input board are really easy to mix up and cause this exact issue - but don't swap yours without checking first.

    While finding the cause you might find it helpful to use insulated wire to extend the connections to that resistor, so you can change it quickly, while you track down the root cause.
     
    Last edited: Jun 9, 2018
  6. RR--

    RR-- Active Member

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    That's interesting because I didn't unplug many cables when I installed the parts that fried the resistor, no capacitors were changed and when I originally unplugged the EQ or Phono cable I marked them with coloured tape so I couldn't mistake them. Also when I did desolder a lot of cables it worked fine when reassembled.
     
    Last edited: Jun 9, 2018

     

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  7. RR--

    RR-- Active Member

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    I did discover I forgot to replace a 2SC1313 transistor at TR612 with a new KSC1815, could it have blown and be causing the new problems?
     
  8. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    It is possible - especially if the resistor that's burning is still R635?
     
  9. RR--

    RR-- Active Member

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    Yep still just R635. I'll swap it out for a new one as well as that rogue 2SC1313 now and report back.
     
  10. RR--

    RR-- Active Member

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    Replaced the transistor and resistor and tested it but the new one began to smoke instantly after powering on. :bs:
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    OK, you need to take the load off the PSU and begin to find out why that resistor is taking such a beating.

    Try disconnecting the power to the Phono board, and then to the Tone Control board - see if you can get to a position where you can power up without burning that resistor.
    BTW: Did you notice the incorrect silk screening on the PSU/Protection PCB for C615 (220µF 35V) - the PSU output filtering capacitor for the -ve supply - if you installed the replacement capacitor as per the silk screening it is back to front! - this could be your problem.
     
    Last edited: Jun 9, 2018

     

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  12. RR--

    RR-- Active Member

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    Thanks for the suggestion, I just tried disconnecting the Tone control board and phono board and powering on, it wouldn't pass protection and I could see that poor resistor turning red very quickly.
    What I don't get is not much at all has changed to cause this issue, originally I accidentally put a KSC1815 where a KSC1845 should have gone at TR17 on the right channel driver board, that's how the original resistor probably burnt up.
     
  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    See additional 'BTW' note in my post above - is that relevant?
     
  14. RR--

    RR-- Active Member

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    Yep I noticed that misprint at the time, I swapped each capacitor with a new one the way they were, it was all original when I got it, plus I recapped this amp back in 2013 and it had been working daily without issue (mostly).
     
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Ok, well, if you have the Tone Board and the Phono Board disconnected and the resistor still burns, it appears you have a fault in the PSU, which could be C608 or still C615 or something else. I have recently had experience of a new electrolytic that turned out to be faulty, this is rare and it was an intermittent fault - but changing it cured the issue. (spookily this was also in an AU-719 - on the driver board)
     
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  16. RR--

    RR-- Active Member

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    Maybe one of the capacitors got cooked a few days ago, I'll replace them now with some spares I have and report back.
    In the meantime here's some pictures in it's current state just in case I've missed anything.
    [​IMG] [​IMG] [​IMG] [​IMG]
     

     

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  17. RR--

    RR-- Active Member

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    Replaced C608 and C615, C615 is correctly in place which is the opposite orientation to the silk screening.
    I think I may have found the issue though, I just noticed that R621 is completely black and burnt out.

    Any idea what rating it used to be? it's not in the schematics.


    [​IMG]

    Edit: here's a picture I took of it before, taking pictures sure comes in handy.
    [​IMG]
     
    Last edited: Jun 9, 2018
  18. Robisme

    Robisme Sansui Enthusiast Subscriber

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    R621 IS in the schematic, as well as in your pic. It is a 100 ohm.

    Rob
     
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  19. RR--

    RR-- Active Member

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    Just replaced the burnt out 100ohm resistor at R621 with a new one and R635 still smokes immediately after powering on.
     
  20. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    TR610 shorted C-B or inserted wrong way round ? - or TR611 shorted C-E or inserted wrong way round ?
     
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