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907 imos .....Blown output mosfets ...need replacements advice

Discussion in 'Exclusively Sansui' started by quadklipsh, Sep 5, 2018.

  1. kevzep

    kevzep Its all about the Music Subscriber

    Hey, you're the guy suggesting it, we are all waiting for your answer.....:D
     

     

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  2. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    Still im hunting on my own .
    I narrowed my search down to
    Either the original
    115/405s.
    Or the
    1529/200 s.
    My local supplier tells me that he can find me up a limited number of original mosfets from china where his father works .
    And he has guaranteed me for original product.
    The prices are quite reasonable .
    But im confused by the discussions that the originals are near end of life . And people use
    200/1529 :
    162/ 1058
    exicon laterals
    Or IrFp 240/9240
    coz they are fresher and have a long life to go.
    Also im perplexed for a full 16 mosfet replacement or just 8 .
    Plz comment
     
  3. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    0_IMG-20180912-WA0006.jpg fake or real ? Are they advisable as replacements
     
  4. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
  5. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
  6. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    These two posts contain golden information, I recommend trying hard to get devices that are the same as the originals. They will need to be matched in sets, if you do it like this you will only need to replace (all) those in the affected channel. However, if you get substitute devices they will still need to be matched, and I think you should replace them all (16) in this case. Most of all you need to find out why this happened, component failure? bias set incorrectly? you'll need to investigate this thoroughly.

    To avoid having to buy large quantities of replacement devices yourself, (enough to get the required matched sets), you may even need to ship the amplifier to someone who has access to quantities of these devices and can match sets for his own use, and do the repair for you. I have no idea who would take on such work. Unfortunately this is the downside of owning vintage equipment containing (almost) unobtanium devices, you need to be prepared for, understand and accept the consequences when things like this happen. Not a nice position to be in, I think many here have empathy for the situation you find yourself in.
     
    Last edited: Sep 13, 2018
    _mano and restorer-john like this.

     

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  7. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    Thanks hyperion,
    For once i had forgotten the sweetnes of the 907s tone as you poured out your concerns for the machine so sweetly .... ;)
    I know the amp wont be the same ,sound quality wise , without emptying my pockets and as of now my worry is to have it working flawlessly after the repair.
    Id like to add to the history of the amp that its bias was double checked by my tech which i regard highly for competence . He had set the bias around 27-28 i dont remember correctly but he didnot have the manual . He also did his homework of searching for the right bias range before doing it .We found it in the 27 range from audiokarma.
    The amp was working great for about a month and was seldom hot . it would only feel warm at the top metal plates .
    But the bias pots ???? i seriously now think that they could be the problem . its like waking up a sleeping dragon . and now wev had it !!!!

    today im expecting input from the sellers of the J200/k1529 mosfets .and if its feasible and original ill have em here in a week.
    im constantly checking for inputs from AKers on this issue . so feel free to keep me busy reading and searching
     
  8. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    26,967
    Location:
    Eastern Bamastan
    I've been using a mosfet amp, 8 per channel, that shows me different drops on the source resistors, I assume they are somewhat mismatched, but the amp sounds fine and doesn't show service since new CA 1995. Was this intentional, should I worrry?
     
  9. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    Found 1058s .
    But my local tech disapproved their use here.
    He says that the 2sk1058s are not to be used instead of 115s .but the vice versa situation is doable .i.e when 1058 crashes you can use 115s .
    Certain circuitry chanfes have to be made too and the sonic character of the amp will change .
    That i definitely cant risk.
    Now is it really that ways ?
     
  10. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    Going all the way with my tech .Finally surrendered for searching myself . Im letting him do the MOSFET arrangements .
    Hes has 4 original 2sk 405s / 2sj 115s.
    and sourcing 4 other from a friend in Karachi.
    Matching the pairs ofcourse and thats where he is charging me heavily .
    I wana get this 907 thingy up and ready, totally original in its outputs . NO hankies.


    NEXT CHECK UP :
    Will bring the amplifier home as soon as it is thoroughly tested in the shop.
    Then hes gonna check the rest of the circuit for flaws , wires ,jumpers ,the speakers their crossovers , power supplies of 100v and any loosing .
    Thorough checkup.

    [​IMG]
     
    smurfer77 likes this.
  11. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan

     

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  12. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    The 1529 /200 mosfets from my local supplier will looks like these .
    Are they fake ???? Or do we have to test em
     

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  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    Well, you'll have to match them which is a test - I hope you have a sufficient number of each to get a good matching set. ;)
     
  14. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    Hyperion.
    So they look like fakes????

    Also
    What parameters have to be matched in MOSFETs.
     
  15. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    The more probable plan is to let my tech arrange and fit in the original value mosfets 115s and 405s.
    He will have to pair em perfectly. He promises it. He doesnot have a pile of mosfets at all . Cmon.... he Just got 8 of em total.
    As for todays find of the 1529/200s im asking if these look fake or real. If they are workable enough then ii order these like 20pcs of each . Then perhaps matching is somewhat of an exercise.
     
  16. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
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    Location:
    Hertfordshire, UK
    Sorry I have no idea if they are fake or not - I simply haven't seen enough pictures of Power Mosfets to make an informed judgement.

    Matching Mosfets - measure Gate-Source voltage vs current - there are some 'How To's' on the wonderful Internet. :)
     
    Last edited: Sep 19, 2018

     

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  17. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
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    Location:
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    Using a curve tracer at full device rated operating voltage should tell the truth.
     
  18. quadklipsh

    quadklipsh Super Member

    Messages:
    1,292
    Location:
    Lahore, Pakistan
    Guys i have good news.

    Sansui 907 imos limited is back to performance.

    Last night we got the amp running . The replacements were original pullouts from a guy who had the four remaining sansui mosfets 1200 km away.total 8 mosfests replaced . The Vgs are a bit different but the tech said ill handle that . You just concentrate on the sound quality and channel differences .
    The amp was up n running getting a bit warmer than expected .
    1) What should be the ideal temperature rise and the bias???

    I doubt the bias at 28,if it is correct or not .
    Coz this amp died spontaneously in my cousins custody where few days earlier which the bias was adjusted to 28 or 27.
    The amp would run warm only but then there was smoke from one side.
    I donot want to have that incident again
    Plz gimmi all those checks and balances for a smooth running.....

    2) Also may i ask if i need a recap or not ????
    This amp is now officially 30 yrs old. From the external/internal looks it is pretty new coz of the patent black glossy shine,that can blanket the age .
     
    Last edited: Oct 8, 2018
  19. smurfer77

    smurfer77 AK Subscriber Subscriber

    Messages:
    1,484
    Location:
    San Francisco
    Sorry, have not read the entire thread carefully and I'm not familiar with the specific model, but,

    (1) 28mV doesn't sound to crazy as related sansui MOSFET amps like the 907Limited and AU-X111MOS ran similar bias voltages (and I assume they used similar emitter resistors; I can't speak for your model, but the other two I mention both use 0.22Ohm).

    (2) Even if 28mV is not spec bias voltage for your amp, I doubt that the corresponding level of idle current, and heat, would (by itself) be the cause of smoke.

    (3) Forget the recap for now. Fix this problem at hand and then think about that later. I have limited experience with this era of Sansui but with the few I've had on the bench over the last month, it looks like original caps are good condition and are of quality (in terms of measurable parameters, not just build) that is hard to replace. Personally, I will start thinking about recapping my amps from this era in another 20 years. That said, nothing wrong with going ahead if you want to avoid a future catastrophe.
     

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