919

Discussion in 'Exclusively Sansui' started by Willy6, Aug 17, 2017.

  1. Willy6

    Willy6 AK Subscriber Subscriber

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    The 500 ohm Trimmers worked on F-2835 (I was able to adjust within specs but only after a voltage drop everytime I check it)but after I put those in F-2833s voltages are all over the place and it went into protection..
     
  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    The F-2835 offset voltage will wander all over the place +/- 100mV or more - it is normal, it doesn't affect sound quality and unless wildly out is not enough to trigger protection.

    Regarding F-2833 - check you have the +/- 10V adjusted correctly.
     
    Last edited: Oct 6, 2017
  3. Willy6

    Willy6 AK Subscriber Subscriber

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    there is either some bad resistors or bad jfets on f-2833 voltages are all over the place..
     
  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Lets hope it isn't any of the JFET's :( - check the +/- 10V regulators - and then do a wiring check (did you completely disconnect the board?), and double check other work done on the board if any. ;)
     
  5. Willy6

    Willy6 AK Subscriber Subscriber

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    Protection problem solved..I found a bad resistor and a couple of bad solder joints on the protection board F-2840 and on the rectifier board F-2928.. 20171009_174108.jpg 20171009_174229.jpg
     
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  6. Willy6

    Willy6 AK Subscriber Subscriber

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    I still have a slight problem with my left channel though..On F-2833 I'm getting a perfect -/+ 10v on the right and a bad -/+ 35mv on the left side..
     
  7. Willy6

    Willy6 AK Subscriber Subscriber

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    I'm wondering which replacements for the differential pair 2SB-527,2SD-357 would be better either KSA-1220AYS,KSC2690AYS or MJE15032G,MJE133G?Or is there something else better?Any suggestions would be appreciated..
     
  8. Willy6

    Willy6 AK Subscriber Subscriber

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    Or would the KSC-2073 be better then the MJE-15032G for the 2SD-357?
     
  9. moefuzz

    moefuzz Active Member

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    Please excuse my poor english, I'm not from your country.



    The 919 is a diamond differential design.

    The transistors responsible for the differential must be faster than fast.


    With their ultra high freq capabilities,
    the (differential) transistors are responsible for pre-driving the power transistors as sets of 2, 4 or 8 NPN PNP power transistor per each side.
    In order for the differential transistors to power themselves, they themselves draw signal from the output of the (pairs/sets of) transistors and feed it back into the front end/input side of those same sets of power transistors.

    I suspect this is similar to the 1920's regeneration circuits (for signal amplification and sensitivity) in a good quality regenerative tube radio.
    -They used to feed some signal from the amplifier circuit/output,
    back into the input, back into itself while still controlling any parasitic oscillations and/or possible feedback loops
    -all with tubes.

    Sansui did it with transistors.

    To say that the 'diamond differential' transistor must be fast is perhaps an understatement?

    Honestly, I just wonder how close to exact the corresponding specs must be between the (differential) transistors
    that Sansui designed/required, and those that we wish to replace them with?
    and what happens if a transistor based feedback loop lacks synchronization and/or timing across the frequency spectrum (sound output)?
    How does that effect the designed flat freq response ~from 10 hertz to 100,000 hertz~ (as Sansui was known for)
    perhaps we think too much and the mind self oscillates?

    ?

    this is too complicated of a theory for me to answer.
    I guess trial and error with a touch of others peoples opinions is the order of the day.

    :confused:

    .
     
    Last edited: Oct 27, 2017
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  10. Overundr1

    Overundr1 AK Subscriber Subscriber

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    these would be fine --- KSA-1220AYS,KSC2690AYS
    -Lee
     
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  11. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Differential pair? These are pass transistors on the supply board, aren't they? The choice is not critical here. I would put the MJE15032/33 here.
     
  12. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    They are OK to use but significant overkill, the suggestion/confirmation by Lee will get the job done, if those are the transistors being referred to.

    When transistors behave like fuses, I'd rather they pass a couple of amps before they go pop! rather than the 8 amps the MJ's will pass. ;)
     
    Last edited: Oct 27, 2017
  13. Willy6

    Willy6 AK Subscriber Subscriber

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    These are on board F-2835..
     
  14. Willy6

    Willy6 AK Subscriber Subscriber

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    If these aren't the differentials what are they? 20171027_151508.jpg
     
  15. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Those are what are called bjt pass transistors, used to regulate current flow through a circuit, the voltages after the transistors are set using resistors (or zener diodes in some cases). I would first be checking voltages at both ends of the connections between the 2833 and 2835 boards, if a ribbon cable was used they can be problematical. You also could probe voltages around those transistors and see if one is truly at fault. Finally, if you replaced any resistors due to glue damage are you sure the correct values were used. Tough one to diagnose without hands on.
    -Lee
     
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  16. Willy6

    Willy6 AK Subscriber Subscriber

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    Ok so my problem is that I'm getting a perfect +,- 10V on the right side of f-2833 and a bad +,- 35mv on the left which led me to these regulators.Im not sure if this is the problem or not but I found one of those regulators with the collector touching the heat sink?so I put some heat shrink tube on it. 20171026_144307.jpg 20171026_144259.jpg 20171026_150107.jpg
     
  17. Overundr1

    Overundr1 AK Subscriber Subscriber

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    If you look at the schematic and locate where the heat sink attaches to the circuit board you will note that the locating solder points are not actually part of the ground plane traces, not to say that having that transistor collector leg touching the heat sink is a game changer but if you actually trust the schematic drawing should not have caused the loss of voltage.
     
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  18. Willy6

    Willy6 AK Subscriber Subscriber

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    I double checked the board itself and the heat sinks are not grounded..I started checking voltages on F-2844 and the schematic shows -40v on pin 06 and I'm getting -38v and +40v on pin 04 and I'm getting +20 on that one. the resistors test good but some are different from what the parts list says for that board..
     
  19. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Well, you know what to do about that ;) - if you feel the need to replace some, copy the values on your board now - ignore the 'parts list'. :)
     
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  20. Willy6

    Willy6 AK Subscriber Subscriber

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    They are not all the same on the schematic as they are on the board either. I guess I'll write everything down the way it is and then change it the way the schematic says it is and see what happens..
     

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