SA-7500 differential pair issues.

Discussion in 'Pioneer Audio' started by Eastham, Jun 10, 2018.

  1. petehall347

    petehall347 the brandy coffee man Subscriber

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    did you look for oscillations on pre out ?
     

     

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  2. Eastham

    Eastham More Class-A than ever!

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    I'm guessing these are a known failure point?

    No but I did pull the pre out / in jumpers, no change. I'll hook my 'scope upto the pre out for the left channel and report back if I find anything
     
  3. LesE

    LesE 110284 Subscriber

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    They are know to be ticking time bombs. If the BE junction is failing, it will affect Q1 and the operation of the differential amplifier.
     
  4. petehall347

    petehall347 the brandy coffee man Subscriber

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    would that not affect the bias current of the outputs ?
     
  5. Eastham

    Eastham More Class-A than ever!

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    I see.

    Good news guy's, I swapped the 2SC1451's and now the left channel is good with the 2SA970's I put in it. I guess it was the 1451 all along... Wonder why it worked for two days with the 992's and only for a few minutes with the 970's
     
  6. LesE

    LesE 110284 Subscriber

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    Entirely possible. Also, if the diff amp is being thrown out of balance by the C1451, the DC offset could go high which would trigger protection. Either way, the C1451's need to go.
     
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  7. petehall347

    petehall347 the brandy coffee man Subscriber

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    overworked maybe
     
  8. Eastham

    Eastham More Class-A than ever!

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    Man, electronics are wierd. I'll get some KSC3503's on order now hopefully they'll be here within the week. Any specific suffix I should go with?
     
  9. Hipocrates

    Hipocrates Anti-Muppet

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    2SC1451 is the work of the devil.
    They did all kind of things on my 7500 too.
     
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  10. LesE

    LesE 110284 Subscriber

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    KSC3503DSTU
     
  11. Eastham

    Eastham More Class-A than ever!

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    Thanks!
     

     

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  12. Eastham

    Eastham More Class-A than ever!

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    The 1451's in my unit won't be able to hurt anyone anymore... Think I'm gonna pop them with some mains voltage in a totally safe and controlled manner...
     
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  13. Eastham

    Eastham More Class-A than ever!

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    Well guy's, I pulled one of the 1451's out of my SX-737 and put it in my 7500 since I'm going to replace them both in both units and all is well so far, gonna reset the DC offset and bias after a few minutes.
     
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  14. Watthour

    Watthour Electron Rancher - JS3600

    As long as you're going to be right there, replace the trimmers (VR1/2) and sleep better.
     
  15. Eastham

    Eastham More Class-A than ever!

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    I already replaced them with two pairs of Bourns 3386F Series trimmers. I like the 3386F trimmers for Pioneers because their lead spacing allows them to fit perfectly in the factory Pioneer holes. I also replaced VR3/4. Trimmers are cheap enough and it's extra reliability.
     
  16. Eastham

    Eastham More Class-A than ever!

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    Well guys the KSA1015's came intoday so I spent a little time pairing them all and got them installed. Just waiting on the C3503's.

    DSC_1771.JPG
     
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  17. rcs16

    rcs16 Super Member

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    I always suggest to put some glue on the leads/covers for the STK diodes, where they enter the package = cheap insurance.
    Also good to have ksc3503 and its complement ksa1381 in your stash, as they are used on so many units.
     
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  18. Eastham

    Eastham More Class-A than ever!

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    Yeah, i'll definitely be buying a few of these in bulk.

    More progress, The KSC3503's came in so I go them matched up and installed. All seems good in the neigborhood now. Thanks for the help guy's, I'll definitely be replacing those dastardly 2sc1451's In all the guy I own and acquire in the future. Just gotta pick up the re-cap where this guy left off, all the sub 10µF caps in the power supply he never replaced and on the Phono pre amp.

    DSC_1772.JPG

    I spent a while with my... precision adjustment tool... straightening the chassis out. Looks like someone dropped it at some point because the transformer was pushed up and the bottom panel is all crumpled. I also had to re adjust the front panel frame which was twisted.
     
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