SA-7500 differential pair issues.

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

  1. Eastham

    Eastham More Class-A than ever!

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    Hi guy's. So I recently ran into a problem, my newly accuired SA-7500's input differentail pair decided to die the other day with a loud bzzzz in the left channel, the differential pair tested fine on my multimeter but seemed to be breaking down in circuit causing DC on the left channel. No problem I thought, I put in a pair of KSA992FBU's, all voltages are good and I was back to the races. Or so I thought...

    About two days later the same thing happens, BZZZZZ *click*. Okay. Maybe one of the 992's was a dud, so I replaced them both again and again after about two days. BZZZZZZ click.

    Now today I decided to rob a pair of 2SA970's out of a spare amp I have, thinking maybe the lower collector current rating of the 992's was my downfall. So I power it up with the 970's in it and it worked on my DBT, powered it directly off the mains all was good. Hooked up my 8ohm test load and it ran for a few minutes idle before dying yet again...

    When it's working all my voltages on the transistors are fine and pretty much match the right channel and the schematic.

    I've got some KSA1015's coming as I was told these would be a better sub for the 726's but I can't help but feel that these are going to pop too. Nothings oscillating and it's nothing in my system that's causing it.

    I'm currently considering stealing a pair of 726's out of my SX-737 but if those blow I won't be able to use it.

    Any insight and suggestions are more than welcome. Thanks.
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    I would suggest NOT stealing the packages from your 737, but find the real problem.

    Incidentally, is the a 7500, or a 7500-II?
     
  3. Eastham

    Eastham More Class-A than ever!

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    Just the 7500.
     
  4. petehall347

    petehall347 the brandy coffee man Subscriber

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    check the solder joints for that channel
     
  5. Eastham

    Eastham More Class-A than ever!

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    They're all good, I re-soldered the whole thing and repaired some damaged traces, the previous owner tried to re-cap it and I had to fix all his mistakes..
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

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    does it affect the idle bias when its not working correctly ?
     

     

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  7. Eastham

    Eastham More Class-A than ever!

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    Nope.
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

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    wondering if a cap is in backwards
     
  9. Eastham

    Eastham More Class-A than ever!

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    All caps apear to be installed correctly.
     
  10. Eastham

    Eastham More Class-A than ever!

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    Seems I have no luck with differential pairs does it Pete. lol
     
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  11. Watthour

    Watthour Electron Rancher - JS3600

    FWIW, a KSA1220AY may be a better choice for those pairs, as well as the KSA1013 (or even the 1015). Given the current carry capacity of the originals, and the fact that the circuit may be taxing the driver (2SC1451), it might be pushing the limits of the 992s. The same applies to the 1451. It may be better off as a KSC3503.
     

     

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  12. Eastham

    Eastham More Class-A than ever!

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    Darn, I used to have a bag full of 1220's. I lost it moving. I'll wait for the 1015's to come in and if they fail, I'll buy another bag of 1220's. I'll also grab some 3503's. Thanks.
     
  13. LesE

    LesE 110284 Subscriber

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    Going by the values on the schematic, the collector current for the differential pair (Q1 & Q3) should be less than 0.5 mA each resulting in a power dissipation of roughly 15 mW. Neither of these results would be a problem for the KSA992 IMO.
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    have any resistors been changed ? or anything else for that matter ?
    or a better question is what has been changed with what ?
     
  15. Eastham

    Eastham More Class-A than ever!

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    Nope, everything looks to be original aside from the caps. The previous owner used Elna Slimic II's and Elna Cerafine's for the re-cap.
     
  16. petehall347

    petehall347 the brandy coffee man Subscriber

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    big question is did it work ok before it was touched ?
     

     

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  17. LesE

    LesE 110284 Subscriber

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    Was Q5 & Q6 (2sc1451) replaced?
     
  18. Watthour

    Watthour Electron Rancher - JS3600

    Agreed. It SHOULDN'T be a problem at all, thus Post # 2.
     
  19. Eastham

    Eastham More Class-A than ever!

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    Yeah, I didn't even know the previous owner had re-capped it. No doubt he want to hide his... Handy work...

    They look original but someone seems to have filled the top of their heatsinks with thermal paste for some reason.

    I'm going to start measuring components then, what's weird is that all the voltages seem fine.
     
  20. LesE

    LesE 110284 Subscriber

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    If the c1451's are still in place, replace them with KSC3503 ASAP.
     

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