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Help me decide: Transitors

Discussion in 'Solid State' started by drumbum, Dec 29, 2017.

  1. drumbum

    drumbum Super Member

    Messages:
    1,207
    I have a couple QSC series three 3800, one has a bad channel; a few bad outputs, so far.

    The amps have Toshiba 2SD424/2SB554. Plan to replace bad in pairs with MJ21193g/94g, don't want to couple an old Toshiba with a new ON semi.

    Three pairs of the ten (20 total per side), hmmmm.....

    Should I replace the entire channel with the On Semi combo and sell the good Toshiba's to recoup cost of new outputs? Would this help someone or should I stash 'em for future needs?

    Should I use the 95/96 instead?

    What would you do? Yes, you...there in the back.

    Thanks for any input on my outputs.
     

     

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  2. drumbum

    drumbum Super Member

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    Seriously, ...I can't edit the title?
     
    Last edited: Dec 30, 2017
  3. drumbum

    drumbum Super Member

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    1,207
    Six of ten bad 2sb554's.:thumbsdown: 424's test good.
     
  4. awillia6

    awillia6 Super Member

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    Some things you just have to get right the first time. Trick is knowing which ones they are beforehand.
     
    DRecovery likes this.
  5. redk9258

    redk9258 Super Member

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    True of so many things in life!

    I think you have to be a paid subscriber to edit a title.
     
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  6. awillia6

    awillia6 Super Member

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    I got another one! Money can solve problems that nothing else will.
     

     

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  7. drumbum

    drumbum Super Member

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    Harry, all output transistors were pulled and checked with dmm set to diode test.
    You wouldn't advise to change all the 2sd424's. Have you coupled new mj21194g's with old 2sd424's?

    Also Harry, is English your first language?
     
  8. avionic

    avionic " Black Knights " Subscriber

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    No.... If your going to replace outputs use complementary pairs.
     
    UncleBingo likes this.
  9. drumbum

    drumbum Super Member

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    1,207
    I have 5 each in hand. Waiting for the other ten from mouser (back ordered).
     
  10. elnaldo

    elnaldo Addicted Member

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    You can use a power amp with less output transistors if you don't need all that power. I usually install just one pair until the amp performs OK, I test it for some days before installing (and risking) an expensive batch of transistors.

    On other "bench amp" I've got for free, I never installed all the outputs, it runs with just 4 transistors (2 each channel), since I never play it loud.

    To match transistors without test circuits or test equipment, you can install them in circuit, connect a dummy load to the output, and play some 100 to 1000Hz sinewave, at mid volume, not stressing the amp, and measure AC (audio is AC) across the emitter resistors, to check if the voltages are balanced. One transistor conducting much more that the rest will show a higher voltage drop. Choose the parts with similar performance.

    I suggest to use a low freq (100-200-500Hz) since standard DMMs are more accurate measuring low freq AC.

    I'd do this test using a Dim Bult Tester just in case something goes wrong.
     
    Last edited: Jan 6, 2018
    dlucy likes this.
  11. drumbum

    drumbum Super Member

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    Thanks for that elnaldo.

    I'll try to digest that and make a plan.
     

     

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  12. elnaldo

    elnaldo Addicted Member

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    Basically, "matched transistors" share the load approx the same between them. This way you check that, an actual load in the amplifier, being shared by the transistors in parallel.

    Anyway nowadays manufacturing tolerantes are tight, so I think you'll be fine just buying new transistors from the same source. I'm sure they will perform very similar out of the box.
     
  13. drumbum

    drumbum Super Member

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    1,207
    The key is going to be finding why one 554 shorted bad enough to burn the board and lift the trace to it, and take out 5 other 554's.

    So far I've found one resistor leg lifted, like not soldered properly in factory, one main filter cap (12 2200uf 100v matsushitas) loose, unsoldered. Bridge rectifiers seem fine as do the rest of the resistors (aside from the 15ohm 1w I just found). Driver transistors seem fine but will double check.

    Once I have the old thermal paste cleaned up and all outputs installed, I plan to fire up channel with DBT.

    What tests can be done with a DMM on capacitors aside from a dead short?
     
    Last edited: Jan 6, 2018
  14. drumbum

    drumbum Super Member

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    Remaining T0-3's came in thurs. Got them installed with about .5" of solder left:rolleyes:.

    DBT fail:mad:. Tested everything...but the one of two bridge rectifiers. Sho nuff bad.

    Replaced and all good:banana:

    Have some re-assembly with thermal paste to do yet and adj. bias.

    QSC series three 3800 up and running. Now on to cosmetic resto of the other working 3800;)

    375w@8ohm 20-20 600w@4ohm 20-20 .1%thd .015%@1k thd
    75 pounds of fanless dual mono construction.:beerchug:
     
    elnaldo likes this.
  15. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Good to hear you have her up and running :).
     
  16. drumbum

    drumbum Super Member

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    Now to figure what the hell to do with...
     

     

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  17. drumbum

    drumbum Super Member

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    Maybe I could rent it out to help pay for an S.:rolleyes:
     
  18. classic carl

    classic carl Without Music, Life Would B FLAT. Subscriber

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    I believe only Subscribers can edit their titles. Click on the report button at the bottom of your post and ask a mod to change it for you.
     

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