Help me decide: Transitors

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

  1. drumbum

    drumbum Well-Known Member

    Messages:
    976
    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.
     
  2. drumbum

    drumbum Well-Known Member

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    976
    Seriously, ...I can't edit the title?
     
    Last edited: Dec 30, 2017
  3. drumbum

    drumbum Well-Known Member

    Messages:
    976
    Six of ten bad 2sb554's.:thumbsdown: 424's test good.
     
  4. awillia6

    awillia6 Active Member

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    460
    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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    Location:
    Illinois, close to St. Louis.
    True of so many things in life!

    I think you have to be a paid subscriber to edit a title.
     
    petehall347 likes this.
  6. awillia6

    awillia6 Active Member

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    460
    I got another one! Money can solve problems that nothing else will.
     
  7. Harryconover

    Harryconover Well-Known Member

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    767
    Location:
    Charlotte NC
    All of one bank should be of the same type even of the same date code if possible the two banks cane be of different types also wene you say they check good how are you checking them a device yoused in a bank need to be matched for gain and beackdown voltage with is had to check with out a good curve tracer (not cheap) so if you want to resale the old ones make shure to sell them as USED and there are worth less !
     
  8. drumbum

    drumbum Well-Known Member

    Messages:
    976
    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?
     
  9. Harryconover

    Harryconover Well-Known Member

    Messages:
    767
    Location:
    Charlotte NC
    Checking with a DMM is ok for good/bad test but little else and you don’t want to mix types for best performance and true English not the American is my language and I am well spoken not writing but in this format that dosent count so I try
     
  10. avionic

    avionic " Black Knights " Subscriber

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

    drumbum Well-Known Member

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    976
    I have 5 each in hand. Waiting for the other ten from mouser (back ordered).
     
  12. elnaldo

    elnaldo Addicted Member

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    Location:
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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
  13. drumbum

    drumbum Well-Known Member

    Messages:
    976
    Thanks for that elnaldo.

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

    Harryconover Well-Known Member

    Messages:
    767
    Location:
    Charlotte NC
    100 percent true just time consuming so manufacturers go with matched sets but this method works great
     
  15. elnaldo

    elnaldo Addicted Member

    Messages:
    6,714
    Location:
    Buenos Aires GMT-3
    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.
     
  16. drumbum

    drumbum Well-Known Member

    Messages:
    976
    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

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